path: root/doc/cephfs/fs-volumes.rst

.. _fs-volumes-and-subvolumes:

FS volumes and subvolumes
=========================

The volumes module of the :term:`Ceph Manager` daemon (ceph-mgr) provides a
single source of truth for CephFS exports. The OpenStack shared file system
service (manila_) and the Ceph Container Storage Interface (CSI_) storage
administrators use the common CLI provided by the ceph-mgr ``volumes`` module
to manage CephFS exports.

The ceph-mgr ``volumes`` module implements the following file system export
abstractions:

* FS volumes, an abstraction for CephFS file systems

* FS subvolume groups, an abstraction for a directory level higher than FS
  subvolumes. Used to effect policies (e.g., :doc:`/cephfs/file-layouts`)
  across a set of subvolumes

* FS subvolumes, an abstraction for independent CephFS directory trees

Possible use-cases for the export abstractions:

* FS subvolumes used as Manila shares or CSI volumes

* FS-subvolume groups used as Manila share groups

Requirements
------------

* Nautilus (14.2.x) or later Ceph release

* Cephx client user (see :doc:`/rados/operations/user-management`) with
  at least the following capabilities::

    mon 'allow r'
    mgr 'allow rw'

FS Volumes
----------

Create a volume by running the following command:

.. prompt:: bash #

   ceph fs volume create <vol_name> [placement]

This creates a CephFS file system and its data and metadata pools. This command
can also deploy MDS daemons for the filesystem using a Ceph Manager orchestrator
module (for example Rook). See :doc:`/mgr/orchestrator`.

``<vol_name>`` is the volume name (an arbitrary string). ``[placement]`` is an
optional string that specifies the :ref:`orchestrator-cli-placement-spec` for
the MDS. See also :ref:`orchestrator-cli-cephfs` for more examples on
placement.

.. note:: Specifying placement via a YAML file is not supported through the
          volume interface.

To remove a volume, run the following command:

.. prompt:: bash #

   ceph fs volume rm <vol_name> [--yes-i-really-mean-it]

This command removes a file system and its data and metadata pools. It also
tries to remove MDS daemons using the enabled Ceph Manager orchestrator module.

.. note:: After volume deletion, we recommend restarting `ceph-mgr` if a new
   file system is created on the same cluster and the subvolume interface is
   being used. See https://tracker.ceph.com/issues/49605#note-5 for more
   details.

.. note:: If the snap-schedule Ceph Manager module is being used for a volume
   and the volume is deleted, then the snap-schedule Ceph Manager module will
   continue to hold references to the old pools. This will lead to the
   snap-schedule Ceph Manager module faulting and logging errors. To remedy
   this scenario, we recommend that the snap-schedule Ceph Manager module
   be restarted after volume deletion. If the faults still persist, then we
   recommend restarting `ceph-mgr`.

List volumes by running the following command:

.. prompt:: bash #

   ceph fs volume ls

Rename a volume by running the following command:

.. prompt:: bash #

   ceph fs volume rename <vol_name> <new_vol_name> [--yes-i-really-mean-it]

Renaming a volume can be an expensive operation that requires the following:

- Renaming the orchestrator-managed MDS service to match the
  ``<new_vol_name>``.  This involves launching a MDS service with
  ``<new_vol_name>`` and bringing down the MDS service with ``<vol_name>``.
- Renaming the file system from ``<vol_name>`` to ``<new_vol_name>``.
- Changing the application tags on the data and metadata pools of the file
  system to ``<new_vol_name>``.
- Renaming the metadata and data pools of the file system.

The CephX IDs that are authorized for ``<vol_name>`` must be reauthorized for
``<new_vol_name>``. Any ongoing operations of the clients that are using these
IDs may be disrupted. Ensure that mirroring is disabled on the volume.

To fetch the information of a CephFS volume, run the following command:

.. prompt:: bash #

   ceph fs volume info vol_name [--human_readable]

The ``--human_readable`` flag shows used and available pool capacities in
KB/MB/GB.

The output format is JSON and contains fields as follows:

* ``pools``: Attributes of data and metadata pools
        * ``avail``: The amount of free space available in bytes
        * ``used``: The amount of storage consumed in bytes
        * ``name``: Name of the pool
* ``mon_addrs``: List of Ceph monitor addresses
* ``used_size``: Current used size of the CephFS volume in bytes
* ``pending_subvolume_deletions``: Number of subvolumes pending deletion

Sample output of the ``volume info`` command:

.. prompt:: bash #

   ceph fs volume info vol_name

::

    {
        "mon_addrs": [
            "192.168.1.7:40977"
        ],
        "pending_subvolume_deletions": 0,
        "pools": {
            "data": [
                {
                    "avail": 106288709632,
                    "name": "cephfs.vol_name.data",
                    "used": 4096
                }
            ],
            "metadata": [
                {
                    "avail": 106288709632,
                    "name": "cephfs.vol_name.meta",
                    "used": 155648
                }
            ]
        },
        "used_size": 0
    }

FS Subvolume groups
-------------------

Create a subvolume group by running the following command:

.. prompt:: bash #

   ceph fs subvolumegroup create <vol_name> <group_name> [--size <size_in_bytes>] [--pool_layout <data_pool_name>] [--uid <uid>] [--gid <gid>] [--mode <octal_mode>]

The command succeeds even if the subvolume group already exists.

