NAME
btrfs-subvolume - manage btrfs subvolumesSYNOPSIS
btrfs subvolume <subcommand> [<args>]DESCRIPTION
btrfs subvolume is used to create/delete/list/show btrfs subvolumes and snapshots.A subvolume in BTRFS is not like an LVM
logical volume, which is block-level snapshot while BTRFS subvolumes are file
extent-based.
- •
- like any other directory that is accessible to the user
- •
- like a separately mounted filesystem (options subvol or subvolid)
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.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.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, ...).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
- •
- fs-specific: compress, autodefrag, nodatacow, nodatasum
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.Inode number is not a filesystem-wide unique
identifier, some applications assume that. Please user pair
subvolumeid:inodenumber for that purpose.
PERFORMANCE
Subvolume creation needs to flush dirty data that belong to the subvolume, 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.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.A subvolume in btrfs is not like an LVM
logical volume, which is block-level snapshot while btrfs subvolumes are file
extent-based.
- •
- like any other directory that is accessible to the user
- •
- like a separately mounted filesystem (options subvol or subvolid)
SUBCOMMAND
- create [-i <qgroupid>] [<dest>/]<name>
- Create a subvolume name in dest. If dest is not given, subvolume name will be created in the current 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 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) 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
- -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>.
- -a
- print all the subvolumes in the filesystem and distinguish between absolute and relative path with respect to the given path.
- -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.
- -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.
- -t
- print the result as a table.
- -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):
- -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. 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)February 28, 2023 | 6.2 |