btrfs-quota - control the global quota status of a btrfs filesystem
btrfs quota <subcommand> <args>
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
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 detailed description.
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 available resources.
In case of files, the solution is quite straightforward. Each file has an
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 qgroups
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.
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 freely.
Each qgroup can contain a set of lower level qgroups, thus creating a hierarchy
of qgroups. Figure 1 shows an example qgroup tree.
/ \ / \
+---+/ \+---+/ \+---+
qgroups |0/1| |0/2| |0/3|
+-+-+ +---+ +---+
| / \ / \
| / \ / \
| / \ / \
extents 1 2 3 4
Figure1: 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
. 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
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.
Below are some usecases that do not mean to be extensive. You can find your own
way how to integrate qgroups.
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.
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,
. 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
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
, but do not assign system snapshots to user’s qgroup.
Disable subvolume quota support for a
Enable subvolume quota support for a
Trash all qgroup numbers and scan the metadata
again with the current config.
show status of a running rescan
wait for rescan operation to finish(can be
already in progress).
returns a zero exit status if it succeeds. Non zero is
returned in case of failure.
is part of btrfs-progs. Please refer to the btrfs wiki
for further details.