btrfs-progs/Documentation/btrfs-qgroup.rst

btrfs-qgroup(8)
===============

SYNOPSIS
--------

**btrfs qgroup** <subcommand> <args>

DESCRIPTION
-----------

**btrfs qgroup** is used to control quota group (qgroup) of a btrfs filesystem.

.. note::
   To use qgroup you need to enable quota first using :command:`btrfs quota enable`
   command.

.. warning::
   Qgroup is not stable yet and will impact performance in current mainline
   kernel (v4.14).

QGROUP
------

Quota groups or qgroup in btrfs make a tree hierarchy, the leaf qgroups are
attached to subvolumes. The size limits are set per qgroup and apply when any
limit is reached in tree that contains a given subvolume.

The limits are separated between shared and exclusive and reflect the extent
ownership. For example a fresh snapshot shares almost all the blocks with the
original subvolume, new writes to either subvolume will raise towards the
exclusive limit.

The qgroup identifiers conform to *level/id* where level 0 is reserved to the
qgroups associated with subvolumes. Such qgroups are created automatically.

The qgroup hierarchy is built by commands :command:`create` and :command:`assign`.

.. note::
   If the qgroup of a subvolume is destroyed, quota about the subvolume will
   not be functional until qgroup *0/<subvolume id>* is created again.

SUBCOMMAND
----------

assign [options] <src> <dst> <path>
        Assign qgroup *src* as the child qgroup of *dst* in the btrfs filesystem
        identified by *path*.

        ``Options``

        --rescan
                (default since: 4.19) Automatically schedule quota rescan if the new qgroup
                assignment would lead to quota inconsistency. See *QUOTA RESCAN* for more
                information.
        --no-rescan
                Explicitly ask not to do a rescan, even if the assignment will make the quotas
                inconsistent. This may be useful for repeated calls where the rescan would add
                unnecessary overhead.

create <qgroupid> <path>
        Create a subvolume quota group.

        For the *0/<subvolume id>* qgroup, a qgroup can be created even before the
        subvolume is created.

destroy <qgroupid> <path>
        Destroy a qgroup.

        If a qgroup is not isolated, meaning it is a parent or child qgroup, then it
        can only be destroyed after the relationship is removed.

clear-stale <path>
	Clear all stale qgroups whose subvolume does not exist anymore, this is the
	level 0 qgroup like 0/subvolid. Higher level qgroups are not deleted even
	if they don't have any child qgroups.

limit [options] <size>|none [<qgroupid>] <path>
        Limit the size of a qgroup to *size* or no limit in the btrfs filesystem
        identified by *path*.

        If *qgroupid* is not given, qgroup of the subvolume identified by *path*
        is used if possible.

        ``Options``

        -c
                limit amount of data after compression. This is the default, it is currently not
                possible to turn off this option.
        -e
                limit space exclusively assigned to this qgroup.

remove <src> <dst> <path>
        Remove the relationship between child qgroup *src* and parent qgroup *dst* in
        the btrfs filesystem identified by *path*.

        ``Options``

        --rescan
                (default since: 4.19) Automatically schedule quota rescan if the removed qgroup
                relation would lead to quota inconsistency. See *QUOTA RESCAN* for more
                information.
        --no-rescan
                Explicitly ask not to do a rescan, even if the removal will make the quotas
                inconsistent. This may be useful for repeated calls where the rescan would add
                unnecessary overhead.

show [options] <path>
        Show all qgroups in the btrfs filesystem identified by <path>.

        ``Options``

        -p
                print parent qgroup id.
        -c
                print child qgroup id.
        -r
                print limit of referenced size of qgroup.
        -e
                print limit of exclusive size of qgroup.
        -F
                list all qgroups which impact the given path(include ancestral qgroups)
        -f
                list all qgroups which impact the given path(exclude ancestral qgroups)
        --raw
                raw numbers in bytes, without the *B* suffix.
        --human-readable
                print human friendly numbers, base 1024, this is the default
        --iec
                select the 1024 base for the following options, according to the IEC standard.
        --si
                select the 1000 base for the following options, according to the SI standard.
        --kbytes
                show sizes in KiB, or kB with --si.
        --mbytes
                show sizes in MiB, or MB with --si.
        --gbytes
                show sizes in GiB, or GB with --si.
        --tbytes
                show sizes in TiB, or TB with --si.

