plucky (8) btrfs-subvolume.8.gz
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
initial content of the original subvolume. A subvolume has always inode number 256 (see more in Inode
numbers (in Subvolumes)).
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 operations described below. Subvolumes
can be renamed or moved, nesting subvolumes is not restricted but has some implications regarding
snapshotting. 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, internally 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 subvolume 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 facility, but otherwise share the
single storage pool of the whole btrfs filesystem. They may even share data between themselves (through
deduplication 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 damaged. 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 themselves
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
subvolume 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, 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 reset the received_uuid. As this is a notable change and could potentially 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 status 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 received_uuid manually. There may exist a convenience tool in the future.
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 subvolumes 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 subvolumes appear in the snapshot. Instead, there's a
stub subvolume, also sometimes called empty subvolume, with the same name as original subvolume and with
inode number 2. This can be used intentionally but could be confusing in case of nested layouts.
$ btrfs subvolume create subvol1
$ btrfs subvolume create subvol1/subvol2
$ btrfs subvolume snapshot subvol1 snap1
$ find -ls
121093 0 drwxr-xr-x 1 user users 24 Jul 30 12:34 .
256 0 drwxr-xr-x 1 user users 14 Jul 30 12:34 ./subvol1
256 0 drwxr-xr-x 1 user users 0 Jul 30 12:34 ./subvol1/subvol2
257 0 -rw-r--r-- 1 user users 0 Jul 30 12:34 ./subvol1/subvol2/file
256 0 drwxr-xr-x 1 user users 14 Jul 30 12:34 ./snap1
2 0 drwxr-xr-x 1 user users 0 Jul 30 12:34 ./snap1/subvol2
The numbers in the first columns are inode numbers, 256 is for a regular subvolume (or snapshot), 2 is
the empty subvolume. The snapshotted directory representing subvol2 does not contain the file.
NOTE:
The empty subvolume will not be sent (btrfs-send(8)) and thus will not be created on the receive side
(btrfs-receive(8)).
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 rollbacks to previous version, as one atomic step. If the entire filesystem
hierarchy starting in / is in one subvolume, taking snapshot will encompass all files. This is easy for
the snapshotting part but has undesirable 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, making each directory that stores data to
be preserved across rollbacks its own subvolume. This could be e.g. /var. Further more fine-grained
partitioning could be done, e.g. adding separate subvolumes for /var/log, /var/cache etc.
The fact that there are separate subvolumes requires separate actions to take the snapshots (here, it
gets disconnected from the system root snapshots). This needs to be taken care of by system tools,
installers, together 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 options that always affect the whole filesystem:
• Generic: noatime/relatime/..., nodev, nosuid, ro, rw, dirsync
• Filesystem-specific: compress, autodefrag, nodatacow, nodatasum
Examples of whole filesystem options are e.g. space_cache, rescue, device, skip_balance, etc. The
exceptional options are subvol and subvolid 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. Also, changes done by remount have immediate 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/mountinfo in the 4th column:
27 21 0:19 /subv1 /mnt rw,relatime - btrfs /dev/sda rw,space_cache
^^^^^^
INODE NUMBERS
A directory representing a subvolume has always inode number 256 (sometimes also called a root of the
subvolume):
$ ls -lis
total 0
389111 0 drwxr-xr-x 1 user users 0 Jan 20 12:13 dir
389110 0 -rw-r--r-- 1 user users 0 Jan 20 12:13 file
256 0 drwxr-xr-x 1 user users 0 Jan 20 12:13 snap1
256 0 drwxr-xr-x 1 user users 0 Jan 20 12:13 subv1
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 applications assume that. Please use the
subvolumeid:inodenumber pair for that purpose. The subvolume id can be read by btrfs inspect-internal
rootid or by the ioctl BTRFS_IOC_INO_LOOKUP.
PERFORMANCE
Subvolume creation needs to flush dirty data that belong to the subvolume and this step may take some
time. Otherwise, once there's nothing else to do, the snapshot is instantaneous and only creates a new
tree root copy in the metadata.
Snapshot deletion has two phases: first its directory is deleted and the subvolume is added to a queuing
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/directory hierarchy. Subvolumes can
share file extents. A snapshot is also subvolume, but with a given initial content of the original
subvolume.
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 operations described below. Subvolumes
can be renamed or moved, nesting subvolumes is not restricted but has some implications regarding
snapshotting.
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, internally 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 [options] [<dest>/]<name> [[<dest2>/]<name2> ...]
Create subvolume(s) at the destination(s).
If dest part of the path is not given, subvolume name will be created in the current directory.
If multiple destinations are given, then the given options are applied to all subvolumes.
If failure happens for any of the destinations, the command would still retry the remaining
destinations, but would return 1 to indicate the failure (similar to what mkdir would do.
Options
-i <qgroupid>
Add the newly created subvolume to a qgroup. This option can be given multiple times.
-p|--parents
Create any missing parent directories for each argument (like mkdir -p).
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 continues 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
subvolume 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(8))
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 subvolume.
-i|--subvolid <subvolid>
subvolume id to be removed instead of the <path> that should point to the filesystem with
the subvolume
-R|--recursive
delete subvolumes beneath each subvolume recursively
This requires either CAP_SYS_ADMIN or the filesystem must be mounted with
user_subvol_rm_allowed mount option. In the unprivileged case, subvolumes which cannot be
accessed are skipped. The deletion is not atomic.
-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 default:
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>. Note that this is not a recursive command,
and won't show nested subvolumes under <path>.
-a print all the subvolumes in the filesystem and distinguish 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 ascending.
-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 error.
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 until all current deletion requests are completed, but do not wait for
subvolumes deleted in the meantime.
If the filesystem status changes to read-only then the waiting is interrupted.
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.
SEE ALSO
btrfs-qgroup(8), btrfs-quota(8), btrfs-send(8), mkfs.btrfs(8), mount(8)