BTRFS-BALANCE(8) BTRFS BTRFS-BALANCE(8)
NAME
btrfs-balance - balance block groups on a btrfs filesystem
SYNOPSIS
btrfs balance <subcommand> <args>
DESCRIPTION
The primary purpose of the balance feature is to spread block groups
across all devices so they match constraints defined by the respective
profiles. See mkfs.btrfs(8) section PROFILES for more details. The
scope of the balancing process can be further tuned by use of filters
that can select the block groups to process. Balance works only on a
mounted filesystem. Extent sharing is preserved and reflinks are not
broken. Files are not defragmented nor recompressed, file extents are
preserved but the physical location on devices will change.
The balance operation is cancellable by the user. The on-disk state of
the filesystem is always consistent so an unexpected interruption (e.g.
system crash, reboot) does not corrupt the filesystem. The progress of
the balance operation is temporarily stored as an internal state and
will be resumed upon mount, unless the mount option skip_balance is
specified.
WARNING:
Running balance without filters will take a lot of time as it basi-
cally move data/metadata from the whole filesystem and needs to up-
date all block pointers.
The filters can be used to perform following actions:
• convert block group profiles (filter convert)
• make block group usage more compact (filter usage)
• perform actions only on a given device (filters devid, drange)
The filters can be applied to a combination of block group types (data,
metadata, system). Note that changing only the system type needs the
force option. Otherwise system gets automatically converted whenever
metadata profile is converted.
When metadata redundancy is reduced (e.g. from RAID1 to single) the
force option is also required and it is noted in system log.
NOTE:
The balance operation needs enough work space, i.e. space that is
completely unused in the filesystem, otherwise this may lead to
ENOSPC reports. See the section ENOSPC for more details.
COMPATIBILITY
NOTE:
The balance subcommand also exists under the btrfs filesystem name-
space. This still works for backward compatibility but is depre-
cated and should not be used any more.
NOTE:
A short syntax btrfs balance <path> works due to backward compati-
bility but is deprecated and should not be used any more. Use btrfs
balance start command instead.
PERFORMANCE IMPLICATIONS
Balancing operations are very IO intensive and can also be quite CPU
intensive, impacting other ongoing filesystem operations. Typically
large amounts of data are copied from one location to another, with
corresponding metadata updates.
Depending upon the block group layout, it can also be seek heavy. Per-
formance on rotational devices is noticeably worse compared to SSDs or
fast arrays.
SUBCOMMAND
cancel <path>
cancels a running or paused balance, the command will block and
wait until the current block group being processed completes
Since kernel 5.7 the response time of the cancellation is sig-
nificantly improved, on older kernels it might take a long time
until currently processed chunk is completely finished.
pause <path>
pause running balance operation, this will store the state of
the balance progress and used filters to the filesystem
resume <path>
resume interrupted balance, the balance status must be stored on
the filesystem from previous run, e.g. after it was paused or
forcibly interrupted and mounted again with skip_balance
start [options] <path>
start the balance operation according to the specified filters,
without any filters the data and metadata from the whole
filesystem are moved. The process runs in the foreground.
NOTE:
The balance command without filters will basically move ev-
erything in the filesystem to a new physical location on de-
vices (i.e. it does not affect the logical properties of file
extents like offsets within files and extent sharing). The
run time is potentially very long, depending on the filesys-
tem size. To prevent starting a full balance by accident, the
user is warned and has a few seconds to cancel the operation
before it starts. The warning and delay can be skipped with
--full-balance option.
Please note that the filters must be written together with the
-d, -m and -s options, because they're optional and bare -d and
-m also work and mean no filters.
NOTE:
When the target profile for conversion filter is raid5 or
raid6, there's a safety timeout of 10 seconds to warn users
about the status of the feature
Options
-d[<filters>]
act on data block groups, see FILTERS section for details
about filters
-m[<filters>]
act on metadata chunks, see FILTERS section for details
about filters
-s[<filters>]
act on system chunks (requires -f), see FILTERS section
for details about filters.
-f force a reduction of metadata integrity, e.g. when going
from raid1 to single, or skip safety timeout when the
target conversion profile is raid5 or raid6
--background|--bg
run the balance operation asynchronously in the back-
ground, uses fork(2) to start the process that calls the
kernel ioctl
--enqueue
wait if there's another exclusive operation running, oth-
erwise continue
-v (deprecated) alias for global '-v' option
status [-v] <path>
Show status of running or paused balance.
