BTRFS(5) BTRFS BTRFS(5)
NAME
btrfs - topics about the BTRFS filesystem (mount options, supported
file attributes and other)
DESCRIPTION
This document describes topics related to BTRFS that are not specific
to the tools. Currently covers:
1. mount options
2. filesystem features
3. checksum algorithms
4. compression
5. sysfs interface
6. filesystem exclusive operations
7. filesystem limits
8. bootloader support
9. file attributes
10. zoned mode
11. control device
12. filesystems with multiple block group profiles
13. seeding device
14. RAID56 status and recommended practices
15. storage model, hardware considerations
MOUNT OPTIONS
BTRFS SPECIFIC MOUNT OPTIONS
This section describes mount options specific to BTRFS. For the
generic mount options please refer to mount(8) manual page. The options
are sorted alphabetically (discarding the no prefix).
NOTE:
Most mount options apply to the whole filesystem and only options in
the first mounted subvolume will take effect. This is due to lack of
implementation and may change in the future. This means that (for
example) you can't set per-subvolume nodatacow, nodatasum, or com-
press using mount options. This should eventually be fixed, but it
has proved to be difficult to implement correctly within the Linux
VFS framework.
Mount options are processed in order, only the last occurrence of an
option takes effect and may disable other options due to constraints
(see e.g. nodatacow and compress). The output of mount command shows
which options have been applied.
acl, noacl
(default: on)
Enable/disable support for POSIX Access Control Lists (ACLs).
See the acl(5) manual page for more information about ACLs.
The support for ACL is build-time configurable
(BTRFS_FS_POSIX_ACL) and mount fails if acl is requested but the
feature is not compiled in.
autodefrag, noautodefrag
(since: 3.0, default: off)
Enable automatic file defragmentation. When enabled, small ran-
dom writes into files (in a range of tens of kilobytes, cur-
rently it's 64KiB) are detected and queued up for the defragmen-
tation process. Not well suited for large database workloads.
The read latency may increase due to reading the adjacent blocks
that make up the range for defragmentation, successive write
will merge the blocks in the new location.
WARNING:
Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2
as well as with Linux stable kernel versions ≥ 3.10.31, ≥
3.12.12 or ≥ 3.13.4 will break up the reflinks of COW data
(for example files copied with cp --reflink, snapshots or
de-duplicated data). This may cause considerable increase of
space usage depending on the broken up reflinks.
barrier, nobarrier
(default: on)
Ensure that all IO write operations make it through the device
cache and are stored permanently when the filesystem is at its
consistency checkpoint. This typically means that a flush com-
mand is sent to the device that will synchronize all pending
data and ordinary metadata blocks, then writes the superblock
and issues another flush.
The write flushes incur a slight hit and also prevent the IO
block scheduler to reorder requests in a more effective way.
Disabling barriers gets rid of that penalty but will most cer-
tainly lead to a corrupted filesystem in case of a crash or
power loss. The ordinary metadata blocks could be yet unwritten
at the time the new superblock is stored permanently, expecting
that the block pointers to metadata were stored permanently be-
fore.
On a device with a volatile battery-backed write-back cache, the
nobarrier option will not lead to filesystem corruption as the
pending blocks are supposed to make it to the permanent storage.
check_int, check_int_data, check_int_print_mask=<value>
(since: 3.0, default: off)
These debugging options control the behavior of the integrity
checking module (the BTRFS_FS_CHECK_INTEGRITY config option re-
quired). The main goal is to verify that all blocks from a given
transaction period are properly linked.
check_int enables the integrity checker module, which examines
all block write requests to ensure on-disk consistency, at a
large memory and CPU cost.
check_int_data includes extent data in the integrity checks, and
implies the check_int option.
check_int_print_mask takes a bitmask of BTRFSIC_PRINT_MASK_*
values as defined in fs/btrfs/check-integrity.c, to control the
integrity checker module behavior.
See comments at the top of fs/btrfs/check-integrity.c for more
information.
clear_cache
Force clearing and rebuilding of the disk space cache if some-
thing has gone wrong. See also: space_cache.
commit=<seconds>
(since: 3.12, default: 30)
Set the interval of periodic transaction commit when data are
synchronized to permanent storage. Higher interval values lead
to larger amount of unwritten data, which has obvious conse-
quences when the system crashes. The upper bound is not forced,
but a warning is printed if it's more than 300 seconds (5 min-
utes). Use with care.
compress, compress=<type[:level]>, compress-force, com-
press-force=<type[:level]>
(default: off, level support since: 5.1)
Control BTRFS file data compression. Type may be specified as
zlib, lzo, zstd or no (for no compression, used for remounting).
If no type is specified, zlib is used. If compress-force is
specified, then compression will always be attempted, but the
data may end up uncompressed if the compression would make them
larger.
Both zlib and zstd (since version 5.1) expose the compression
level as a tunable knob with higher levels trading speed and
memory (zstd) for higher compression ratios. This can be set by
appending a colon and the desired level. ZLIB accepts the range
[1, 9] and ZSTD accepts [1, 15]. If no level is set, both cur-
rently use a default level of 3. The value 0 is an alias for the
default level.
Otherwise some simple heuristics are applied to detect an incom-
pressible file. If the first blocks written to a file are not
compressible, the whole file is permanently marked to skip com-
pression. As this is too simple, the compress-force is a work-
around that will compress most of the files at the cost of some
wasted CPU cycles on failed attempts. Since kernel 4.15, a set
of heuristic algorithms have been improved by using frequency
sampling, repeated pattern detection and Shannon entropy calcu-
lation to avoid that.
NOTE:
If compression is enabled, nodatacow and nodatasum are dis-
abled.
datacow, nodatacow
(default: on)
Enable data copy-on-write for newly created files. Nodatacow
implies nodatasum, and disables compression. All files created
under nodatacow are also set the NOCOW file attribute (see
chattr(1)).
NOTE:
If nodatacow or nodatasum are enabled, compression is dis-
abled.
Updates in-place improve performance for workloads that do fre-
quent overwrites, at the cost of potential partial writes, in
case the write is interrupted (system crash, device failure).
datasum, nodatasum
(default: on)
Enable data checksumming for newly created files. Datasum im-
plies datacow, i.e. the normal mode of operation. All files cre-
ated under nodatasum inherit the "no checksums" property, how-
ever there's no corresponding file attribute (see chattr(1)).
NOTE:
If nodatacow or nodatasum are enabled, compression is dis-
abled.
There is a slight performance gain when checksums are turned
off, the corresponding metadata blocks holding the checksums do
not need to updated. The cost of checksumming of the blocks in
memory is much lower than the IO, modern CPUs feature hardware
support of the checksumming algorithm.
degraded
(default: off)
Allow mounts with less devices than the RAID profile constraints
require. A read-write mount (or remount) may fail when there
are too many devices missing, for example if a stripe member is
completely missing from RAID0.
Since 4.14, the constraint checks have been improved and are
verified on the chunk level, not at the device level. This al-
lows degraded mounts of filesystems with mixed RAID profiles for
data and metadata, even if the device number constraints would
not be satisfied for some of the profiles.
Example: metadata -- raid1, data -- single, devices -- /dev/sda,
/dev/sdb
Suppose the data are completely stored on sda, then missing sdb
will not prevent the mount, even if 1 missing device would nor-
mally prevent (any) single profile to mount. In case some of the
data chunks are stored on sdb, then the constraint of sin-
gle/data is not satisfied and the filesystem cannot be mounted.
device=<devicepath>
Specify a path to a device that will be scanned for BTRFS
filesystem during mount. This is usually done automatically by a
device manager (like udev) or using the btrfs device scan com-
mand (e.g. run from the initial ramdisk). In cases where this is
not possible the device mount option can help.
NOTE:
Booting e.g. a RAID1 system may fail even if all filesystem's
device paths are provided as the actual device nodes may not
be discovered by the system at that point.
discard, discard=sync, discard=async, nodiscard
(default: off, async support since: 5.6)
Enable discarding of freed file blocks. This is useful for SSD
devices, thinly provisioned LUNs, or virtual machine images;
however, every storage layer must support discard for it to
work.
In the synchronous mode (sync or without option value), lack of
asynchronous queued TRIM on the backing device TRIM can severely
degrade performance, because a synchronous TRIM operation will
be attempted instead. Queued TRIM requires newer than SATA revi-
sion 3.1 chipsets and devices.
The asynchronous mode (async) gathers extents in larger chunks
before sending them to the devices for TRIM. The overhead and
performance impact should be negligible compared to the previous
mode and it's supposed to be the preferred mode if needed.
