btrfs-qgroup - control the quota group of a btrfs filesystem
btrfs qgroup <subcommand> <args>
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 enable command.
WARNING:
Qgroup is not stable yet and will impact performance in
current mainline kernel (v4.14).
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
apply 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
automatically.
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.
- 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 inconsistency. 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 default, it is
currently not possible to turn off this option.
- -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 inconsistency. 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 ancestral
qgroups)
- -f
- list all qgroups which impact the given path(exclude ancestral
qgroups)
- --raw
- raw numbers in bytes, without the B suffix.
- --human-readable
- print human friendly numbers, base 1024, this is the default
- --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 order 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.
For btrfs qgroup show subcommand, the path column
may has some special strings:
- <toplevel>
- The toplevel subvolume
- <under deletion>
- The subvolume has been deleted (it's directory removed), but the subvolume
metadata not not yet fully cleaned.
- <squota space holder>
- For simple quota mode only. By its design, a fully deleted subvolume may
still have accounting on it, so even the subvolume is gone, the numbers
are still here for future accounting.
- <stale>
- The qgroup has no corresponding subvolume anymore, and the qgroup can be
cleaned up under most cases. The only exception is that, if the qgroup
numbers are inconsistent and the qgroup numbers are not all zeros, some
older kernels may refuse to delete such qgroups until a full 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
relationship 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
subvolume 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.
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
btrfs qgroup returns a zero exit status if it succeeds. Non
zero is returned in case of failure.
btrfs is part of btrfs-progs. Please refer to the
documentation at https://btrfs.readthedocs.io.
btrfs-quota(8), btrfs-subvolume(8),
mkfs.btrfs(8)