path: root/upstream/debian-bookworm/man8/btrfs-subvolume.8

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.TH "BTRFS-SUBVOLUME" "8" "Feb 28, 2023" "6.2" "BTRFS"
.SH NAME
btrfs-subvolume \- manage btrfs subvolumes
.SH SYNOPSIS
.sp
\fBbtrfs subvolume\fP <subcommand> [<args>]
.SH DESCRIPTION
.sp
\fBbtrfs subvolume\fP is used to create/delete/list/show btrfs subvolumes and
snapshots.
.sp
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.
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
A subvolume in BTRFS is not like an LVM logical volume, which is block\-level
snapshot while BTRFS subvolumes are file extent\-based.
.UNINDENT
.UNINDENT
.sp
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 \fIsubvolid\fP or \fIrootid\fP) of the subvolume is persistent and cannot be
changed.
.sp
A subvolume in BTRFS can be accessed in two ways:
.INDENT 0.0
.IP \(bu 2
like any other directory that is accessible to the user
.IP \(bu 2
like a separately mounted filesystem (options \fIsubvol\fP or \fIsubvolid\fP)
.UNINDENT
.sp
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.
.sp
A freshly created filesystem is also a subvolume, called \fItop\-level\fP,
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 \fBbtrfs subvolume set\-default\fP).
.sp
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.
.sp
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).
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
A snapshot is not a backup: snapshots work by use of BTRFS\(aq 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
\(dqcopies\(dq 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.
.UNINDENT
.UNINDENT
.SH SUBVOLUME FLAGS
.sp
The subvolume flag currently implemented is the \fIro\fP property (read\-only
status). Read\-write subvolumes have that set to \fIfalse\fP, snapshots as \fItrue\fP\&.
In addition to that, a plain snapshot will also have last change generation and
creation generation equal.
.sp
Read\-only snapshots are building blocks of incremental send (see
\fI\%btrfs\-send(8)\fP) 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.
.sp
A snapshot that was created by send/receive will be read\-only, with different
last change generation, read\-only and with set \fIreceived_uuid\fP 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 \fIreceived_uuid\fP\&. As this is a notable
change and could potentially break the incremental send use case, performing
it by \fBbtrfs property set\fP requires force if that is really desired by user.
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
The safety checks have been implemented in 5.14.2, any subvolumes previously
received (with a valid \fIreceived_uuid\fP) and read\-write status may exist and
could still lead to problems with send/receive. You can use \fBbtrfs subvolume
show\fP to identify them. Flipping the flags to read\-only and back to
read\-write will reset the \fIreceived_uuid\fP manually.  There may exist a
convenience tool in the future.
.UNINDENT
.UNINDENT
.SH NESTED SUBVOLUMES
.sp
There are no restrictions for subvolume creation, so it\(aqs 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.
.sp
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\(aqs a stub subvolume (also sometimes
\fBempty subvolume\fP with the same name as original subvolume, with inode number
2).  This can be used intentionally but could be confusing in case of nested
layouts.
.SS Case study: system root layouts
.sp
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 \(dq/\(dq
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, ...).
.sp
Here we could utilize the snapshotting barrier mentioned above, each directory
that stores data to be preserved across rollbacks is it\(aqs own subvolume. This
could be e.g. \fB/var\fP\&. Further more\-fine grained partitioning could be done, e.g.
adding separate subvolumes for \fB/var/log\fP, \fB/var/cache\fP etc.
.sp
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.
.SH MOUNT OPTIONS
.sp
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:
.INDENT 0.0
.IP \(bu 2
generic: noatime/relatime/..., nodev, nosuid, ro, rw, dirsync
.IP \(bu 2
fs\-specific: compress, autodefrag, nodatacow, nodatasum
.UNINDENT
.sp
An example of whole filesystem options is e.g. \fIspace_cache\fP, \fIrescue\fP, \fIdevice\fP,
\fIskip_balance\fP, etc. The exceptional options are \fIsubvol\fP and \fIsubvolid\fP that
are actually used for mounting a given subvolume and can be specified only once
for the mount.
.sp
Subvolumes belong to a single filesystem and as implemented now all share the
same specific mount options, changes done by remount have immediate effect. This
may change in the future.
.sp
Mounting a read\-write snapshot as read\-only is possible and will not change the
\fIro\fP property and flag of the subvolume.
.sp
The name of the mounted subvolume is stored in file \fB/proc/self/mounts\fP in the
4th column:
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
27 21 0:19 /subv1 /mnt rw,relatime \- btrfs /dev/sda rw,space_cache
           ^^^^^^
.ft P
.fi
.UNINDENT
.UNINDENT
.SH INODE NUMBERS
.sp
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.
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
Inode number is not a filesystem\-wide unique identifier, some applications
assume that. Please user pair \fIsubvolumeid:inodenumber\fP for that purpose.