If it is not necessary to immediately discard freed blocks, then
the fstrim tool can be used to discard all free blocks in a
batch. Scheduling a TRIM during a period of low system activity
will prevent latent interference with the performance of other
operations. Also, a device may ignore the TRIM command if the
range is too small, so running a batch discard has a greater
probability of actually discarding the blocks.
enospc_debug, noenospc_debug
(default: off)
Enable verbose output for some ENOSPC conditions. It's safe to
use but can be noisy if the system reaches near-full state.
fatal_errors=<action>
(since: 3.4, default: bug)
Action to take when encountering a fatal error.
bug BUG() on a fatal error, the system will stay in the
crashed state and may be still partially usable, but re-
boot is required for full operation
panic panic() on a fatal error, depending on other system con-
figuration, this may be followed by a reboot. Please re-
fer to the documentation of kernel boot parameters, e.g.
panic, oops or crashkernel.
flushoncommit, noflushoncommit
(default: off)
This option forces any data dirtied by a write in a prior trans-
action to commit as part of the current commit, effectively a
full filesystem sync.
This makes the committed state a fully consistent view of the
file system from the application's perspective (i.e. it includes
all completed file system operations). This was previously the
behavior only when a snapshot was created.
When off, the filesystem is consistent but buffered writes may
last more than one transaction commit.
fragment=<type>
(depends on compile-time option BTRFS_DEBUG, since: 4.4, de-
fault: off)
A debugging helper to intentionally fragment given type of block
groups. The type can be data, metadata or all. This mount option
should not be used outside of debugging environments and is not
recognized if the kernel config option BTRFS_DEBUG is not en-
abled.
nologreplay
(default: off, even read-only)
The tree-log contains pending updates to the filesystem until
the full commit. The log is replayed on next mount, this can be
disabled by this option. See also treelog. Note that nologre-
play is the same as norecovery.
WARNING:
Currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, mount also with nologre-
play.
max_inline=<bytes>
(default: min(2048, page size) )
Specify the maximum amount of space, that can be inlined in a
metadata b-tree leaf. The value is specified in bytes, option-
ally with a K suffix (case insensitive). In practice, this
value is limited by the filesystem block size (named sectorsize
at mkfs time), and memory page size of the system. In case of
sectorsize limit, there's some space unavailable due to leaf
headers. For example, a 4KiB sectorsize, maximum size of inline
data is about 3900 bytes.
Inlining can be completely turned off by specifying 0. This will
increase data block slack if file sizes are much smaller than
block size but will reduce metadata consumption in return.
NOTE:
The default value has changed to 2048 in kernel 4.6.
metadata_ratio=<value>
(default: 0, internal logic)
Specifies that 1 metadata chunk should be allocated after every
value data chunks. Default behaviour depends on internal logic,
some percent of unused metadata space is attempted to be main-
tained but is not always possible if there's not enough space
left for chunk allocation. The option could be useful to over-
ride the internal logic in favor of the metadata allocation if
the expected workload is supposed to be metadata intense (snap-
shots, reflinks, xattrs, inlined files).
norecovery
(since: 4.5, default: off)
Do not attempt any data recovery at mount time. This will dis-
able logreplay and avoids other write operations. Note that this
option is the same as nologreplay.
NOTE:
The opposite option recovery used to have different meaning
but was changed for consistency with other filesystems, where
norecovery is used for skipping log replay. BTRFS does the
same and in general will try to avoid any write operations.
rescan_uuid_tree
(since: 3.12, default: off)
Force check and rebuild procedure of the UUID tree. This should
not normally be needed.
rescue (since: 5.9)
Modes allowing mount with damaged filesystem structures.
• usebackuproot (since: 5.9, replaces standalone option useback-
uproot)
• nologreplay (since: 5.9, replaces standalone option nologre-
play)
• ignorebadroots, ibadroots (since: 5.11)
• ignoredatacsums, idatacsums (since: 5.11)
• all (since: 5.9)
skip_balance
(since: 3.3, default: off)
Skip automatic resume of an interrupted balance operation. The
operation can later be resumed with btrfs balance resume, or the
paused state can be removed with btrfs balance cancel. The de-
fault behaviour is to resume an interrupted balance immediately
after a volume is mounted.
space_cache, space_cache=<version>, nospace_cache
(nospace_cache since: 3.2, space_cache=v1 and space_cache=v2
since 4.5, default: space_cache=v1)
Options to control the free space cache. The free space cache
greatly improves performance when reading block group free space
into memory. However, managing the space cache consumes some re-
sources, including a small amount of disk space.
There are two implementations of the free space cache. The orig-
inal one, referred to as v1, is the safe default. The v1 space
cache can be disabled at mount time with nospace_cache without
clearing.
On very large filesystems (many terabytes) and certain work-
loads, the performance of the v1 space cache may degrade drasti-
cally. The v2 implementation, which adds a new b-tree called the
free space tree, addresses this issue. Once enabled, the v2
space cache will always be used and cannot be disabled unless it
is cleared. Use clear_cache,space_cache=v1 or
clear_cache,nospace_cache to do so. If v2 is enabled, kernels
without v2 support will only be able to mount the filesystem in
read-only mode.
The btrfs-check(8) and :doc:`mkfs.btrfs(8)<mkfs.btrfs> commands
have full v2 free space cache support since v4.19.
If a version is not explicitly specified, the default implemen-
tation will be chosen, which is v1.
ssd, ssd_spread, nossd, nossd_spread
(default: SSD autodetected)
Options to control SSD allocation schemes. By default, BTRFS
will enable or disable SSD optimizations depending on status of
a device with respect to rotational or non-rotational type. This
is determined by the contents of /sys/block/DEV/queue/rota-
tional). If it is 0, the ssd option is turned on. The option
nossd will disable the autodetection.
The optimizations make use of the absence of the seek penalty
that's inherent for the rotational devices. The blocks can be
typically written faster and are not offloaded to separate
threads.
NOTE:
Since 4.14, the block layout optimizations have been dropped.
This used to help with first generations of SSD devices.
Their FTL (flash translation layer) was not effective and the
optimization was supposed to improve the wear by better
aligning blocks. This is no longer true with modern SSD de-
vices and the optimization had no real benefit. Furthermore
it caused increased fragmentation. The layout tuning has been
kept intact for the option ssd_spread.
The ssd_spread mount option attempts to allocate into bigger and
aligned chunks of unused space, and may perform better on
low-end SSDs. ssd_spread implies ssd, enabling all other SSD
heuristics as well. The option nossd will disable all SSD op-
tions while nossd_spread only disables ssd_spread.
subvol=<path>
Mount subvolume from path rather than the toplevel subvolume.
The path is always treated as relative to the toplevel subvol-
ume. This mount option overrides the default subvolume set for
the given filesystem.
subvolid=<subvolid>
Mount subvolume specified by a subvolid number rather than the
toplevel subvolume. You can use btrfs subvolume list of btrfs
subvolume show to see subvolume ID numbers. This mount option
overrides the default subvolume set for the given filesystem.
NOTE:
If both subvolid and subvol are specified, they must point at
the same subvolume, otherwise the mount will fail.
thread_pool=<number>
(default: min(NRCPUS + 2, 8) )
The number of worker threads to start. NRCPUS is number of
on-line CPUs detected at the time of mount. Small number leads
to less parallelism in processing data and metadata, higher num-
bers could lead to a performance hit due to increased locking
contention, process scheduling, cache-line bouncing or costly
data transfers between local CPU memories.
treelog, notreelog
(default: on)
Enable the tree logging used for fsync and O_SYNC writes. The
tree log stores changes without the need of a full filesystem
sync. The log operations are flushed at sync and transaction
commit. If the system crashes between two such syncs, the pend-
ing tree log operations are replayed during mount.
WARNING:
Currently, the tree log is replayed even with a read-only
mount! To disable that behaviour, also mount with nologre-
play.
The tree log could contain new files/directories, these would
not exist on a mounted filesystem if the log is not replayed.
usebackuproot
(since: 4.6, default: off)
Enable autorecovery attempts if a bad tree root is found at
mount time. Currently this scans a backup list of several pre-
vious tree roots and tries to use the first readable. This can
be used with read-only mounts as well.
NOTE:
This option has replaced recovery.
user_subvol_rm_allowed
(default: off)
Allow subvolumes to be deleted by their respective owner. Other-
wise, only the root user can do that.
NOTE:
Historically, any user could create a snapshot even if he was
not owner of the source subvolume, the subvolume deletion has
been restricted for that reason. The subvolume creation has
been restricted but this mount option is still required. This
is a usability issue. Since 4.18, the rmdir(2) syscall can
delete an empty subvolume just like an ordinary directory.
Whether this is possible can be detected at runtime, see
rmdir_subvol feature in FILESYSTEM FEATURES.
DEPRECATED MOUNT OPTIONS
List of mount options that have been removed, kept for backward compat-
ibility.
recovery
(since: 3.2, default: off, deprecated since: 4.5)
NOTE:
This option has been replaced by usebackuproot and should not
be used but will work on 4.5+ kernels.
inode_cache, noinode_cache
(removed in: 5.11, since: 3.0, default: off)
NOTE:
The functionality has been removed in 5.11, any stale data
created by previous use of the inode_cache option can be re-
moved by btrfs check --clear-ino-cache.