Options
-v (deprecated) alias for global -v option
FILTERS
From kernel 3.3 onwards, btrfs balance can limit its action to a subset
of the whole filesystem, and can be used to change the replication con-
figuration (e.g. moving data from single to RAID1). This functionality
is accessed through the -d, -m or -s options to btrfs balance start,
which filter on data, metadata and system blocks respectively.
A filter has the following structure: type[=params][,type=...]
The available types are:
profiles=<profiles>
Balances only block groups with the given profiles. Parameters
are a list of profile names separated by "|" (pipe).
usage=<percent>, usage=<range>
Balances only block groups with usage under the given percent-
age. The value of 0 is allowed and will clean up completely un-
used block groups, this should not require any new work space
allocated. You may want to use usage=0 in case balance is re-
turning ENOSPC and your filesystem is not too full.
The argument may be a single value or a range. The single value
N means at most N percent used, equivalent to ..N range syntax.
Kernels prior to 4.4 accept only the single value format. The
minimum range boundary is inclusive, maximum is exclusive.
devid=<id>
Balances only block groups which have at least one chunk on the
given device. To list devices with ids use btrfs filesystem
show.
drange=<range>
Balance only block groups which overlap with the given byte
range on any device. Use in conjunction with devid to filter on
a specific device. The parameter is a range specified as
start..end.
vrange=<range>
Balance only block groups which overlap with the given byte
range in the filesystem's internal virtual address space. This
is the address space that most reports from btrfs in the kernel
log use. The parameter is a range specified as start..end.
convert=<profile>
Convert each selected block group to the given profile name
identified by parameters.
NOTE:
Starting with kernel 4.5, the data chunks can be converted
to/from the DUP profile on a single device.
NOTE:
Starting with kernel 4.6, all profiles can be converted
to/from DUP on multi-device filesystems.
limit=<number>, limit=<range>
Process only given number of chunks, after all filters are ap-
plied. This can be used to specifically target a chunk in con-
nection with other filters (drange, vrange) or just simply limit
the amount of work done by a single balance run.
The argument may be a single value or a range. The single value
N means at most N chunks, equivalent to ..N range syntax. Ker-
nels prior to 4.4 accept only the single value format. The
range minimum and maximum are inclusive.
stripes=<range>
Balance only block groups which have the given number of
stripes. The parameter is a range specified as start..end. Makes
sense for block group profiles that utilize striping, i.e.
RAID0/10/5/6. The range minimum and maximum are inclusive.
soft Takes no parameters. Only has meaning when converting between
profiles. When doing convert from one profile to another and
soft mode is on, chunks that already have the target profile are
left untouched. This is useful e.g. when half of the filesystem
was converted earlier but got cancelled.
The soft mode switch is (like every other filter) per-type. For
example, this means that we can convert metadata chunks the
"hard" way while converting data chunks selectively with soft
switch.
Profile names, used in profiles and convert are one of: raid0, raid1,
raid1c3, raid1c4, raid10, raid5, raid6, dup, single. The mixed
data/metadata profiles can be converted in the same way, but it's con-
version between mixed and non-mixed is not implemented. For the con-
straints of the profiles please refer to mkfs.btrfs(8), section PRO-
FILES.
ENOSPC
The way balance operates, it usually needs to temporarily create a new
block group and move the old data there, before the old block group can
be removed. For that it needs the work space, otherwise it fails for
ENOSPC reasons. This is not the same ENOSPC as if the free space is
exhausted. This refers to the space on the level of block groups, which
are bigger parts of the filesystem that contain many file extents.
The free work space can be calculated from the output of the btrfs
filesystem show command:
Label: 'BTRFS' uuid: 8a9d72cd-ead3-469d-b371-9c7203276265
Total devices 2 FS bytes used 77.03GiB
devid 1 size 53.90GiB used 51.90GiB path /dev/sdc2
devid 2 size 53.90GiB used 51.90GiB path /dev/sde1
size - used = free work space
53.90GiB - 51.90GiB = 2.00GiB
An example of a filter that does not require workspace is usage=0. This
will scan through all unused block groups of a given type and will re-
claim the space. After that it might be possible to run other filters.
CONVERSIONS ON MULTIPLE DEVICES
Conversion to profiles based on striping (RAID0, RAID5/6) require the
work space on each device. An interrupted balance may leave partially
filled block groups that consume the work space.