.UNINDENT
.UNINDENT
.SH PERFORMANCE
.sp
Subvolume creation needs to flush dirty data that belong to the subvolume, this
step may take some time, otherwise once there\(aqs nothing else to do, the snapshot
is instant and in the metadata it only creates a new tree root copy.
.sp
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 \fIcleaning\fP 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.
.SH SUBVOLUME AND SNAPSHOT
.sp
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.
.sp
\fBNOTE:\fP
.INDENT 0.0
.INDENT 3.5
A subvolume in btrfs is not like an LVM logical volume, which is block\-level
snapshot while btrfs subvolumes are file extent\-based.
.UNINDENT
.UNINDENT
.sp
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.
.sp
A subvolume in btrfs can be accessed in two ways:
.INDENT 0.0
.IP \(bu 2
like any other directory that is accessible to the user
.IP \(bu 2
like a separately mounted filesystem (options \fIsubvol\fP or \fIsubvolid\fP)
.UNINDENT
.sp
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.
.sp
A freshly created filesystem is also a subvolume, called \fItop\-level\fP,
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 \fIset\-default\fP).
.sp
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.
.SH SUBCOMMAND
.INDENT 0.0
.TP
.B create [\-i <qgroupid>] [<dest>/]<name>
Create a subvolume \fIname\fP in \fIdest\fP\&.
.sp
If \fIdest\fP is not given, subvolume \fIname\fP will be created in the current
directory.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.BI \-i \ <qgroupid>
Add the newly created subvolume to a qgroup. This option can be given multiple
times.
.UNINDENT
.TP
.B delete [options] [<subvolume> [<subvolume>...]], delete \-i|\-\-subvolid <subvolid> <path>
Delete the subvolume(s) from the filesystem.
.sp
If \fIsubvolume\fP is not a subvolume, btrfs returns an error but continues if
there are more arguments to process.
.sp
If \fI\-\-subvolid\fP is used, \fIpath\fP must point to a btrfs filesystem. See \fBbtrfs
subvolume list\fP or \fBbtrfs inspect\-internal rootid\fP how to get the subvolume id.
.sp
The corresponding directory is removed instantly but the data blocks are
removed later in the background. The command returns immediately. See \fBbtrfs
subvolume sync\fP how to wait until the subvolume gets completely removed.
.sp
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 \fI\-\-commit\fP options to wait until the operation is
safely stored on the device.
.sp
Deleting subvolume needs sufficient permissions, by default the owner
cannot delete it unless it\(aqs enabled by a mount option
\fIuser_subvol_rm_allowed\fP, or deletion is run as root.
The default subvolume (see \fBbtrfs subvolume set\-default\fP) cannot be deleted and
returns error (EPERM) and this is logged to the system log. A subvolume that\(aqs
currently involved in send (see \fBbtrfs send\fP) also cannot be deleted until the
send is finished. This is also logged in the system log.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B \-c|\-\-commit\-after
wait for transaction commit at the end of the operation.
.TP
.B \-C|\-\-commit\-each
wait for transaction commit after deleting each subvolume.
.TP
.B \-i|\-\-subvolid <subvolid>
subvolume id to be removed instead of the <path> that should point to the
filesystem with the subvolume
.TP
.B \-v|\-\-verbose
(deprecated) alias for global \fI\-v\fP option
.UNINDENT
.TP
.B find\-new <subvolume> <last_gen>
List the recently modified files in a subvolume, after \fIlast_gen\fP generation.
.TP
.B get\-default <path>
Get the default subvolume of the filesystem \fIpath\fP\&.
.sp
The output format is similar to \fBsubvolume list\fP command.
.TP
.B list [options] [\-G [+|\-]<value>] [\-C [+|\-]<value>] [\-\-sort=rootid,gen,ogen,path] <path>
List the subvolumes present in the filesystem \fIpath\fP\&.
.sp
For every subvolume the following information is shown by default:
.sp
ID \fIID\fP gen \fIgeneration\fP top level \fIparent_ID\fP path \fIpath\fP
.sp
where \fIID\fP is subvolume\(aqs (root)id, \fIgeneration\fP is an internal counter which is
updated every transaction, \fIparent_ID\fP is the same as the parent subvolume\(aqs id,
and \fIpath\fP is the relative path of the subvolume to the top level subvolume.
The subvolume\(aqs ID may be used by the subvolume set\-default command,
or at mount time via the \fIsubvolid=\fP option.
.sp
\fBOptions\fP
.sp
Path filtering:
.INDENT 7.0
.TP
.B  \-o
print only subvolumes below specified <path>.
.TP
.B  \-a
print all the subvolumes in the filesystem and distinguish between
absolute and relative path with respect to the given \fIpath\fP\&.
.UNINDENT
.sp
Field selection:
.INDENT 7.0
.TP
.B  \-p
print the parent ID
(\fIparent\fP here means the subvolume which contains this subvolume).
.TP
.B  \-c
print the ogeneration of the subvolume, aliases: ogen or origin generation.
