Provided by: btrfs-progs_4.15.1-1build1_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 (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.
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, 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.
+---+
|2/1|
+---+
/ \
+---+/ \+---+
|1/1| |1/2|
+---+ +---+
/ \ / \
+---+/ \+---+/ \+---+
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 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 usecases 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 , 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.
SUBCOMMAND
disable <path>
Disable subvolume quota support for a filesystem.
enable <path>
Enable subvolume quota support for a filesystem.
rescan [-s] <path>
Trash all qgroup numbers and scan the metadata again with the current config.
Options
-s
show status of a running rescan operation.
-w
wait for rescan operation to finish(can be already in progress).
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 btrfs wiki http://btrfs.wiki.kernel.org for further
details.
SEE ALSO
mkfs.btrfs(8), btrfs-subvolume(8), btrfs-qgroup(8)