When you create a subvolume group, you can specify its data pool layout (see
:doc:`/cephfs/file-layouts`), uid, gid, file mode in octal numerals, and
size in bytes. The size of the subvolume group is specified by setting
a quota on it (see :doc:`/cephfs/quota`). By default, the subvolume group
is created with octal file mode ``755``, uid ``0``, gid ``0`` and the data pool
layout of its parent directory.

Remove a subvolume group by running a command of the following form:

.. prompt:: bash #

   ceph fs subvolumegroup rm <vol_name> <group_name> [--force]

The removal of a subvolume group fails if the subvolume group is not empty or
is non-existent. The ``--force`` flag allows the command to succeed when its
argument is a non-existent subvolume group.

Fetch the absolute path of a subvolume group by running a command of the
following form:

.. prompt:: bash #

   ceph fs subvolumegroup getpath <vol_name> <group_name>

List subvolume groups by running a command of the following form:

.. prompt:: bash #

   ceph fs subvolumegroup ls <vol_name>

.. note:: Subvolume group snapshot feature is no longer supported in mainline CephFS (existing group
          snapshots can still be listed and deleted)

Fetch the metadata of a subvolume group by running a command of the following
form:

.. prompt:: bash #

   ceph fs subvolumegroup info <vol_name> <group_name>

The output format is JSON and contains fields as follows:

* ``atime``: access time of the subvolume group path in the format ``YYYY-MM-DD
  HH:MM:SS``
* ``mtime``: time of the most recent modification of the subvolume group path
  in the format
  ``YYYY-MM-DD HH:MM:SS``
* ``ctime``: time of the most recent change of the subvolume group path in the
  format ``YYYY-MM-DD HH:MM:SS``
* ``uid``: uid of the subvolume group path
* ``gid``: gid of the subvolume group path
* ``mode``: mode of the subvolume group path
* ``mon_addrs``: list of monitor addresses
* ``bytes_pcent``: quota used in percentage if quota is set, else displays "undefined"
* ``bytes_quota``: quota size in bytes if quota is set, else displays "infinite"
* ``bytes_used``: current used size of the subvolume group in bytes
* ``created_at``: creation time of the subvolume group in the format "YYYY-MM-DD HH:MM:SS"
* ``data_pool``: data pool to which the subvolume group belongs

Check for the presence of a given subvolume group by running a command of the
following form:

.. prompt:: bash #

   ceph fs subvolumegroup exist <vol_name>

The ``exist`` command outputs:

* ``subvolumegroup exists``: if any subvolumegroup is present
* ``no subvolumegroup exists``: if no subvolumegroup is present

.. note:: This command checks for the presence of custom groups and not
   presence of the default one. A subvolumegroup-existence check alone is not
   sufficient to validate the emptiness of the volume. Subvolume existence must
   also be checked, as there might be subvolumes in the default group.

Resize a subvolume group by running a command of the following form:

.. prompt:: bash #

   ceph fs subvolumegroup resize <vol_name> <group_name> <new_size> [--no_shrink]

This command resizes the subvolume group quota, using the size specified by
``new_size``.  The ``--no_shrink`` flag prevents the subvolume group from
shrinking below the current used size.

The subvolume group may be resized to an infinite size by passing ``inf`` or
``infinite`` as the ``new_size``.

Remove a snapshot of a subvolume group by running a command of the following
form:

.. prompt:: bash #

   ceph fs subvolumegroup snapshot rm <vol_name> <group_name> <snap_name> [--force]

Supplying the ``--force`` flag allows the command to succeed when it would
otherwise fail due to the nonexistence of the snapshot.

List snapshots of a subvolume group by running a command of the following form:

.. prompt:: bash #

   ceph fs subvolumegroup snapshot ls <vol_name> <group_name>


FS Subvolumes
-------------

Creating a subvolume
~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to create a subvolume:

.. prompt:: bash #

   ceph fs subvolume create <vol_name> <subvol_name> [--size <size_in_bytes>] [--group_name <subvol_group_name>] [--pool_layout <data_pool_name>] [--uid <uid>] [--gid <gid>] [--mode <octal_mode>] [--namespace-isolated] [--earmark <earmark>]


The command succeeds even if the subvolume already exists.

When creating a subvolume, you can specify its subvolume group, data pool
layout, uid, gid, file mode in octal numerals, and size in bytes. The size of
the subvolume is specified by setting a quota on it (see :doc:`/cephfs/quota`).
The subvolume can be created in a separate RADOS namespace by specifying the
``--namespace-isolated`` option. By default, a subvolume is created within the
default subvolume group with an octal file mode of ``755``, a uid of its
subvolume group, a gid of its subvolume group, a data pool layout of its parent
directory, and no size limit.
You can also assign an earmark to a subvolume using the ``--earmark`` option.
The earmark is a unique identifier that tags the subvolume for specific purposes,
such as NFS or SMB services. By default, no earmark is set, allowing for flexible
assignment based on administrative needs. An empty string ("") can be used to remove
any existing earmark from a subvolume.

