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).
the 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
When the quotas are turned on, they affect all extent processing,
taking a performance hit. It is not recommended to turn on qgroups
unless the user intends to actually use them.
The qgroup implementation has turned out to be quite difficult as it
affects the core of the filesystem operation. The users have hit
various corner cases over time, eg. wrong accounting or system
instability. The situation is gradually improving but currently (4.7)
there are still issues found and fixed.
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
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 (eg. /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
Each qgroup primarily tracks two numbers, the amount of total
referenced space and the amount of exclusively referenced space.
space is the amount of data that can be reached from any of the
subvolumes contained in the qgroup, while
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, eg. 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
Each qgroup can contain a set of lower level qgroups, thus creating a
hierarchy of qgroups. Figure 1 shows an example qgroup tree.
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
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.
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
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.
Below are some usecases that do not mean to be extensive. You can
find your own way how to integrate qgroups.
SINGLE-USER MACHINEReplacement 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
MULTI-USER MACHINERestricting 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
, eg. 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.
Disable subvolume quota support for a filesystem.
Enable subvolume quota support for a filesystem.
rescan [-s] <path>
Trash all qgroup numbers and scan the metadata again with the
show status of a running rescan operation.
wait for rescan operation to finish(can be already in
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Btrfs v4.6.1 03/12/2017 BTRFS-QUOTA(8)