NOTES ON GENERIC MOUNT OPTIONS
Some of the general mount options from mount(8) that affect BTRFS and
are worth mentioning.
noatime
under read intensive work-loads, specifying noatime signifi-
cantly improves performance because no new access time informa-
tion needs to be written. Without this option, the default is
relatime, which only reduces the number of inode atime updates
in comparison to the traditional strictatime. The worst case for
atime updates under relatime occurs when many files are read
whose atime is older than 24 h and which are freshly snapshot-
ted. In that case the atime is updated and COW happens - for
each file - in bulk. See also https://lwn.net/Articles/499293/ -
Atime and btrfs: a bad combination? (LWN, 2012-05-31).
Note that noatime may break applications that rely on atime up-
times like the venerable Mutt (unless you use maildir mail-
boxes).
FILESYSTEM FEATURES
The basic set of filesystem features gets extended over time. The back-
ward compatibility is maintained and the features are optional, need to
be explicitly asked for so accidental use will not create incompatibil-
ities.
There are several classes and the respective tools to manage the fea-
tures:
at mkfs time only
This is namely for core structures, like the b-tree nodesize or
checksum algorithm, see mkfs.btrfs(8) for more details.
after mkfs, on an unmounted filesystem
Features that may optimize internal structures or add new struc-
tures to support new functionality, see btrfstune(8). The com-
mand btrfs inspect-internal dump-super /dev/sdx will dump a su-
perblock, you can map the value of incompat_flags to the fea-
tures listed below
after mkfs, on a mounted filesystem
The features of a filesystem (with a given UUID) are listed in
/sys/fs/btrfs/UUID/features/, one file per feature. The status
is stored inside the file. The value 1 is for enabled and ac-
tive, while 0 means the feature was enabled at mount time but
turned off afterwards.
Whether a particular feature can be turned on a mounted filesys-
tem can be found in the directory /sys/fs/btrfs/features/, one
file per feature. The value 1 means the feature can be enabled.
List of features (see also mkfs.btrfs(8) section FILESYSTEM FEATURES):
big_metadata
(since: 3.4)
the filesystem uses nodesize for metadata blocks, this can be
bigger than the page size
compress_lzo
(since: 2.6.38)
the lzo compression has been used on the filesystem, either as a
mount option or via btrfs filesystem defrag.
compress_zstd
(since: 4.14)
the zstd compression has been used on the filesystem, either as
a mount option or via btrfs filesystem defrag.
default_subvol
(since: 2.6.34)
the default subvolume has been set on the filesystem
extended_iref
(since: 3.7)
increased hardlink limit per file in a directory to 65536, older
kernels supported a varying number of hardlinks depending on the
sum of all file name sizes that can be stored into one metadata
block
free_space_tree
(since: 4.5)
free space representation using a dedicated b-tree, successor of
v1 space cache
metadata_uuid
(since: 5.0)
the main filesystem UUID is the metadata_uuid, which stores the
new UUID only in the superblock while all metadata blocks still
have the UUID set at mkfs time, see btrfstune(8) for more
mixed_backref
(since: 2.6.31)
the last major disk format change, improved backreferences, now
default
mixed_groups
(since: 2.6.37)
mixed data and metadata block groups, i.e. the data and metadata
are not separated and occupy the same block groups, this mode is
suitable for small volumes as there are no constraints how the
remaining space should be used (compared to the split mode,
where empty metadata space cannot be used for data and vice
versa)
on the other hand, the final layout is quite unpredictable and
possibly highly fragmented, which means worse performance
no_holes
(since: 3.14)
improved representation of file extents where holes are not ex-
plicitly stored as an extent, saves a few percent of metadata if
sparse files are used
raid1c34
(since: 5.5)
extended RAID1 mode with copies on 3 or 4 devices respectively
RAID56 (since: 3.9)
the filesystem contains or contained a RAID56 profile of block
groups
rmdir_subvol
(since: 4.18)
indicate that rmdir(2) syscall can delete an empty subvolume
just like an ordinary directory. Note that this feature only de-
pends on the kernel version.
skinny_metadata
(since: 3.10)
reduced-size metadata for extent references, saves a few percent
of metadata
send_stream_version
(since: 5.10)
number of the highest supported send stream version
supported_checksums
(since: 5.5)
list of checksum algorithms supported by the kernel module, the
respective modules or built-in implementing the algorithms need
to be present to mount the filesystem, see CHECKSUM ALGORITHMS
supported_sectorsizes
(since: 5.13)
list of values that are accepted as sector sizes (mkfs.btrfs
--sectorsize) by the running kernel
supported_rescue_options
(since: 5.11)
list of values for the mount option rescue that are supported by
the running kernel, see btrfs(5)
zoned (since: 5.12)
zoned mode is allocation/write friendly to host-managed zoned
devices, allocation space is partitioned into fixed-size zones
that must be updated sequentially, see ZONED MODE
SWAPFILE SUPPORT
A swapfile is file-backed memory that the system uses to temporarily
offload the RAM. It is supported since kernel 5.0. Use swapon(8) to
activate the swapfile. There are some limitations of the implementation
in BTRFS and Linux swap subsystem:
• filesystem - must be only single device
• filesystem - must have only single data profile
• swapfile - the containing subvolume cannot be snapshotted
• swapfile - must be preallocated (i.e. no holes)
• swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)
The limitations come namely from the COW-based design and mapping layer
of blocks that allows the advanced features like relocation and
multi-device filesystems. However, the swap subsystem expects simpler
mapping and no background changes of the file block location once
they've been assigned to swap.
With active swapfiles, the following whole-filesystem operations will
skip swapfile extents or may fail:
• balance - block groups with swapfile extents are skipped and re-
ported, the rest will be processed normally
• resize grow - unaffected
• resize shrink - works as long as the extents are outside of the
shrunk range
• device add - a new device does not interfere with existing swapfile
and this operation will work, though no new swapfile can be activated
afterwards
• device delete - if the device has been added as above, it can be also
deleted
• device replace - ditto
When there are no active swapfiles and a whole-filesystem exclusive op-
eration is running (e.g. balance, device delete, shrink), the swapfiles
cannot be temporarily activated. The operation must finish first.
To create and activate a swapfile run the following commands:
# truncate -s 0 swapfile
# chattr +C swapfile
# fallocate -l 2G swapfile
# chmod 0600 swapfile
# mkswap swapfile
# swapon swapfile
Since version 6.1 it's possible to create the swapfile in a single com-
mand (except the activation):
# btrfs filesystem mkswapfile swapfile
# swapon swapfile
Please note that the UUID returned by the mkswap utility identifies the
swap "filesystem" and because it's stored in a file, it's not generally
visible and usable as an identifier unlike if it was on a block device.
The file will appear in /proc/swaps:
# cat /proc/swaps
Filename Type Size Used Priority
/path/swapfile file 2097152 0 -2
The swapfile can be created as one-time operation or, once properly
created, activated on each boot by the swapon -a command (usually
started by the service manager). Add the following entry to /etc/fstab,
assuming the filesystem that provides the /path has been already
mounted at this point. Additional mount options relevant for the swap-
file can be set too (like priority, not the BTRFS mount options).
/path/swapfile none swap defaults 0 0
HIBERNATION
A swapfile can be used for hibernation but it's not straightforward.
Before hibernation a resume offset must be written to file
/sys/power/resume_offset or the kernel command line parameter re-
sume_offset must be set.
The value is the physical offset on the device. Note that this is not
the same value that filefrag prints as physical offset!
Btrfs filesystem uses mapping between logical and physical addresses
but here the physical can still map to one or more device-specific
physical block addresses. It's the device-specific physical offset that
is suitable as resume offset.
Since version 6.1 there's a command btrfs inspect-internal map-swapfile
that will print the device physical offset and the adjusted value for
/sys/power/resume_offset. Note that the value is divided by page size,
i.e. it's not the offset itself.
# btrfs filesystem mkswapfile swapfile
# btrfs inspect-internal map-swapfile swapfile
Physical start: 811511726080
Resume offset: 198122980
For scripting and convenience the option -r will print just the offset:
# btrfs inspect-internal map-swapfile -r swapfile
198122980
The command map-swapfile also verifies all the requirements, i.e. no
holes, single device, etc.
TROUBLESHOOTING
If the swapfile activation fails please verify that you followed all
the steps above or check the system log (e.g. dmesg or journalctl) for
more information.
Notably, the swapon utility exits with a message that does not say what
failed:
# swapon /path/swapfile
swapon: /path/swapfile: swapon failed: Invalid argument
The specific reason is likely to be printed to the system log by the
btrfs module:
# journalctl -t kernel | grep swapfile
kernel: BTRFS warning (device sda): swapfile must have single data profile
CHECKSUM ALGORITHMS
Data and metadata are checksummed by default, the checksum is calcu-
lated before write and verified after reading the blocks from devices.
The whole metadata block has a checksum stored inline in the b-tree
node header, each data block has a detached checksum stored in the
checksum tree.
