BTRFS-SUBVOLUME(8) BTRFS BTRFS-SUBVOLUME(8)
NAME
btrfs-subvolume - manage btrfs subvolumes
SYNOPSIS
btrfs subvolume <subcommand> [<args>]
DESCRIPTION
btrfs subvolume is used to create/delete/list/show btrfs subvolumes and
snapshots.
A BTRFS subvolume is a part of filesystem with its own independent
file/directory hierarchy and inode number namespace. Subvolumes can
share file extents. A snapshot is also subvolume, but with a given ini-
tial content of the original subvolume. A subvolume has always inode
number 256.
NOTE:
A subvolume in BTRFS is not like an LVM logical volume, which is
block-level snapshot while BTRFS subvolumes are file extent-based.
A subvolume looks like a normal directory, with some additional opera-
tions described below. Subvolumes can be renamed or moved, nesting sub-
volumes is not restricted but has some implications regarding snapshot-
ting. The numeric id (called subvolid or rootid) of the subvolume is
persistent and cannot be changed.
A subvolume in BTRFS can be accessed in two ways:
• like any other directory that is accessible to the user
• like a separately mounted filesystem (options subvol or subvolid)
In the latter case the parent directory is not visible and accessible.
This is similar to a bind mount, and in fact the subvolume mount does
exactly that.
A freshly created filesystem is also a subvolume, called top-level, in-
ternally has an id 5. This subvolume cannot be removed or replaced by
another subvolume. This is also the subvolume that will be mounted by
default, unless the default subvolume has been changed (see btrfs sub-
volume set-default).
A snapshot is a subvolume like any other, with given initial content.
By default, snapshots are created read-write. File modifications in a
snapshot do not affect the files in the original subvolume.
Subvolumes can be given capacity limits, through the qgroups/quota fa-
cility, but otherwise share the single storage pool of the whole btrfs
filesystem. They may even share data between themselves (through dedu-
plication or snapshotting).
NOTE:
A snapshot is not a backup: snapshots work by use of BTRFS'
copy-on-write behaviour. A snapshot and the original it was taken
from initially share all of the same data blocks. If that data is
damaged in some way (cosmic rays, bad disk sector, accident with dd
to the disk), then the snapshot and the original will both be dam-
aged. Snapshots are useful to have local online "copies" of the
filesystem that can be referred back to, or to implement a form of
deduplication, or to fix the state of a filesystem for making a full
backup without anything changing underneath it. They do not in them-
selves make your data any safer.
SUBVOLUME FLAGS
The subvolume flag currently implemented is the ro property (read-only
status). Read-write subvolumes have that set to false, snapshots as
true. In addition to that, a plain snapshot will also have last change
generation and creation generation equal.
Read-only snapshots are building blocks of incremental send (see
btrfs-send(8)) and the whole use case relies on unmodified snapshots
where the relative changes are generated from. Thus, changing the sub-
volume flags from read-only to read-write will break the assumptions
and may lead to unexpected changes in the resulting incremental stream.
A snapshot that was created by send/receive will be read-only, with
different last change generation, read-only and with set received_uuid
which identifies the subvolume on the filesystem that produced the
stream. The use case relies on matching data on both sides. Changing
the subvolume to read-write after it has been received requires to re-
set the received_uuid. As this is a notable change and could poten-
tially break the incremental send use case, performing it by btrfs
property set requires force if that is really desired by user.
NOTE:
The safety checks have been implemented in 5.14.2, any subvolumes
previously received (with a valid received_uuid) and read-write sta-
tus may exist and could still lead to problems with send/receive.
You can use btrfs subvolume show to identify them. Flipping the
flags to read-only and back to read-write will reset the re-
ceived_uuid manually. There may exist a convenience tool in the fu-
ture.
NESTED SUBVOLUMES
There are no restrictions for subvolume creation, so it's up to the
user how to organize them, whether to have a flat layout (all subvol-
umes are direct descendants of the toplevel one), or nested.
What should be mentioned early is that a snapshotting is not recursive,
so a subvolume or a snapshot is effectively a barrier and no files in
the nested appear in the snapshot. Instead there's a stub subvolume
(also sometimes empty subvolume with the same name as original subvol-
ume, with inode number 2). This can be used intentionally but could be
confusing in case of nested layouts.
Case study: system root layouts
There are two ways how the system root directory and subvolume layout
could be organized. The interesting use case for root is to allow roll-
backs to previous version, as one atomic step. If the entire filesystem
hierarchy starting in "/" is in one subvolume, taking snapshot will en-
compass all files. This is easy for the snapshotting part but has unde-
sirable consequences for rollback. For example, log files would get
rolled back too, or any data that are stored on the root filesystem but
are not meant to be rolled back either (database files, VM images,
...).
Here we could utilize the snapshotting barrier mentioned above, each
directory that stores data to be preserved across rollbacks is it's own
subvolume. This could be e.g. /var. Further more-fine grained parti-
tioning could be done, e.g. adding separate subvolumes for /var/log,
/var/cache etc.
