BTRFS-QUOTA(8) BTRFS BTRFS-QUOTA(8)
NAME
btrfs-quota - control the global quota status of a btrfs filesystem
SYNOPSIS
btrfs quota <subcommand> <args>
DESCRIPTION
The commands under btrfs quota are used to affect the global status of
quotas of a btrfs filesystem. The quota groups (qgroups) are managed by
the subcommand btrfs-qgroup(8).
NOTE:
Qgroups are different than the traditional user quotas and designed
to track shared and exclusive data per-subvolume. Please refer to
the section HIERARCHICAL QUOTA GROUP CONCEPTS for a detailed de-
scription.
PERFORMANCE IMPLICATIONS
When quotas are activated, they affect all extent processing, which
takes a performance hit. Activation of qgroups is not recommended un-
less the user intends to actually use them.
STABILITY STATUS
The qgroup implementation has turned out to be quite difficult as it
affects the core of the filesystem operation. Qgroup users have hit
various corner cases over time, such as incorrect accounting or system
instability. The situation is gradually improving and issues found and
fixed.
HIERARCHICAL QUOTA GROUP CONCEPTS
The concept of quota has a long-standing tradition in the Unix world.
Ever since computers allow multiple users to work simultaneously in one
filesystem, there is the need to prevent one user from using up the en-
tire space. Every user should get his fair share of the available re-
sources.
In case of files, the solution is quite straightforward. Each file has
an owner recorded along with it, and it has a size. Traditional quota
just restricts the total size of all files that are owned by a user.
The concept is quite flexible: if a user hits his quota limit, the ad-
ministrator can raise it on the fly.
On the other hand, the traditional approach has only a poor solution to
restrict directories. At installation time, the harddisk can be parti-
tioned so that every directory (e.g. /usr, /var, ...) that needs a
limit gets its own partition. The obvious problem is that those limits
cannot be changed without a reinstallation. The btrfs subvolume fea-
ture builds a bridge. Subvolumes correspond in many ways to parti-
tions, as every subvolume looks like its own filesystem. With subvol-
ume quota, it is now possible to restrict each subvolume like a parti-
tion, but keep the flexibility of quota. The space for each subvolume
can be expanded or restricted on the fly.
As subvolumes are the basis for snapshots, interesting questions arise
as to how to account used space in the presence of snapshots. If you
have a file shared between a subvolume and a snapshot, whom to account
the file to? The creator? Both? What if the file gets modified in the
snapshot, should only these changes be accounted to it? But wait, both
the snapshot and the subvolume belong to the same user home. I just
want to limit the total space used by both! But somebody else might not
want to charge the snapshots to the users.
Btrfs subvolume quota solves these problems by introducing groups of
subvolumes and let the user put limits on them. It is even possible to
have groups of groups. In the following, we refer to them as qgroups.
Each qgroup primarily tracks two numbers, the amount of total refer-
enced space and the amount of exclusively referenced space.
referenced
space is the amount of data that can be reached from any of the
subvolumes contained in the qgroup, while
exclusive
is the amount of data where all references to this data can be
reached from within this qgroup.
Subvolume quota groups
The basic notion of the Subvolume Quota feature is the quota group,
short qgroup. Qgroups are notated as level/id, e.g. the qgroup 3/2 is
a qgroup of level 3. For level 0, the leading 0/ can be omitted.
Qgroups of level 0 get created automatically when a subvolume/snapshot
gets created. The ID of the qgroup corresponds to the ID of the sub-
volume, so 0/5 is the qgroup for the root subvolume. For the btrfs
qgroup command, the path to the subvolume can also be used instead of
0/ID. For all higher levels, the ID can be chosen freely.
Each qgroup can contain a set of lower level qgroups, thus creating a
hierarchy of qgroups. Figure 1 shows an example qgroup tree.
+---+
|2/1|
+---+
/ \
+---+/ \+---+
|1/1| |1/2|
+---+ +---+
/ \ / \
+---+/ \+---+/ \+---+
qgroups |0/1| |0/2| |0/3|
+-+-+ +---+ +---+
| / \ / \
| / \ / \
| / \ / \
extents 1 2 3 4
Figure 1: Sample qgroup hierarchy
At the bottom, some extents are depicted showing which qgroups refer-
ence which extents. It is important to understand the notion of refer-
enced vs exclusive. In the example, qgroup 0/2 references extents 2
and 3, while 1/2 references extents 2-4, 2/1 references all extents.