There are several checksum algorithms supported. The default and back-
ward compatible is crc32c. Since kernel 5.5 there are three more with
different characteristics and trade-offs regarding speed and strength.
The following list may help you to decide which one to select.
CRC32C (32bit digest)
default, best backward compatibility, very fast, modern CPUs
have instruction-level support, not collision-resistant but
still good error detection capabilities
XXHASH (64bit digest)
can be used as CRC32C successor, very fast, optimized for modern
CPUs utilizing instruction pipelining, good collision resistance
and error detection
SHA256 (256bit digest)
a cryptographic-strength hash, relatively slow but with possible
CPU instruction acceleration or specialized hardware cards, FIPS
certified and in wide use
BLAKE2b (256bit digest)
a cryptographic-strength hash, relatively fast with possible CPU
acceleration using SIMD extensions, not standardized but based
on BLAKE which was a SHA3 finalist, in wide use, the algorithm
used is BLAKE2b-256 that's optimized for 64bit platforms
The digest size affects overall size of data block checksums stored in
the filesystem. The metadata blocks have a fixed area up to 256 bits
(32 bytes), so there's no increase. Each data block has a separate
checksum stored, with additional overhead of the b-tree leaves.
Approximate relative performance of the algorithms, measured against
CRC32C using reference software implementations on a 3.5GHz intel CPU:
┌────────┬─────────────┬───────┬─────────────────┐
│Digest │ Cycles/4KiB │ Ratio │ Implementation │
├────────┼─────────────┼───────┼─────────────────┤
│CRC32C │ 1700 │ 1.00 │ CPU instruction │
├────────┼─────────────┼───────┼─────────────────┤
│XXHASH │ 2500 │ 1.44 │ reference impl. │
├────────┼─────────────┼───────┼─────────────────┤
│SHA256 │ 105000 │ 61 │ reference impl. │
├────────┼─────────────┼───────┼─────────────────┤
│SHA256 │ 36000 │ 21 │ libgcrypt/AVX2 │
├────────┼─────────────┼───────┼─────────────────┤
│SHA256 │ 63000 │ 37 │ libsodium/AVX2 │
├────────┼─────────────┼───────┼─────────────────┤
│BLAKE2b │ 22000 │ 13 │ reference impl. │
├────────┼─────────────┼───────┼─────────────────┤
│BLAKE2b │ 19000 │ 11 │ libgcrypt/AVX2 │
├────────┼─────────────┼───────┼─────────────────┤
│BLAKE2b │ 19000 │ 11 │ libsodium/AVX2 │
└────────┴─────────────┴───────┴─────────────────┘
Many kernels are configured with SHA256 as built-in and not as a mod-
ule. The accelerated versions are however provided by the modules and
must be loaded explicitly (modprobe sha256) before mounting the
filesystem to make use of them. You can check in
/sys/fs/btrfs/FSID/checksum which one is used. If you see
sha256-generic, then you may want to unmount and mount the filesystem
again, changing that on a mounted filesystem is not possible. Check
the file /proc/crypto, when the implementation is built-in, you'd find
name : sha256
driver : sha256-generic
module : kernel
priority : 100
...
while accelerated implementation is e.g.
name : sha256
driver : sha256-avx2
module : sha256_ssse3
priority : 170
...
COMPRESSION
Btrfs supports transparent file compression. There are three algorithms
available: ZLIB, LZO and ZSTD (since v4.14), with various levels. The
compression happens on the level of file extents and the algorithm is
selected by file property, mount option or by a defrag command. You
can have a single btrfs mount point that has some files that are uncom-
pressed, some that are compressed with LZO, some with ZLIB, for in-
stance (though you may not want it that way, it is supported).
Once the compression is set, all newly written data will be compressed,
i.e. existing data are untouched. Data are split into smaller chunks
(128KiB) before compression to make random rewrites possible without a
high performance hit. Due to the increased number of extents the meta-
data consumption is higher. The chunks are compressed in parallel.
The algorithms can be characterized as follows regarding the speed/ra-
tio trade-offs:
ZLIB
• slower, higher compression ratio
• levels: 1 to 9, mapped directly, default level is 3
• good backward compatibility
LZO
• faster compression and decompression than ZLIB, worse compres-
sion ratio, designed to be fast
• no levels
• good backward compatibility
ZSTD
• compression comparable to ZLIB with higher compression/decom-
pression speeds and different ratio
• levels: 1 to 15, mapped directly (higher levels are not avail-
able)
• since 4.14, levels since 5.1
The differences depend on the actual data set and cannot be expressed
by a single number or recommendation. Higher levels consume more CPU
time and may not bring a significant improvement, lower levels are
close to real time.
HOW TO ENABLE COMPRESSION
Typically the compression can be enabled on the whole filesystem, spec-
ified for the mount point. Note that the compression mount options are
shared among all mounts of the same filesystem, either bind mounts or
subvolume mounts. Please refer to section MOUNT OPTIONS.
$ mount -o compress=zstd /dev/sdx /mnt
This will enable the zstd algorithm on the default level (which is 3).
The level can be specified manually too like zstd:3. Higher levels com-
press better at the cost of time. This in turn may cause increased
write latency, low levels are suitable for real-time compression and on
reasonably fast CPU don't cause noticeable performance drops.
$ btrfs filesystem defrag -czstd file
The command above will start defragmentation of the whole file and ap-
ply the compression, regardless of the mount option. (Note: specifying
level is not yet implemented). The compression algorithm is not persis-
tent and applies only to the defragmentation command, for any other
writes other compression settings apply.
Persistent settings on a per-file basis can be set in two ways:
$ chattr +c file
$ btrfs property set file compression zstd
The first command is using legacy interface of file attributes inher-
ited from ext2 filesystem and is not flexible, so by default the zlib
compression is set. The other command sets a property on the file with
the given algorithm. (Note: setting level that way is not yet imple-
mented.)
COMPRESSION LEVELS
The level support of ZLIB has been added in v4.14, LZO does not support
levels (the kernel implementation provides only one), ZSTD level sup-
port has been added in v5.1.
There are 9 levels of ZLIB supported (1 to 9), mapping 1:1 from the
mount option to the algorithm defined level. The default is level 3,
which provides the reasonably good compression ratio and is still rea-
sonably fast. The difference in compression gain of levels 7, 8 and 9
is comparable but the higher levels take longer.
The ZSTD support includes levels 1 to 15, a subset of full range of
what ZSTD provides. Levels 1-3 are real-time, 4-8 slower with improved
compression and 9-15 try even harder though the resulting size may not
be significantly improved.
Level 0 always maps to the default. The compression level does not af-
fect compatibility.
INCOMPRESSIBLE DATA
Files with already compressed data or with data that won't compress
well with the CPU and memory constraints of the kernel implementations
are using a simple decision logic. If the first portion of data being
compressed is not smaller than the original, the compression of the
file is disabled -- unless the filesystem is mounted with com-
press-force. In that case compression will always be attempted on the
file only to be later discarded. This is not optimal and subject to op-
timizations and further development.
If a file is identified as incompressible, a flag is set (NOCOMPRESS)
and it's sticky. On that file compression won't be performed unless
forced. The flag can be also set by chattr +m (since e2fsprogs 1.46.2)
or by properties with value no or none. Empty value will reset it to
the default that's currently applicable on the mounted filesystem.
There are two ways to detect incompressible data:
• actual compression attempt - data are compressed, if the result is
not smaller, it's discarded, so this depends on the algorithm and
level
• pre-compression heuristics - a quick statistical evaluation on the
data is performed and based on the result either compression is per-
formed or skipped, the NOCOMPRESS bit is not set just by the heuris-
tic, only if the compression algorithm does not make an improvement
$ lsattr file
---------------------m file
Using the forcing compression is not recommended, the heuristics are
supposed to decide that and compression algorithms internally detect
incompressible data too.
PRE-COMPRESSION HEURISTICS
The heuristics aim to do a few quick statistical tests on the com-
pressed data in order to avoid probably costly compression that would
turn out to be inefficient. Compression algorithms could have internal
detection of incompressible data too but this leads to more overhead as
the compression is done in another thread and has to write the data
anyway. The heuristic is read-only and can utilize cached memory.
The tests performed based on the following: data sampling, long re-
peated pattern detection, byte frequency, Shannon entropy.
COMPATIBILITY
Compression is done using the COW mechanism so it's incompatible with
nodatacow. Direct IO works on compressed files but will fall back to
buffered writes and leads to recompression. Currently nodatasum and
compression don't work together.
The compression algorithms have been added over time so the version
compatibility should be also considered, together with other tools that
may access the compressed data like bootloaders.
SYSFS INTERFACE
Btrfs has a sysfs interface to provide extra knobs.