That there are separate subvolumes requires separate actions to take
the snapshots (here it gets disconnected from the system root snap-
shots). This needs to be taken care of by system tools, installers to-
gether with selection of which directories are highly recommended to be
separate subvolumes.
MOUNT OPTIONS
Mount options are of two kinds, generic (that are handled by VFS layer)
and specific, handled by the filesystem. The following list shows which
are applicable to individual subvolume mounts, while there are more op-
tions that always affect the whole filesystem:
• generic: noatime/relatime/..., nodev, nosuid, ro, rw, dirsync
• fs-specific: compress, autodefrag, nodatacow, nodatasum
An example of whole filesystem options is e.g. space_cache, rescue, de-
vice, skip_balance, etc. The exceptional options are subvol and sub-
volid that are actually used for mounting a given subvolume and can be
specified only once for the mount.
Subvolumes belong to a single filesystem and as implemented now all
share the same specific mount options, changes done by remount have im-
mediate effect. This may change in the future.
Mounting a read-write snapshot as read-only is possible and will not
change the ro property and flag of the subvolume.
The name of the mounted subvolume is stored in file /proc/self/mounts
in the 4th column:
27 21 0:19 /subv1 /mnt rw,relatime - btrfs /dev/sda rw,space_cache
^^^^^^
INODE NUMBERS
A proper subvolume has always inode number 256. If a subvolume is
nested and then a snapshot is taken, then the cloned directory entry
representing the subvolume becomes empty and the inode has number 2.
All other files and directories in the target snapshot preserve their
original inode numbers.
NOTE:
Inode number is not a filesystem-wide unique identifier, some appli-
cations assume that. Please user pair subvolumeid:inodenumber for
that purpose.
PERFORMANCE
Subvolume creation needs to flush dirty data that belong to the subvol-
ume, this step may take some time, otherwise once there's nothing else
to do, the snapshot is instant and in the metadata it only creates a
new tree root copy.
Snapshot deletion has two phases: first its directory is deleted and
the subvolume is added to a list, then the list is processed one by one
and the data related to the subvolume get deleted. This is usually
called cleaning and can take some time depending on the amount of
shared blocks (can be a lot of metadata updates), and the number of
currently queued deleted subvolumes.
SUBVOLUME AND SNAPSHOT
A subvolume is a part of filesystem with its own independent file/di-
rectory hierarchy. Subvolumes can share file extents. A snapshot is
also subvolume, but with a given initial content of the original sub-
volume.
NOTE:
A subvolume in btrfs is not like an LVM logical volume, which is
block-level snapshot while btrfs subvolumes are file extent-based.
A subvolume looks like a normal directory, with some additional opera-
tions described below. Subvolumes can be renamed or moved, nesting sub-
volumes is not restricted but has some implications regarding snapshot-
ting.
A subvolume in btrfs can be accessed in two ways:
• like any other directory that is accessible to the user
• like a separately mounted filesystem (options subvol or subvolid)
In the latter case the parent directory is not visible and accessible.
This is similar to a bind mount, and in fact the subvolume mount does
exactly that.
A freshly created filesystem is also a subvolume, called top-level, in-
ternally has an id 5. This subvolume cannot be removed or replaced by
another subvolume. This is also the subvolume that will be mounted by
default, unless the default subvolume has been changed (see subcommand
set-default).
A snapshot is a subvolume like any other, with given initial content.
By default, snapshots are created read-write. File modifications in a
snapshot do not affect the files in the original subvolume.
SUBCOMMAND
create [-i <qgroupid>] [<dest>/]<name>
Create a subvolume name in dest.
If dest is not given, subvolume name will be created in the cur-
rent directory.
Options
-i <qgroupid>
Add the newly created subvolume to a qgroup. This option
can be given multiple times.
delete [options] [<subvolume> [<subvolume>...]], delete -i|--subvolid
<subvolid> <path>
Delete the subvolume(s) from the filesystem.
If subvolume is not a subvolume, btrfs returns an error but con-
tinues if there are more arguments to process.
If --subvolid is used, path must point to a btrfs filesystem.
See btrfs subvolume list or btrfs inspect-internal rootid how to
get the subvolume id.
The corresponding directory is removed instantly but the data
blocks are removed later in the background. The command returns
immediately. See btrfs subvolume sync how to wait until the sub-
volume gets completely removed.
The deletion does not involve full transaction commit by default
due to performance reasons. As a consequence, the subvolume may
appear again after a crash. Use one of the --commit options to
wait until the operation is safely stored on the device.
Deleting subvolume needs sufficient permissions, by default the
owner cannot delete it unless it's enabled by a mount option
user_subvol_rm_allowed, or deletion is run as root. The default
subvolume (see btrfs subvolume set-default) cannot be deleted
and returns error (EPERM) and this is logged to the system log.
A subvolume that's currently involved in send (see btrfs send)
also cannot be deleted until the send is finished. This is also
logged in the system log.