EXAMPLES
A more comprehensive example when going from one to multiple devices,
and back, can be found in section TYPICAL USECASES of btrfs-device(8).
MAKING BLOCK GROUP LAYOUT MORE COMPACT
The layout of block groups is not normally visible; most tools report
only summarized numbers of free or used space, but there are still some
hints provided.
Let's use the following real life example and start with the output:
$ btrfs filesystem df /path
Data, single: total=75.81GiB, used=64.44GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.84GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
Roughly calculating for data, 75G - 64G = 11G, the used/total ratio is
about 85%. How can we can interpret that:
• chunks are filled by 85% on average, i.e. the usage filter with any-
thing smaller than 85 will likely not affect anything
• in a more realistic scenario, the space is distributed unevenly, we
can assume there are completely used chunks and the remaining are
partially filled
Compacting the layout could be used on both. In the former case it
would spread data of a given chunk to the others and removing it. Here
we can estimate that roughly 850 MiB of data have to be moved (85% of a
1 GiB chunk).
In the latter case, targeting the partially used chunks will have to
move less data and thus will be faster. A typical filter command would
look like:
# btrfs balance start -dusage=50 /path
Done, had to relocate 2 out of 97 chunks
$ btrfs filesystem df /path
Data, single: total=74.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.84GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
As you can see, the total amount of data is decreased by just 1 GiB,
which is an expected result. Let's see what will happen when we in-
crease the estimated usage filter.
# btrfs balance start -dusage=85 /path
Done, had to relocate 13 out of 95 chunks
$ btrfs filesystem df /path
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=15.87GiB, used=8.85GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
Now the used/total ratio is about 94% and we moved about 74G - 68G = 6G
of data to the remaining block groups, i.e. the 6GiB are now free of
filesystem structures, and can be reused for new data or metadata block
groups.
We can do a similar exercise with the metadata block groups, but this
should not typically be necessary, unless the used/total ratio is re-
ally off. Here the ratio is roughly 50% but the difference as an abso-
lute number is "a few gigabytes", which can be considered normal for a
workload with snapshots or reflinks updated frequently.
# btrfs balance start -musage=50 /path
Done, had to relocate 4 out of 89 chunks
$ btrfs filesystem df /path
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=14.87GiB, used=8.85GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
Just 1 GiB decrease, which possibly means there are block groups with
good utilization. Making the metadata layout more compact would in turn
require updating more metadata structures, i.e. lots of IO. As running
out of metadata space is a more severe problem, it's not necessary to
keep the utilization ratio too high. For the purpose of this example,
let's see the effects of further compaction:
# btrfs balance start -musage=70 /path
Done, had to relocate 13 out of 88 chunks
$ btrfs filesystem df .
Data, single: total=68.03GiB, used=64.43GiB
System, RAID1: total=32.00MiB, used=20.00KiB
Metadata, RAID1: total=11.97GiB, used=8.83GiB
GlobalReserve, single: total=512.00MiB, used=0.00B
GETTING RID OF COMPLETELY UNUSED BLOCK GROUPS
Normally the balance operation needs a work space, to temporarily move
the data before the old block groups gets removed. If there's no work
space, it ends with no space left.
There's a special case when the block groups are completely unused,
possibly left after removing lots of files or deleting snapshots. Re-
moving empty block groups is automatic since 3.18. The same can be
achieved manually with a notable exception that this operation does not
require the work space. Thus it can be used to reclaim unused block
groups to make it available.
# btrfs balance start -dusage=0 /path
This should lead to decrease in the total numbers in the btrfs filesys-
tem df output.
EXIT STATUS
Unless indicated otherwise below, all btrfs balance subcommands return
a zero exit status if they succeed, and non zero in case of failure.
The pause, cancel, and resume subcommands exit with a status of 2 if
they fail because a balance operation was not running.
The status subcommand exits with a status of 0 if a balance operation
is not running, 1 if the command-line usage is incorrect or a balance
operation is still running, and 2 on other errors.
AVAILABILITY
btrfs is part of btrfs-progs. Please refer to the documentation at
https://btrfs.readthedocs.io or wiki http://btrfs.wiki.kernel.org for
further information.
SEE ALSO
mkfs.btrfs(8), btrfs-device(8)
6.2 Feb 28, 2023 BTRFS-BALANCE(8)
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