The top level path is /sys/fs/btrfs/, and the main directory layout is
the following:
┌─────────────────────────────┬─────────────────────┬─────────┐
│Relative Path │ Description │ Version │
├─────────────────────────────┼─────────────────────┼─────────┤
│features/ │ All supported fea- │ 3.14+ │
│ │ tures │ │
├─────────────────────────────┼─────────────────────┼─────────┤
│<UUID>/ │ Mounted fs UUID │ 3.14+ │
├─────────────────────────────┼─────────────────────┼─────────┤
│<UUID>/allocation/ │ Space allocation │ 3.14+ │
│ │ info │ │
├─────────────────────────────┼─────────────────────┼─────────┤
│<UUID>/features/ │ Features of the │ 3.14+ │
│ │ filesystem │ │
├─────────────────────────────┼─────────────────────┼─────────┤
│<UUID>/devices/<DE- │ Symlink to each │ 5.6+ │
│VID>/ │ block device sysfs │ │
├─────────────────────────────┼─────────────────────┼─────────┤
│<UUID>/devinfo/<DE- │ Btrfs specific info │ 5.6+ │
│VID>/ │ for each device │ │
├─────────────────────────────┼─────────────────────┼─────────┤
│<UUID>/qgroups/ │ Global qgroup info │ 5.9+ │
├─────────────────────────────┼─────────────────────┼─────────┤
│<UUID>/qgroups/<LEVEL>_<ID>/ │ Info for each │ 5.9+ │
│ │ qgroup │ │
└─────────────────────────────┴─────────────────────┴─────────┘
For /sys/fs/btrfs/features/ directory, each file means a supported fea-
ture for the current kernel.
For /sys/fs/btrfs/<UUID>/features/ directory, each file means an en-
abled feature for the mounted filesystem.
The features shares the same name in section FILESYSTEM FEATURES.
Files in /sys/fs/btrfs/<UUID>/ directory are:
bg_reclaim_threshold
(RW, since: 5.19)
Used space percentage of total device space to start auto block
group claim. Mostly for zoned devices.
checksum
(RO, since: 5.5)
The checksum used for the mounted filesystem. This includes
both the checksum type (see section CHECKSUM ALGORITHMS) and the
implemented driver (mostly shows if it's hardware accelerated).
clone_alignment
(RO, since: 3.16)
The bytes alignment for clone and dedupe ioctls.
commit_stats
(RW, since: 6.0)
The performance statistics for btrfs transaction commit. Mostly
for debug purposes.
Writing into this file will reset the maximum commit duration to
the input value.
exclusive_operation
(RO, since: 5.10)
Shows the running exclusive operation. Check section FILESYSTEM
EXCLUSIVE OPERATIONS for details.
generation
(RO, since: 5.11)
Show the generation of the mounted filesystem.
label (RW, since: 3.14)
Show the current label of the mounted filesystem.
metadata_uuid
(RO, since: 5.0)
Shows the metadata uuid of the mounted filesystem. Check meta-
data_uuid feature for more details.
nodesize
(RO, since: 3.14)
Show the nodesize of the mounted filesystem.
quota_override
(RW, since: 4.13)
Shows the current quota override status. 0 means no quota over-
ride. 1 means quota override, quota can ignore the existing
limit settings.
read_policy
(RW, since: 5.11)
Shows the current balance policy for reads. Currently only
"pid" (balance using pid value) is supported.
sectorsize
(RO, since: 3.14)
Shows the sectorsize of the mounted filesystem.
Files and directories in /sys/fs/btrfs/<UUID>/allocations directory
are:
global_rsv_reserved
(RO, since: 3.14)
The used bytes of the global reservation.
global_rsv_size
(RO, since: 3.14)
The total size of the global reservation.
data/, metadata/ and system/ directories
(RO, since: 5.14)
Space info accounting for the 3 chunk types. Mostly for debug
purposes.
Files in /sys/fs/btrfs/<UUID>/allocations/{data,metadata,system} direc-
tory are:
bg_reclaim_threshold
(RW, since: 5.19)
Reclaimable space percentage of block group's size (excluding
permanently unusable space) to reclaim the block group. Can be
used on regular or zoned devices.
chunk_size
(RW, since: 6.0)
Shows the chunk size. Can be changed for data and metadata.
Cannot be set for zoned devices.
Files in /sys/fs/btrfs/<UUID>/devinfo/<DEVID> directory are:
error_stats:
(RO, since: 5.14)
Shows all the history error numbers of the device.
fsid: (RO, since: 5.17)
Shows the fsid which the device belongs to. It can be different
than the <UUID> if it's a seed device.
in_fs_metadata
(RO, since: 5.6)
Shows whether we have found the device. Should always be 1, as
if this turns to 0, the <DEVID> directory would get removed au-
tomatically.
missing
(RO, since: 5.6)
Shows whether the device is missing.
replace_target
(RO, since: 5.6)
Shows whether the device is the replace target. If no dev-re-
place is running, this value should be 0.
scrub_speed_max
(RW, since: 5.14)
Shows the scrub speed limit for this device. The unit is
Bytes/s. 0 means no limit.
writeable
(RO, since: 5.6)
Show if the device is writeable.
Files in /sys/fs/btrfs/<UUID>/qgroups/ directory are:
enabled
(RO, since: 6.1)
Shows if qgroup is enabled. Also, if qgroup is disabled, the
qgroups directory would be removed automatically.
inconsistent
(RO, since: 6.1)
Shows if the qgroup numbers are inconsistent. If 1, it's recom-
mended to do a qgroup rescan.
drop_subtree_threshold
(RW, since: 6.1)
Shows the subtree drop threshold to automatically mark qgroup
inconsistent.
When dropping large subvolumes with qgroup enabled, there would
be a huge load for qgroup accounting. If we have a subtree
whose level is larger than or equal to this value, we will not
trigger qgroup account at all, but mark qgroup inconsistent to
avoid the huge workload.
Default value is 8, where no subtree drop can trigger qgroup.
Lower value can reduce qgroup workload, at the cost of extra
qgroup rescan to re-calculate the numbers.
Files in /sys/fs/btrfs/<UUID>/<LEVEL>_<ID>/ directory are:
exclusive
(RO, since: 5.9)
Shows the exclusively owned bytes of the qgroup.
limit_flags
(RO, since: 5.9)
Shows the numeric value of the limit flags. If 0, means no
limit implied.
max_exclusive
(RO, since: 5.9)
Shows the limits on exclusively owned bytes.
max_referenced
(RO, since: 5.9)
Shows the limits on referenced bytes.
referenced
(RO, since: 5.9)
Shows the referenced bytes of the qgroup.
rsv_data
(RO, since: 5.9)
Shows the reserved bytes for data.
rsv_meta_pertrans
(RO, since: 5.9)
Shows the reserved bytes for per transaction metadata.
rsv_meta_prealloc
(RO, since: 5.9)
Shows the reserved bytes for preallocated metadata.
FILESYSTEM EXCLUSIVE OPERATIONS
There are several operations that affect the whole filesystem and can-
not be run in parallel. Attempt to start one while another is running
will fail (see exceptions below).
Since kernel 5.10 the currently running operation can be obtained from
/sys/fs/UUID/exclusive_operation with following values and operations:
• balance
• balance paused (since 5.17)
• device add
• device delete
• device replace
• resize
• swapfile activate
• none
Enqueuing is supported for several btrfs subcommands so they can be
started at once and then serialized.
There's an exception when a paused balance allows to start a device add
operation as they don't really collide and this can be used to add more
space for the balance to finish.
FILESYSTEM LIMITS
maximum file name length
255
This limit is imposed by Linux VFS, the structures of BTRFS
could store larger file names.
maximum symlink target length
depends on the nodesize value, for 4KiB it's 3949 bytes, for
larger nodesize it's 4095 due to the system limit PATH_MAX
The symlink target may not be a valid path, i.e. the path name
components can exceed the limits (NAME_MAX), there's no content
validation at symlink(3) creation.
maximum number of inodes
264 but depends on the available metadata space as the inodes
are created dynamically
Each subvolume is an independent namespace of inodes and thus
their numbers, so the limit is per subvolume, not for the whole
filesystem.
inode numbers
minimum number: 256 (for subvolumes), regular files and directo-
ries: 257, maximum number: (2:sup:64 - 256)
The inode numbers that can be assigned to user created files are
from the whole 64bit space except first 256 and last 256 in that
range that are reserved for internal b-tree identifiers.
maximum file length
inherent limit of BTRFS is 264 (16 EiB) but the practical limit
of Linux VFS is 263 (8 EiB)
maximum number of subvolumes
the subvolume ids can go up to 248 but the number of actual sub-
volumes depends on the available metadata space
The space consumed by all subvolume metadata includes bookkeep-
ing of shared extents can be large (MiB, GiB). The range is not
the full 64bit range because of qgroups that use the upper 16
bits for another purposes.
maximum number of hardlinks of a file in a directory
65536 when the extref feature is turned on during mkfs (de-
fault), roughly 100 otherwise
minimum filesystem size
the minimal size of each device depends on the mixed-bg feature,
without that (the default) it's about 109MiB, with mixed-bg it's
is 16MiB
BOOTLOADER SUPPORT
GRUB2 (https://www.gnu.org/software/grub) has the most advanced support
of booting from BTRFS with respect to features.