.TP
.B  \-g
print the generation of the subvolume (default).
.TP
.B  \-u
print the UUID of the subvolume.
.TP
.B  \-q
print the parent UUID of the subvolume
(\fIparent\fP here means subvolume of which this subvolume is a snapshot).
.TP
.B  \-R
print the UUID of the sent subvolume, where the subvolume is the result of a receive operation.
.UNINDENT
.sp
Type filtering:
.INDENT 7.0
.TP
.B  \-s
only snapshot subvolumes in the filesystem will be listed.
.TP
.B  \-r
only readonly subvolumes in the filesystem will be listed.
.TP
.B  \-d
list deleted subvolumes that are not yet cleaned.
.UNINDENT
.sp
Other:
.INDENT 7.0
.TP
.B  \-t
print the result as a table.
.UNINDENT
.sp
Sorting:
.sp
By default the subvolumes will be sorted by subvolume ID ascending.
.INDENT 7.0
.TP
.B \-G [+|\-]<value>
list subvolumes in the filesystem that its generation is
>=, <= or = value. \(aq+\(aq means >= value, \(aq\-\(aq means <= value, If there is
neither \(aq+\(aq nor \(aq\-\(aq, it means = value.
.TP
.B \-C [+|\-]<value>
list subvolumes in the filesystem that its ogeneration is
>=, <= or = value. The usage is the same to \fI\-G\fP option.
.TP
.B \-\-sort=rootid,gen,ogen,path
list subvolumes in order by specified items.
you can add \fI+\fP or \fI\-\fP in front of each items, \fI+\fP means ascending,
\fI\-\fP means descending. The default is ascending.
.sp
for \fI\-\-sort\fP you can combine some items together by \fI,\fP, just like
\fI\-\-sort=+ogen,\-gen,path,rootid\fP\&.
.UNINDENT
.TP
.B set\-default [<subvolume>|<id> <path>]
Set the default subvolume for the (mounted) filesystem.
.sp
Set the default subvolume for the (mounted) filesystem at \fIpath\fP\&. This will hide
the top\-level subvolume (i.e. the one mounted with \fIsubvol=/\fP or \fIsubvolid=5\fP).
Takes action on next mount.
.sp
There are two ways how to specify the subvolume, by \fIid\fP or by the \fIsubvolume\fP
path.
The id can be obtained from \fBbtrfs subvolume list\fP, \fBbtrfs subvolume show\fP or
\fBbtrfs inspect\-internal rootid\fP\&.
.TP
.B show [options] <path>
Show more information about a subvolume (UUIDs, generations, times, flags,
related snapshots).
.INDENT 7.0
.INDENT 3.5
.sp
.nf
.ft C
/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):
.ft P
.fi
.UNINDENT
.UNINDENT
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B \-r|\-\-rootid <ID>
show details about subvolume with root \fIID\fP, looked up in \fIpath\fP
.TP
.B \-u|\-\-uuid UUID
show details about subvolume with the given \fIUUID\fP, looked up in \fIpath\fP
.UNINDENT
.TP
.B snapshot [\-r] [\-i <qgroupid>] <source> <dest>|[<dest>/]<name>
Create a snapshot of the subvolume \fIsource\fP with the
name \fIname\fP in the \fIdest\fP directory.
.sp
If only \fIdest\fP is given, the subvolume will be named the basename of \fIsource\fP\&.
If \fIsource\fP is not a subvolume, btrfs returns an error.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.B  \-r
Make the new snapshot read only.
.TP
.BI \-i \ <qgroupid>
Add the newly created subvolume to a qgroup. This option can be given multiple
times.
.UNINDENT
.TP
.B 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.
.sp
\fBOptions\fP
.INDENT 7.0
.TP
.BI \-s \ <N>
sleep N seconds between checks (default: 1)
.UNINDENT
.UNINDENT
.SH EXAMPLES
.SS Deleting a subvolume
.sp
If we want to delete a subvolume called \fIfoo\fP from a btrfs volume mounted at
\fI/mnt/bar\fP we could run the following:
.INDENT 0.0
.INDENT 3.5
.sp
.nf
.ft C
btrfs subvolume delete /mnt/bar/foo
.ft P
.fi
.UNINDENT
.UNINDENT
.SH EXIT STATUS
.sp
\fBbtrfs subvolume\fP returns a zero exit status if it succeeds. A non\-zero value is
returned in case of failure.
.SH AVAILABILITY
.sp
\fBbtrfs\fP is part of btrfs\-progs.  Please refer to the documentation at
\fI\%https://btrfs.readthedocs.io\fP or wiki \fI\%http://btrfs.wiki.kernel.org\fP for further
information.
.SH SEE ALSO
.sp
\fI\%btrfs\-qgroup(8)\fP,
\fI\%btrfs\-quota(8)\fP,
\fI\%btrfs\-send(8)\fP,
\fI\%mkfs.btrfs(8)\fP,
\fBmount(8)\fP
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