On the other hand, extent 1 is exclusive to 0/1, extent 2 is exclusive
to 0/2, while extent 3 is neither exclusive to 0/2 nor to 0/3. But be-
cause both references can be reached from 1/2, extent 3 is exclusive to
1/2. All extents are exclusive to 2/1.
So exclusive does not mean there is no other way to reach the extent,
but it does mean that if you delete all subvolumes contained in a
qgroup, the extent will get deleted.
Exclusive of a qgroup conveys the useful information how much space
will be freed in case all subvolumes of the qgroup get deleted.
All data extents are accounted this way. Metadata that belongs to a
specific subvolume (i.e. its filesystem tree) is also accounted.
Checksums and extent allocation information are not accounted.
In turn, the referenced count of a qgroup can be limited. All writes
beyond this limit will lead to a 'Quota Exceeded' error.
Inheritance
Things get a bit more complicated when new subvolumes or snapshots are
created. The case of (empty) subvolumes is still quite easy. If a
subvolume should be part of a qgroup, it has to be added to the qgroup
at creation time. To add it at a later time, it would be necessary to
at least rescan the full subvolume for a proper accounting.
Creation of a snapshot is the hard case. Obviously, the snapshot will
reference the exact amount of space as its source, and both source and
destination now have an exclusive count of 0 (the filesystem nodesize
to be precise, as the roots of the trees are not shared). But what
about qgroups of higher levels? If the qgroup contains both the source
and the destination, nothing changes. If the qgroup contains only the
source, it might lose some exclusive.
But how much? The tempting answer is, subtract all exclusive of the
source from the qgroup, but that is wrong, or at least not enough.
There could have been an extent that is referenced from the source and
another subvolume from that qgroup. This extent would have been exclu-
sive to the qgroup, but not to the source subvolume. With the creation
of the snapshot, the qgroup would also lose this extent from its exclu-
sive set.
So how can this problem be solved? In the instant the snapshot gets
created, we already have to know the correct exclusive count. We need
to have a second qgroup that contains all the subvolumes as the first
qgroup, except the subvolume we want to snapshot. The moment we create
the snapshot, the exclusive count from the second qgroup needs to be
copied to the first qgroup, as it represents the correct value. The
second qgroup is called a tracking qgroup. It is only there in case a
snapshot is needed.
Use cases
Below are some use cases that do not mean to be extensive. You can find
your own way how to integrate qgroups.
Single-user machine
Replacement for partitions. The simplest use case is to use qgroups as
simple replacement for partitions. Btrfs takes the disk as a whole,
and /, /usr, /var, etc. are created as subvolumes. As each subvolume
gets it own qgroup automatically, they can simply be restricted. No
hierarchy is needed for that.
Track usage of snapshots. When a snapshot is taken, a qgroup for it
will automatically be created with the correct values. Referenced will
show how much is in it, possibly shared with other subvolumes. Exclu-
sive will be the amount of space that gets freed when the subvolume is
deleted.
Multi-user machine
Restricting homes. When you have several users on a machine, with home
directories probably under /home, you might want to restrict /home as a
whole, while restricting every user to an individual limit as well.
This is easily accomplished by creating a qgroup for /home , e.g. 1/1,
and assigning all user subvolumes to it. Restricting this qgroup will
limit /home, while every user subvolume can get its own (lower) limit.
Accounting snapshots to the user. Let's say the user is allowed to
create snapshots via some mechanism. It would only be fair to account
space used by the snapshots to the user. This does not mean the user
doubles his usage as soon as he takes a snapshot. Of course, files
that are present in his home and the snapshot should only be accounted
once. This can be accomplished by creating a qgroup for each user, say
1/UID. The user home and all snapshots are assigned to this qgroup.
Limiting it will extend the limit to all snapshots, counting files only
once. To limit /home as a whole, a higher level group 2/1 replacing
1/1 from the previous example is needed, with all user qgroups assigned
to it.