U-boot (https://www.denx.de/wiki/U-Boot/) has decent support for boot-
ing but not all BTRFS features are implemented, check the documenta-
tion.
EXTLINUX (from the https://syslinux.org project) has limited support
for BTRFS boot and hasn't been updated for for a long time so is not
recommended as bootloader.
In general, the first 1MiB on each device is unused with the exception
of primary superblock that is on the offset 64KiB and spans 4KiB. The
rest can be freely used by bootloaders or for other system information.
Note that booting from a filesystem on zoned device is not supported.
FILE ATTRIBUTES
The btrfs filesystem supports setting file attributes or flags. Note
there are old and new interfaces, with confusing names. The following
list should clarify that:
• attributes: chattr(1) or lsattr(1) utilities (the ioctls are
FS_IOC_GETFLAGS and FS_IOC_SETFLAGS), due to the ioctl names the at-
tributes are also called flags
• xflags: to distinguish from the previous, it's extended flags, with
tunable bits similar to the attributes but extensible and new bits
will be added in the future (the ioctls are FS_IOC_FSGETXATTR and
FS_IOC_FSSETXATTR but they are not related to extended attributes
that are also called xattrs), there's no standard tool to change the
bits, there's support in xfs_io(8) as command xfs_io -c chattr
Attributes
a append only, new writes are always written at the end of the
file
A no atime updates
c compress data, all data written after this attribute is set will
be compressed. Please note that compression is also affected by
the mount options or the parent directory attributes.
When set on a directory, all newly created files will inherit
this attribute. This attribute cannot be set with 'm' at the
same time.
C no copy-on-write, file data modifications are done in-place
When set on a directory, all newly created files will inherit
this attribute.
NOTE:
Due to implementation limitations, this flag can be set/unset
only on empty files.
d no dump, makes sense with 3rd party tools like dump(8), on BTRFS
the attribute can be set/unset but no other special handling is
done
D synchronous directory updates, for more details search open(2)
for O_SYNC and O_DSYNC
i immutable, no file data and metadata changes allowed even to the
root user as long as this attribute is set (obviously the excep-
tion is unsetting the attribute)
m no compression, permanently turn off compression on the given
file. Any compression mount options will not affect this file.
(chattr support added in 1.46.2)
When set on a directory, all newly created files will inherit
this attribute. This attribute cannot be set with c at the same
time.
S synchronous updates, for more details search open(2) for O_SYNC
and O_DSYNC
No other attributes are supported. For the complete list please refer
to the chattr(1) manual page.
XFLAGS
There's overlap of letters assigned to the bits with the attributes,
this list refers to what xfs_io(8) provides:
i immutable, same as the attribute
a append only, same as the attribute
s synchronous updates, same as the attribute S
A no atime updates, same as the attribute
d no dump, same as the attribute
ZONED MODE
Since version 5.12 btrfs supports so called zoned mode. This is a spe-
cial on-disk format and allocation/write strategy that's friendly to
zoned devices. In short, a device is partitioned into fixed-size zones
and each zone can be updated by append-only manner, or reset. As btrfs
has no fixed data structures, except the super blocks, the zoned mode
only requires block placement that follows the device constraints. You
can learn about the whole architecture at https://zonedstorage.io .
The devices are also called SMR/ZBC/ZNS, in host-managed mode. Note
that there are devices that appear as non-zoned but actually are, this
is drive-managed and using zoned mode won't help.
The zone size depends on the device, typical sizes are 256MiB or 1GiB.
In general it must be a power of two. Emulated zoned devices like
null_blk allow to set various zone sizes.
Requirements, limitations
• all devices must have the same zone size
• maximum zone size is 8GiB
• minimum zone size is 4MiB
• mixing zoned and non-zoned devices is possible, the zone writes are
emulated, but this is namely for testing
•
the super block is handled in a special way and is at different loca-
tions than on a non-zoned filesystem:
• primary: 0B (and the next two zones)
• secondary: 512GiB (and the next two zones)
• tertiary: 4TiB (4096GiB, and the next two zones)
Incompatible features
The main constraint of the zoned devices is lack of in-place update of
the data. This is inherently incompatible with some features:
• NODATACOW - overwrite in-place, cannot create such files
• fallocate - preallocating space for in-place first write
• mixed-bg - unordered writes to data and metadata, fixing that means
using separate data and metadata block groups
• booting - the zone at offset 0 contains superblock, resetting the
zone would destroy the bootloader data
Initial support lacks some features but they're planned:
• only single profile is supported
• fstrim - due to dependency on free space cache v1
Super block
As said above, super block is handled in a special way. In order to be
crash safe, at least one zone in a known location must contain a valid
superblock. This is implemented as a ring buffer in two consecutive
zones, starting from known offsets 0B, 512GiB and 4TiB.
The values are different than on non-zoned devices. Each new super
block is appended to the end of the zone, once it's filled, the zone is
reset and writes continue to the next one. Looking up the latest super
block needs to read offsets of both zones and determine the last writ-
ten version.
The amount of space reserved for super block depends on the zone size.
The secondary and tertiary copies are at distant offsets as the capac-
ity of the devices is expected to be large, tens of terabytes. Maximum
zone size supported is 8GiB, which would mean that e.g. offset 0-16GiB
would be reserved just for the super block on a hypothetical device of
that zone size. This is wasteful but required to guarantee crash
safety.
CONTROL DEVICE
There's a character special device /dev/btrfs-control with major and
minor numbers 10 and 234 (the device can be found under the 'misc' cat-
egory).
$ ls -l /dev/btrfs-control
crw------- 1 root root 10, 234 Jan 1 12:00 /dev/btrfs-control
The device accepts some ioctl calls that can perform following actions
on the filesystem module:
• scan devices for btrfs filesystem (i.e. to let multi-device filesys-
tems mount automatically) and register them with the kernel module
• similar to scan, but also wait until the device scanning process is
finished for a given filesystem
• get the supported features (can be also found under
/sys/fs/btrfs/features)
The device is created when btrfs is initialized, either as a module or
a built-in functionality and makes sense only in connection with that.
Running e.g. mkfs without the module loaded will not register the de-
vice and will probably warn about that.
In rare cases when the module is loaded but the device is not present
(most likely accidentally deleted), it's possible to recreate it by
# mknod --mode=600 /dev/btrfs-control c 10 234
or (since 5.11) by a convenience command
# btrfs rescue create-control-device
The control device is not strictly required but the device scanning
will not work and a workaround would need to be used to mount a
multi-device filesystem. The mount option device can trigger the de-
vice scanning during mount, see also btrfs device scan.
FILESYSTEM WITH MULTIPLE PROFILES
It is possible that a btrfs filesystem contains multiple block group
profiles of the same type. This could happen when a profile conversion
using balance filters is interrupted (see btrfs-balance(8)). Some
btrfs commands perform a test to detect this kind of condition and
print a warning like this:
WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1
The corresponding output of btrfs filesystem df might look like:
WARNING: Multiple block group profiles detected, see 'man btrfs(5)'.
WARNING: Data: single, raid1
WARNING: Metadata: single, raid1
Data, RAID1: total=832.00MiB, used=0.00B
Data, single: total=1.63GiB, used=0.00B
System, single: total=4.00MiB, used=16.00KiB
Metadata, single: total=8.00MiB, used=112.00KiB
Metadata, RAID1: total=64.00MiB, used=32.00KiB
GlobalReserve, single: total=16.25MiB, used=0.00B
There's more than one line for type Data and Metadata, while the pro-
files are single and RAID1.
This state of the filesystem OK but most likely needs the user/adminis-
trator to take an action and finish the interrupted tasks. This cannot
be easily done automatically, also the user knows the expected final
profiles.
In the example above, the filesystem started as a single device and
single block group profile. Then another device was added, followed by
balance with convert=raid1 but for some reason hasn't finished.
Restarting the balance with convert=raid1 will continue and end up with
filesystem with all block group profiles RAID1.
NOTE:
If you're familiar with balance filters, you can use con-
vert=raid1,profiles=single,soft, which will take only the uncon-
verted single profiles and convert them to raid1. This may speed up
the conversion as it would not try to rewrite the already convert
raid1 profiles.
Having just one profile is desired as this also clearly defines the
profile of newly allocated block groups, otherwise this depends on in-
ternal allocation policy. When there are multiple profiles present, the
order of selection is RAID56, RAID10, RAID1, RAID0 as long as the de-
vice number constraints are satisfied.
Commands that print the warning were chosen so they're brought to user
attention when the filesystem state is being changed in that regard.
This is: device add, device delete, balance cancel, balance pause. Com-
mands that report space usage: filesystem df, device usage. The command
filesystem usage provides a line in the overall summary:
Multiple profiles: yes (data, metadata)
SEEDING DEVICE
The COW mechanism and multiple devices under one hood enable an inter-
esting concept, called a seeding device: extending a read-only filesys-
tem on a device with another device that captures all writes. For exam-
ple imagine an immutable golden image of an operating system enhanced
with another device that allows to use the data from the golden image
and normal operation. This idea originated on CD-ROMs with base OS and
allowing to use them for live systems, but this became obsolete. There
are technologies providing similar functionality, like unionmount,
overlayfs or qcow2 image snapshot.
