Provided by: btrfs-progs_6.6.3-1.2_amd64 
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 description.
PERFORMANCE IMPLICATIONS
When quotas are activated, they affect all extent processing, which takes a performance
hit. Activation of qgroups is not recommended unless 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 entire space. Every user should get his fair share
of the available resources.
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 administrator 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 partitioned 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 feature builds a bridge. Subvolumes correspond in many ways to partitions, as
every subvolume looks like its own filesystem. With subvolume quota, it is now possible
to restrict each subvolume like a partition, 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 referenced 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
subvolume, 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 reference which extents.
It is important to understand the notion of referenced 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 because 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 exclusive to the qgroup, but not to the source subvolume. With the creation of
the snapshot, the qgroup would also lose this extent from its exclusive 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. Exclusive 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 previous 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 similar 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 latencies, 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. exclusive usage. Instead, they account all extents to
the subvolume that first allocated it. With a bit of new bookkeeping, this allows all
accounting 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 mutable
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
section for more details.
Options
-s|--simple
use simple quotas (squotas) instead of full qgroup accounting
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 returned 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)