Do not account snapshots. On the other hand, when the snapshots get
created automatically, the user has no chance to control them, so the
space used by them should not be accounted to him. This is already the
case when creating snapshots in the example from the previous section.
Snapshots for backup purposes. This scenario is a mixture of the pre-
vious two. The user can create snapshots, but some snapshots for
backup purposes are being created by the system. The user's snapshots
should be accounted to the user, not the system. The solution is simi-
lar to the one from section Accounting snapshots to the user, but do
not assign system snapshots to user's qgroup.
Simple quotas (squota)
As detailed in this document, qgroups can handle many complex extent
sharing and unsharing scenarios while maintaining an accurate count of
exclusive and shared usage. However, this flexibility comes at a cost:
many of the computations are global, in the sense that we must count up
the number of trees referring to an extent after its references change.
This can slow down transaction commits and lead to unacceptable laten-
cies, especially in cases where snapshots scale up.
To work around this limitation of qgroups, btrfs also supports a second
set of quota semantics: simple quotas or squotas. Squotas fully share
the qgroups API and hierarchical model, but do not track shared vs. ex-
clusive usage. Instead, they account all extents to the subvolume that
first allocated it. With a bit of new bookkeeping, this allows all ac-
counting decisions to be local to the allocation or freeing operation
that deals with the extents themselves, and fully avoids the complex
and costly back-reference resolutions.
Example
To illustrate the difference between squotas and qgroups, consider the
following basic example assuming a nodesize of 16KiB.
1. create subvolume 256
2. rack up 1GiB of data and metadata usage in 256
3. snapshot 256, creating subvolume 257
4. COW 512MiB of the data and metadata in 257
5. delete everything in 256
At each step, qgroups would have the following accounting:
1. 0/256: 16KiB excl 0 shared
2. 0/256: 1GiB excl 0 shared
3. 0/256: 0 excl 1GiB shared; 0/257: 0 excl 1GiB shared
4. 0/256: 512MiB excl 512MiB shared; 0/257: 512MiB excl 512MiB shared
5. 0/256: 16KiB excl 0 shared; 0/257: 1GiB excl 0 shared
Whereas under squotas, the accounting would look like:
1. 0/256: 16KiB excl 16KiB shared
2. 0/256: 1GiB excl 1GiB shared
3. 0/256: 1GiB excl 1GiB shared; 0/257: 16KiB excl 16KiB shared
4. 0/256: 1GiB excl 1GiB shared; 0/257: 512MiB excl 512MiB shared
5. 0/256: 512MiB excl 512MiB shared; 0/257: 512MiB excl 512MiB shared
Note that since the original snapshotted 512MiB are still referenced by
257, they cannot be freed from 256, even after 256 is emptied, or even
deleted.
Summary
If you want some of power and flexibility of quotas for tracking and
limiting subvolume usage, but want to avoid the performance penalty of
accurately tracking extent ownership life cycles, then squotas can be a
useful option.
Furthermore, squotas is targeted at use cases where the original extent
is immutable, like image snapshotting for container startup, in which
case we avoid these awkward scenarios where a subvolume is empty or
deleted but still has significant extents accounted to it. However, as
long as you are aware of the accounting semantics, they can handle mu-
table original extents.
SUBCOMMAND
disable <path>
Disable subvolume quota support for a filesystem.
enable [options] <path>
Enable subvolume quota support for a filesystem. At this point
it's possible the two modes of accounting. The full means that
extent ownership by subvolumes will be tracked all the time,
simple will account everything to the first owner. See the sec-
tion for more details.
Options
-s|--simple
use simple quotas (squotas) instead of full qgroup ac-
counting
rescan [options] <path>
Trash all qgroup numbers and scan the metadata again with the
current config.
Options
-s|--status
show status of a running rescan operation.
-w|--wait
start rescan and wait for it to finish (can be already in
progress)
-W|--wait-norescan
wait for rescan to finish without starting it
EXIT STATUS
btrfs quota 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.
SEE ALSO
btrfs-qgroup(8), btrfs-subvolume(8), mkfs.btrfs(8)
6.6.3 Mar 31, 2024 BTRFS-QUOTA(8)
Generated by dwww version 1.16 on Tue Dec 16 14:25:50 CET 2025.