BTRFS-QGROUP(8) BTRFS BTRFS-QGROUP(8)
NAME
btrfs-qgroup - control the quota group of a btrfs filesystem
SYNOPSIS
btrfs qgroup <subcommand> <args>
DESCRIPTION
btrfs qgroup is used to control quota group (qgroup) of a btrfs
filesystem.
NOTE:
To use qgroup you need to enable quota first using btrfs quota en-
able command.
WARNING:
Qgroup is not stable yet and will impact performance in current
mainline kernel (v4.14).
QGROUP
Quota groups or qgroup in btrfs make a tree hierarchy, the leaf qgroups
are attached to subvolumes. The size limits are set per qgroup and ap-
ply when any limit is reached in tree that contains a given subvolume.
The limits are separated between shared and exclusive and reflect the
extent ownership. For example a fresh snapshot shares almost all the
blocks with the original subvolume, new writes to either subvolume will
raise towards the exclusive limit.
The qgroup identifiers conform to level/id where level 0 is reserved to
the qgroups associated with subvolumes. Such qgroups are created auto-
matically.
The qgroup hierarchy is built by commands create and assign.
NOTE:
If the qgroup of a subvolume is destroyed, quota about the subvolume
will not be functional until qgroup 0/<subvolume id> is created
again.
SUBCOMMAND
assign [options] <src> <dst> <path>
Assign qgroup src as the child qgroup of dst in the btrfs
filesystem identified by path.
Options
--rescan
(default since: 4.19) Automatically schedule quota rescan
if the new qgroup assignment would lead to quota incon-
sistency. See QUOTA RESCAN for more information.
--no-rescan
Explicitly ask not to do a rescan, even if the assignment
will make the quotas inconsistent. This may be useful for
repeated calls where the rescan would add unnecessary
overhead.
create <qgroupid> <path>
Create a subvolume quota group.
For the 0/<subvolume id> qgroup, a qgroup can be created even
before the subvolume is created.
destroy <qgroupid> <path>
Destroy a qgroup.
If a qgroup is not isolated, meaning it is a parent or child
qgroup, then it can only be destroyed after the relationship is
removed.
clear-stale <path>
Clear all stale qgroups whose subvolume does not exist anymore,
this is the level 0 qgroup like 0/subvolid. Higher level qgroups
are not deleted even if they don't have any child qgroups.
limit [options] <size>|none [<qgroupid>] <path>
Limit the size of a qgroup to size or no limit in the btrfs
filesystem identified by path.
If qgroupid is not given, qgroup of the subvolume identified by
path is used if possible.
Options
-c limit amount of data after compression. This is the de-
fault, it is currently not possible to turn off this op-
tion.
-e limit space exclusively assigned to this qgroup.
remove <src> <dst> <path>
Remove the relationship between child qgroup src and parent
qgroup dst in the btrfs filesystem identified by path.
Options
--rescan
(default since: 4.19) Automatically schedule quota rescan
if the removed qgroup relation would lead to quota incon-
sistency. See QUOTA RESCAN for more information.
--no-rescan
Explicitly ask not to do a rescan, even if the removal
will make the quotas inconsistent. This may be useful for
repeated calls where the rescan would add unnecessary
overhead.
show [options] <path>
Show all qgroups in the btrfs filesystem identified by <path>.
Options
-p print parent qgroup id.
-c print child qgroup id.
-r print limit of referenced size of qgroup.
-e print limit of exclusive size of qgroup.
-F list all qgroups which impact the given path(include an-
cestral qgroups)
-f list all qgroups which impact the given path(exclude an-
cestral qgroups)
--raw raw numbers in bytes, without the B suffix.
--human-readable
print human friendly numbers, base 1024, this is the de-
fault
--iec select the 1024 base for the following options, according
to the IEC standard.
--si select the 1000 base for the following options, according
to the SI standard.
--kbytes
show sizes in KiB, or kB with --si.
--mbytes
show sizes in MiB, or MB with --si.
--gbytes
show sizes in GiB, or GB with --si.
--tbytes
show sizes in TiB, or TB with --si.
--sort=[+/-]<attr>[,[+/-]<attr>]...
list qgroups in order of <attr>.
<attr> can be one or more of
qgroupid,rfer,excl,max_rfer,max_excl.
Prefix + means ascending order and - means descending or-
der of attr. If no prefix is given, use ascending order
by default.
If multiple attr values are given, use comma to separate.
--sync To retrieve information after updating the state of
qgroups, force sync of the filesystem identified by path
before getting information.
QUOTA RESCAN
The rescan reads all extent sharing metadata and updates the respective
qgroups accordingly.
The information consists of bytes owned exclusively (excl) or
shared/referred to (rfer). There's no explicit information about which
extents are shared or owned exclusively. This means when qgroup rela-
tionship changes, extent owners change and qgroup numbers are no longer
consistent unless we do a full rescan.
However there are cases where we can avoid a full rescan, if a subvol-
ume whose rfer number equals its excl number, which means all bytes are
exclusively owned, then assigning/removing this subvolume only needs to
add/subtract rfer number from its parent qgroup. This can speed up the
rescan.
EXAMPLES
Make a parent group that has two quota group children
Given the following filesystem mounted at /mnt/my-vault
Label: none uuid: 60d2ab3b-941a-4f22-8d1a-315f329797b2
Total devices 1 FS bytes used 128.00KiB
devid 1 size 5.00GiB used 536.00MiB path /dev/vdb
Enable quota and create subvolumes. Check subvolume ids.
$ cd /mnt/my-vault
$ btrfs quota enable .
$ btrfs subvolume create a
$ btrfs subvolume create b
$ btrfs subvolume list .
ID 261 gen 61 top level 5 path a
ID 262 gen 62 top level 5 path b
Create qgroup and set limit to 10MiB.
$ btrfs qgroup create 1/100 .
$ btrfs qgroup limit 10M 1/100 .
$ btrfs qgroup assign 0/261 1/100 .
$ btrfs qgroup assign 0/262 1/100 .
And check qgroups.
$ btrfs qgroup show .
qgroupid rfer excl
-------- ---- ----
0/5 16.00KiB 16.00KiB
0/261 16.00KiB 16.00KiB
0/262 16.00KiB 16.00KiB
1/100 32.00KiB 32.00KiB
EXIT STATUS
btrfs qgroup returns a zero exit status if it succeeds. Non zero is re-
turned 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-quota(8), btrfs-subvolume(8), mkfs.btrfs(8),
6.2 Feb 28, 2023 BTRFS-QGROUP(8)
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