Options
-c|--commit-after
wait for transaction commit at the end of the operation.
-C|--commit-each
wait for transaction commit after deleting each subvol-
ume.
-i|--subvolid <subvolid>
subvolume id to be removed instead of the <path> that
should point to the filesystem with the subvolume
-v|--verbose
(deprecated) alias for global -v option
find-new <subvolume> <last_gen>
List the recently modified files in a subvolume, after last_gen
generation.
get-default <path>
Get the default subvolume of the filesystem path.
The output format is similar to subvolume list command.
list [options] [-G [+|-]<value>] [-C [+|-]<value>]
[--sort=rootid,gen,ogen,path] <path>
List the subvolumes present in the filesystem path.
For every subvolume the following information is shown by de-
fault:
ID ID gen generation top level parent_ID path path
where ID is subvolume's (root)id, generation is an internal
counter which is updated every transaction, parent_ID is the
same as the parent subvolume's id, and path is the relative path
of the subvolume to the top level subvolume. The subvolume's ID
may be used by the subvolume set-default command, or at mount
time via the subvolid= option.
Options
Path filtering:
-o print only subvolumes below specified <path>.
-a print all the subvolumes in the filesystem and distin-
guish between absolute and relative path with respect to
the given path.
Field selection:
-p print the parent ID (parent here means the subvolume
which contains this subvolume).
-c print the ogeneration of the subvolume, aliases: ogen or
origin generation.
-g print the generation of the subvolume (default).
-u print the UUID of the subvolume.
-q print the parent UUID of the subvolume (parent here means
subvolume of which this subvolume is a snapshot).
-R print the UUID of the sent subvolume, where the subvolume
is the result of a receive operation.
Type filtering:
-s only snapshot subvolumes in the filesystem will be
listed.
-r only readonly subvolumes in the filesystem will be
listed.
-d list deleted subvolumes that are not yet cleaned.
Other:
-t print the result as a table.
Sorting:
By default the subvolumes will be sorted by subvolume ID ascend-
ing.
-G [+|-]<value>
list subvolumes in the filesystem that its generation is
>=, <= or = value. '+' means >= value, '-' means <=
value, If there is neither '+' nor '-', it means = value.
-C [+|-]<value>
list subvolumes in the filesystem that its ogeneration is
>=, <= or = value. The usage is the same to -G option.
--sort=rootid,gen,ogen,path
list subvolumes in order by specified items. you can add
+ or - in front of each items, + means ascending, - means
descending. The default is ascending.
for --sort you can combine some items together by ,, just
like --sort=+ogen,-gen,path,rootid.
set-default [<subvolume>|<id> <path>]
Set the default subvolume for the (mounted) filesystem.
Set the default subvolume for the (mounted) filesystem at path.
This will hide the top-level subvolume (i.e. the one mounted
with subvol=/ or subvolid=5). Takes action on next mount.
There are two ways how to specify the subvolume, by id or by the
subvolume path. The id can be obtained from btrfs subvolume
list, btrfs subvolume show or btrfs inspect-internal rootid.
show [options] <path>
Show more information about a subvolume (UUIDs, generations,
times, flags, related snapshots).
/mnt/btrfs/subvolume
Name: subvolume
UUID: 5e076a14-4e42-254d-ac8e-55bebea982d1
Parent UUID: -
Received UUID: -
Creation time: 2018-01-01 12:34:56 +0000
Subvolume ID: 79
Generation: 2844
Gen at creation: 2844
Parent ID: 5
Top level ID: 5
Flags: -
Snapshot(s):
Options
-r|--rootid <ID>
show details about subvolume with root ID, looked up in
path
-u|--uuid UUID
show details about subvolume with the given UUID, looked
up in path
snapshot [-r] [-i <qgroupid>] <source> <dest>|[<dest>/]<name>
Create a snapshot of the subvolume source with the name name in
the dest directory.
If only dest is given, the subvolume will be named the basename
of source. If source is not a subvolume, btrfs returns an er-
ror.
Options
-r Make the new snapshot read only.
-i <qgroupid>
Add the newly created subvolume to a qgroup. This option
can be given multiple times.
sync <path> [subvolid...]
Wait until given subvolume(s) are completely removed from the
filesystem after deletion. If no subvolume id is given, wait un-
til all current deletion requests are completed, but do not wait
for subvolumes deleted in the meantime.
Options
-s <N> sleep N seconds between checks (default: 1)
EXAMPLES
Deleting a subvolume
If we want to delete a subvolume called foo from a btrfs volume mounted
at /mnt/bar we could run the following:
btrfs subvolume delete /mnt/bar/foo
EXIT STATUS
btrfs subvolume returns a zero exit status if it succeeds. A non-zero
value is returned in case of failure.
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
btrfs-qgroup(8), btrfs-quota(8), btrfs-send(8), mkfs.btrfs(8), mount(8)
6.2 Feb 28, 2023 BTRFS-SUBVOLUME(8)
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