The seeding device starts as a normal filesystem, once the contents is
ready, btrfstune -S 1 is used to flag it as a seeding device. Mounting
such device will not allow any writes, except adding a new device by
btrfs device add. Then the filesystem can be remounted as read-write.
Given that the filesystem on the seeding device is always recognized as
read-only, it can be used to seed multiple filesystems from one device
at the same time. The UUID that is normally attached to a device is au-
tomatically changed to a random UUID on each mount.
Once the seeding device is mounted, it needs the writable device. After
adding it, something like remount -o remount,rw /path makes the
filesystem at /path ready for use. The simplest use case is to throw
away all changes by unmounting the filesystem when convenient.
Alternatively, deleting the seeding device from the filesystem can turn
it into a normal filesystem, provided that the writable device can also
contain all the data from the seeding device.
The seeding device flag can be cleared again by btrfstune -f -S 0, e.g.
allowing to update with newer data but please note that this will in-
validate all existing filesystems that use this particular seeding de-
vice. This works for some use cases, not for others, and the forcing
flag to the command is mandatory to avoid accidental mistakes.
Example how to create and use one seeding device:
# mkfs.btrfs /dev/sda
# mount /dev/sda /mnt/mnt1
... fill mnt1 with data
# umount /mnt/mnt1
# btrfstune -S 1 /dev/sda
# mount /dev/sda /mnt/mnt1
# btrfs device add /dev/sdb /mnt/mnt1
# mount -o remount,rw /mnt/mnt1
... /mnt/mnt1 is now writable
Now /mnt/mnt1 can be used normally. The device /dev/sda can be mounted
again with a another writable device:
# mount /dev/sda /mnt/mnt2
# btrfs device add /dev/sdc /mnt/mnt2
# mount -o remount,rw /mnt/mnt2
... /mnt/mnt2 is now writable
The writable device (/dev/sdb) can be decoupled from the seeding device
and used independently:
# btrfs device delete /dev/sda /mnt/mnt1
As the contents originated in the seeding device, it's possible to turn
/dev/sdb to a seeding device again and repeat the whole process.
A few things to note:
• it's recommended to use only single device for the seeding device, it
works for multiple devices but the single profile must be used in or-
der to make the seeding device deletion work
• block group profiles single and dup support the use cases above
• the label is copied from the seeding device and can be changed by
btrfs filesystem label
• each new mount of the seeding device gets a new random UUID
Chained seeding devices
Though it's not recommended and is rather an obscure and untested use
case, chaining seeding devices is possible. In the first example, the
writable device /dev/sdb can be turned onto another seeding device
again, depending on the unchanged seeding device /dev/sda. Then using
/dev/sdb as the primary seeding device it can be extended with another
writable device, say /dev/sdd, and it continues as before as a simple
tree structure on devices.
# mkfs.btrfs /dev/sda
# mount /dev/sda /mnt/mnt1
... fill mnt1 with data
# umount /mnt/mnt1
# btrfstune -S 1 /dev/sda
# mount /dev/sda /mnt/mnt1
# btrfs device add /dev/sdb /mnt/mnt1
# mount -o remount,rw /mnt/mnt1
... /mnt/mnt1 is now writable
# umount /mnt/mnt1
# btrfstune -S 1 /dev/sdb
# mount /dev/sdb /mnt/mnt1
# btrfs device add /dev/sdc /mnt
# mount -o remount,rw /mnt/mnt1
... /mnt/mnt1 is now writable
# umount /mnt/mnt1
As a result we have:
• sda is a single seeding device, with its initial contents
• sdb is a seeding device but requires sda, the contents are from the
time when sdb is made seeding, i.e. contents of sda with any later
changes
• sdc last writable, can be made a seeding one the same way as was sdb,
preserving its contents and depending on sda and sdb
As long as the seeding devices are unmodified and available, they can
be used to start another branch.
RAID56 STATUS AND RECOMMENDED PRACTICES
The RAID56 feature provides striping and parity over several devices,
same as the traditional RAID5/6. There are some implementation and de-
sign deficiencies that make it unreliable for some corner cases and the
feature should not be used in production, only for evaluation or test-
ing. The power failure safety for metadata with RAID56 is not 100%.
Metadata
Do not use raid5 nor raid6 for metadata. Use raid1 or raid1c3 respec-
tively.
The substitute profiles provide the same guarantees against loss of 1
or 2 devices, and in some respect can be an improvement. Recovering
from one missing device will only need to access the remaining 1st or
2nd copy, that in general may be stored on some other devices due to
the way RAID1 works on btrfs, unlike on a striped profile (similar to
raid0) that would need all devices all the time.
The space allocation pattern and consumption is different (e.g. on N
devices): for raid5 as an example, a 1GiB chunk is reserved on each de-
vice, while with raid1 there's each 1GiB chunk stored on 2 devices. The
consumption of each 1GiB of used metadata is then N * 1GiB for vs 2 *
1GiB. Using raid1 is also more convenient for balancing/converting to
other profile due to lower requirement on the available chunk space.
Missing/incomplete support
When RAID56 is on the same filesystem with different raid profiles, the
space reporting is inaccurate, e.g. df, btrfs filesystem df or btrfs
filesystem usage. When there's only a one profile per block group type
(e.g. RAID5 for data) the reporting is accurate.
When scrub is started on a RAID56 filesystem, it's started on all de-
vices that degrade the performance. The workaround is to start it on
each device separately. Due to that the device stats may not match the
actual state and some errors might get reported multiple times.
The write hole problem. An unclean shutdown could leave a partially
written stripe in a state where the some stripe ranges and the parity
are from the old writes and some are new. The information which is
which is not tracked. Write journal is not implemented. Alternatively a
full read-modify-write would make sure that a full stripe is always
written, avoiding the write hole completely, but performance in that
case turned out to be too bad for use.
The striping happens on all available devices (at the time the chunks
were allocated), so in case a new device is added it may not be uti-
lized immediately and would require a rebalance. A fixed configured
stripe width is not implemented.
STORAGE MODEL, HARDWARE CONSIDERATIONS
Storage model
A storage model is a model that captures key physical aspects of data
structure in a data store. A filesystem is the logical structure orga-
nizing data on top of the storage device.
The filesystem assumes several features or limitations of the storage
device and utilizes them or applies measures to guarantee reliability.
BTRFS in particular is based on a COW (copy on write) mode of writing,
i.e. not updating data in place but rather writing a new copy to a dif-
ferent location and then atomically switching the pointers.
In an ideal world, the device does what it promises. The filesystem as-
sumes that this may not be true so additional mechanisms are applied to
either detect misbehaving hardware or get valid data by other means.
The devices may (and do) apply their own detection and repair mecha-
nisms but we won't assume any.
The following assumptions about storage devices are considered (sorted
by importance, numbers are for further reference):
1. atomicity of reads and writes of blocks/sectors (the smallest unit
of data the device presents to the upper layers)
2. there's a flush command that instructs the device to forcibly order
writes before and after the command; alternatively there's a barrier
command that facilitates the ordering but may not flush the data
3. data sent to write to a given device offset will be written without
further changes to the data and to the offset
4. writes can be reordered by the device, unless explicitly serialized
by the flush command
5. reads and writes can be freely reordered and interleaved
The consistency model of BTRFS builds on these assumptions. The logical
data updates are grouped, into a generation, written on the device, se-
rialized by the flush command and then the super block is written end-
ing the generation. All logical links among metadata comprising a con-
sistent view of the data may not cross the generation boundary.
When things go wrong
No or partial atomicity of block reads/writes (1)
• Problem: a partial block contents is written (torn write), e.g. due
to a power glitch or other electronics failure during the read/write
• Detection: checksum mismatch on read
• Repair: use another copy or rebuild from multiple blocks using some
encoding scheme
The flush command does not flush (2)
This is perhaps the most serious problem and impossible to mitigate by
filesystem without limitations and design restrictions. What could hap-
pen in the worst case is that writes from one generation bleed to an-
other one, while still letting the filesystem consider the generations
isolated. Crash at any point would leave data on the device in an in-
consistent state without any hint what exactly got written, what is
missing and leading to stale metadata link information.
Devices usually honor the flush command, but for performance reasons
may do internal caching, where the flushed data are not yet persis-
tently stored. A power failure could lead to a similar scenario as
above, although it's less likely that later writes would be written be-
fore the cached ones. This is beyond what a filesystem can take into
account. Devices or controllers are usually equipped with batteries or
capacitors to write the cache contents even after power is cut. (Bat-
tery backed write cache)
Data get silently changed on write (3)
Such thing should not happen frequently, but still can happen spuri-
ously due the complex internal workings of devices or physical effects
of the storage media itself.
