Manpages - btrfs-quota.8

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.

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.

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.

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.

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.