        --sort=[\+/-]<attr>[,[+/-]<attr>]...
                list qgroups in order of <attr>.

                <attr> can be one or more of qgroupid,rfer,excl,max_rfer,max_excl.

                Prefix *+* means ascending order and *-* means descending order of *attr*.
                If no prefix is given, use ascending order by default.

                If multiple *attr* values are given, use comma to separate.

        --sync
                To retrieve information after updating the state of qgroups,
                force sync of the filesystem identified by *path* before getting information.

SPECIAL PATHS
-------------
For `btrfs qgroup show` subcommand, the ``path`` column may has some special
strings:

`<toplevel>`
	The toplevel subvolume

`<under deletion>`
        The subvolume has been deleted (it's directory removed), but the
        subvolume metadata not not yet fully cleaned.

`<squota space holder>`
	For simple quota mode only.
	By its design, a fully deleted subvolume may still have accounting on
	it, so even the subvolume is gone, the numbers are still here for future
	accounting.

`<stale>`
	The qgroup has no corresponding subvolume anymore, and the qgroup
	can be cleaned up under most cases.
	The only exception is that, if the qgroup numbers are inconsistent and
	the qgroup numbers are not all zeros, some older kernels may refuse to
	delete such qgroups until a full rescan.

QUOTA RESCAN
------------

The rescan reads all extent sharing metadata and updates the respective qgroups
accordingly.

The information consists of bytes owned exclusively (*excl*) or shared/referred
to (*rfer*). There's no explicit information about which extents are shared or
owned exclusively.  This means when qgroup relationship changes, extent owners
change and qgroup numbers are no longer consistent unless we do a full rescan.

However there are cases where we can avoid a full rescan, if a subvolume whose
*rfer* number equals its *excl* number, which means all bytes are exclusively
owned, then assigning/removing this subvolume only needs to add/subtract *rfer*
number from its parent qgroup. This can speed up the rescan.

EXAMPLES
--------

Make a parent group that has two quota group children
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Given the following filesystem mounted at :file:`/mnt/my-vault`

.. code-block:: none

        Label: none  uuid: 60d2ab3b-941a-4f22-8d1a-315f329797b2
               Total devices 1 FS bytes used 128.00KiB
               devid    1 size 5.00GiB used 536.00MiB path /dev/vdb

Enable quota and create subvolumes.  Check subvolume ids.

.. code-block:: bash

        $ cd /mnt/my-vault
        $ btrfs quota enable .
        $ btrfs subvolume create a
        $ btrfs subvolume create b
        $ btrfs subvolume list .

        ID 261 gen 61 top level 5 path a
        ID 262 gen 62 top level 5 path b

Create qgroup and set limit to 10MiB.

.. code-block:: bash

        $ btrfs qgroup create 1/100 .
        $ btrfs qgroup limit 10M 1/100 .
        $ btrfs qgroup assign 0/261 1/100 .
        $ btrfs qgroup assign 0/262 1/100 .

And check qgroups.

.. code-block:: bash

        $ btrfs qgroup show .

        qgroupid         rfer         excl
        --------         ----         ----
        0/5          16.00KiB     16.00KiB
        0/261        16.00KiB     16.00KiB
        0/262        16.00KiB     16.00KiB
        1/100        32.00KiB     32.00KiB


EXIT STATUS
-----------

**btrfs qgroup** 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 documentation at
`https://btrfs.readthedocs.io <https://btrfs.readthedocs.io>`_.

SEE ALSO
--------

:doc:`btrfs-quota`,
:doc:`btrfs-subvolume`,
:doc:`mkfs.btrfs`