• Problem: while the data are written atomically, the contents get
changed
• Detection: checksum mismatch on read
• Repair: use another copy or rebuild from multiple blocks using some
encoding scheme
Data get silently written to another offset (3)
This would be another serious problem as the filesystem has no informa-
tion when it happens. For that reason the measures have to be done
ahead of time. This problem is also commonly called ghost write.
The metadata blocks have the checksum embedded in the blocks, so a cor-
rect atomic write would not corrupt the checksum. It's likely that af-
ter reading such block the data inside would not be consistent with the
rest. To rule that out there's embedded block number in the metadata
block. It's the logical block number because this is what the logical
structure expects and verifies.
The following is based on information publicly available, user feed-
back, community discussions or bug report analyses. It's not complete
and further research is encouraged when in doubt.
Main memory
The data structures and raw data blocks are temporarily stored in com-
puter memory before they get written to the device. It is critical that
memory is reliable because even simple bit flips can have vast conse-
quences and lead to damaged structures, not only in the filesystem but
in the whole operating system.
Based on experience in the community, memory bit flips are more common
than one would think. When it happens, it's reported by the
tree-checker or by a checksum mismatch after reading blocks. There are
some very obvious instances of bit flips that happen, e.g. in an or-
dered sequence of keys in metadata blocks. We can easily infer from the
other data what values get damaged and how. However, fixing that is not
straightforward and would require cross-referencing data from the en-
tire filesystem to see the scope.
If available, ECC memory should lower the chances of bit flips, but
this type of memory is not available in all cases. A memory test should
be performed in case there's a visible bit flip pattern, though this
may not detect a faulty memory module because the actual load of the
system could be the factor making the problems appear. In recent years
attacks on how the memory modules operate have been demonstrated
(rowhammer) achieving specific bits to be flipped. While these were
targeted, this shows that a series of reads or writes can affect unre-
lated parts of memory.
Further reading:
• https://en.wikipedia.org/wiki/Row_hammer
What to do:
• run memtest, note that sometimes memory errors happen only when the
system is under heavy load that the default memtest cannot trigger
• memory errors may appear as filesystem going read-only due to "pre
write" check, that verify meta data before they get written but fail
some basic consistency checks
Direct memory access (DMA)
Another class of errors is related to DMA (direct memory access) per-
formed by device drivers. While this could be considered a software er-
ror, the data transfers that happen without CPU assistance may acciden-
tally corrupt other pages. Storage devices utilize DMA for performance
reasons, the filesystem structures and data pages are passed back and
forth, making errors possible in case page life time is not properly
tracked.
There are lots of quirks (device-specific workarounds) in Linux kernel
drivers (regarding not only DMA) that are added when found. The quirks
may avoid specific errors or disable some features to avoid worse prob-
lems.
What to do:
• use up-to-date kernel (recent releases or maintained long term sup-
port versions)
• as this may be caused by faulty drivers, keep the systems up-to-date
Rotational disks (HDD)
Rotational HDDs typically fail at the level of individual sectors or
small clusters. Read failures are caught on the levels below the
filesystem and are returned to the user as EIO - Input/output error.
Reading the blocks repeatedly may return the data eventually, but this
is better done by specialized tools and filesystem takes the result of
the lower layers. Rewriting the sectors may trigger internal remapping
but this inevitably leads to data loss.
Disk firmware is technically software but from the filesystem perspec-
tive is part of the hardware. IO requests are processed, and caching or
various other optimizations are performed, which may lead to bugs under
high load or unexpected physical conditions or unsupported use cases.
Disks are connected by cables with two ends, both of which can cause
problems when not attached properly. Data transfers are protected by
checksums and the lower layers try hard to transfer the data correctly
or not at all. The errors from badly-connecting cables may manifest as
large amount of failed read or write requests, or as short error bursts
depending on physical conditions.
What to do:
• check smartctl for potential issues
Solid state drives (SSD)
The mechanism of information storage is different from HDDs and this
affects the failure mode as well. The data are stored in cells grouped
in large blocks with limited number of resets and other write con-
straints. The firmware tries to avoid unnecessary resets and performs
optimizations to maximize the storage media lifetime. The known tech-
niques are deduplication (blocks with same fingerprint/hash are mapped
to same physical block), compression or internal remapping and garbage
collection of used memory cells. Due to the additional processing there
are measures to verity the data e.g. by ECC codes.
The observations of failing SSDs show that the whole electronic fails
at once or affects a lot of data (e.g. stored on one chip). Recovering
such data may need specialized equipment and reading data repeatedly
does not help as it's possible with HDDs.
There are several technologies of the memory cells with different char-
acteristics and price. The lifetime is directly affected by the type
and frequency of data written. Writing "too much" distinct data (e.g.
encrypted) may render the internal deduplication ineffective and lead
to a lot of rewrites and increased wear of the memory cells.
There are several technologies and manufacturers so it's hard to de-
scribe them but there are some that exhibit similar behaviour:
• expensive SSD will use more durable memory cells and is optimized for
reliability and high load
• cheap SSD is projected for a lower load ("desktop user") and is opti-
mized for cost, it may employ the optimizations and/or extended error
reporting partially or not at all
It's not possible to reliably determine the expected lifetime of an SSD
due to lack of information about how it works or due to lack of reli-
able stats provided by the device.
Metadata writes tend to be the biggest component of lifetime writes to
a SSD, so there is some value in reducing them. Depending on the device
class (high end/low end) the features like DUP block group profiles may
affect the reliability in both ways:
• high end are typically more reliable and using single for data and
metadata could be suitable to reduce device wear
• low end could lack ability to identify errors so an additional redun-
dancy at the filesystem level (checksums, DUP) could help
Only users who consume 50 to 100% of the SSD's actual lifetime writes
need to be concerned by the write amplification of btrfs DUP metadata.
Most users will be far below 50% of the actual lifetime, or will write
the drive to death and discover how many writes 100% of the actual
lifetime was. SSD firmware often adds its own write multipliers that
can be arbitrary and unpredictable and dependent on application behav-
ior, and these will typically have far greater effect on SSD lifespan
than DUP metadata. It's more or less impossible to predict when a SSD
will run out of lifetime writes to within a factor of two, so it's hard
to justify wear reduction as a benefit.
Further reading:
• https://www.snia.org/educational-library/ssd-and-deduplication-end-spinning-disk-2012
• https://www.snia.org/educational-library/realities-solid-state-storage-2013-2013
• https://www.snia.org/educational-library/ssd-performance-primer-2013
• https://www.snia.org/educational-library/how-controllers-maximize-ssd-life-2013
What to do:
• run smartctl or self-tests to look for potential issues
• keep the firmware up-to-date
NVM express, non-volatile memory (NVMe)
NVMe is a type of persistent memory usually connected over a system bus
(PCIe) or similar interface and the speeds are an order of magnitude
faster than SSD. It is also a non-rotating type of storage, and is not
typically connected by a cable. It's not a SCSI type device either but
rather a complete specification for logical device interface.
In a way the errors could be compared to a combination of SSD class and
regular memory. Errors may exhibit as random bit flips or IO failures.
There are tools to access the internal log (nvme log and nvme-cli) for
a more detailed analysis.
There are separate error detection and correction steps performed e.g.
on the bus level and in most cases never making in to the filesystem
level. Once this happens it could mean there's some systematic error
like overheating or bad physical connection of the device. You may want
to run self-tests (using smartctl).
• https://en.wikipedia.org/wiki/NVM_Express
• https://www.smartmontools.org/wiki/NVMe_Support
Drive firmware
Firmware is technically still software but embedded into the hardware.
As all software has bugs, so does firmware. Storage devices can update
the firmware and fix known bugs. In some cases the it's possible to
avoid certain bugs by quirks (device-specific workarounds) in Linux
kernel.
A faulty firmware can cause wide range of corruptions from small and
localized to large affecting lots of data. Self-repair capabilities may
not be sufficient.
What to do:
• check for firmware updates in case there are known problems, note
that updating firmware can be risky on itself
• use up-to-date kernel (recent releases or maintained long term sup-
port versions)
SD flash cards
There are a lot of devices with low power consumption and thus using
storage media based on low power consumption too, typically flash mem-
ory stored on a chip enclosed in a detachable card package. An improp-
erly inserted card may be damaged by electrical spikes when the device
is turned on or off. The chips storing data in turn may be damaged per-
manently. All types of flash memory have a limited number of rewrites,
so the data are internally translated by FTL (flash translation layer).
This is implemented in firmware (technically a software) and prone to
bugs that manifest as hardware errors.
Adding redundancy like using DUP profiles for both data and metadata
can help in some cases but a full backup might be the best option once
problems appear and replacing the card could be required as well.
Hardware as the main source of filesystem corruptions
If you use unreliable hardware and don't know about that, don't blame
the filesystem when it tells you.
SEE ALSO
acl(5), btrfs(8), chattr(1), fstrim(8), ioctl(2), mkfs.btrfs(8),
mount(8), swapon(8)
6.2 Feb 28, 2023 BTRFS(5)
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