man > btrfs-quota(8)

BTRFS-QUOTA(8)                              Btrfs Manual                              BTRFS-QUOTA(8)



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)



Btrfs v5.16.2                                02/16/2022                               BTRFS-QUOTA(8)