The earmarking mechanism ensures that subvolumes are correctly tagged and managed,
helping to avoid conflicts and ensuring that each subvolume is associated
with the intended service or use case.

Valid Earmarks
~~~~~~~~~~~~~~~~~~~~

- **For NFS:**
   - The valid earmark format is the top-level scope: ``'nfs'``.

- **For SMB:**
   - The valid earmark formats are:
      - The top-level scope: ``'smb'``.
      - The top-level scope with an intra-module level scope: ``'smb.cluster.{cluster_id}'``, where ``cluster_id`` is a short string uniquely identifying the cluster.
      - Example without intra-module scope: ``smb``
      - Example with intra-module scope: ``smb.cluster.cluster_1``

.. note:: If you are changing an earmark from one scope to another (e.g., from nfs to smb or vice versa),
   be aware that user permissions and ACLs associated with the previous scope might still apply. Ensure that
   any necessary permissions are updated as needed to maintain proper access control.


Removing a subvolume
~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to remove a subvolume:

.. prompt:: bash #

   ceph fs subvolume rm <vol_name> <subvol_name> [--group_name <subvol_group_name>] [--force] [--retain-snapshots]

This command removes the subvolume and its contents. This is done in two steps.
First, the subvolume is moved to a trash folder. Second, the contents of that
trash folder are purged asynchronously.

Subvolume removal fails if the subvolume has snapshots or is non-existent.  The
``--force`` flag allows the "non-existent subvolume remove" command to succeed.

To remove a subvolume while retaining snapshots of the subvolume, use the
``--retain-snapshots`` flag. If snapshots associated with a given subvolume are
retained, then the subvolume is considered empty for all operations that do not
involve the retained snapshots.

.. note:: Snapshot-retained subvolumes can be recreated using ``ceph fs
   subvolume create``.

.. note:: Retained snapshots can be used as clone sources for recreating the
   subvolume or for cloning to a newer subvolume.

Resizing a subvolume
~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to resize a subvolume:

.. prompt:: bash #

   ceph fs subvolume resize <vol_name> <subvol_name> <new_size> [--group_name <subvol_group_name>] [--no_shrink]

This command resizes the subvolume quota, using the size specified by
``new_size``.  The ``--no_shrink`` flag prevents the subvolume from shrinking
below the current "used size" of the subvolume.

The subvolume can be resized to an unlimited (but sparse) logical size by
passing ``inf`` or ``infinite`` as ``<new_size>``.

Authorizing CephX auth IDs
~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to authorize CephX auth IDs. This provides
the read/read-write access to file system subvolumes:

.. prompt:: bash #

   ceph fs subvolume authorize <vol_name> <sub_name> <auth_id> [--group_name=<group_name>] [--access_level=<access_level>]

The ``<access_level>`` option takes either ``r`` or ``rw`` as a value.

De-authorizing CephX auth IDs
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to deauthorize CephX auth IDs. This removes
the read/read-write access to file system subvolumes:

.. prompt:: bash #

   ceph fs subvolume deauthorize <vol_name> <sub_name> <auth_id> [--group_name=<group_name>]

Listing CephX auth IDs
~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to list CephX auth IDs authorized to access
the file system subvolume:

.. prompt:: bash #

   ceph fs subvolume authorized_list <vol_name> <sub_name> [--group_name=<group_name>]

Evicting File System Clients (Auth ID)
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to evict file system clients based on the
auth ID and the subvolume mounted:

.. prompt:: bash #

   ceph fs subvolume evict <vol_name> <sub_name> <auth_id> [--group_name=<group_name>]

Fetching the Absolute Path of a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to fetch the absolute path of a subvolume:

.. prompt:: bash #

   ceph fs subvolume getpath <vol_name> <subvol_name> [--group_name <subvol_group_name>]

Fetching a Subvolume's Information
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to fetch a subvolume's information:

.. prompt:: bash #

   ceph fs subvolume info <vol_name> <subvol_name> [--group_name <subvol_group_name>]

The output format is JSON and contains the following fields.

* ``atime``: access time of the subvolume path in the format ``YYYY-MM-DD
  HH:MM:SS``
* ``mtime``: modification time of the subvolume path in the format ``YYYY-MM-DD
  HH:MM:SS``
* ``ctime``: change time of the subvolume path in the format ``YYYY-MM-DD
  HH:MM:SS``
* ``uid``: uid of the subvolume path
* ``gid``: gid of the subvolume path
* ``mode``: mode of the subvolume path
* ``mon_addrs``: list of monitor addresses
* ``bytes_pcent``: quota used in percentage if quota is set; else displays
  ``undefined``
* ``bytes_quota``: quota size in bytes if quota is set; else displays
  ``infinite``
* ``bytes_used``: current used size of the subvolume in bytes
* ``created_at``: creation time of the subvolume in the format ``YYYY-MM-DD
  HH:MM:SS``
* ``data_pool``: data pool to which the subvolume belongs
* ``path``: absolute path of a subvolume
* ``type``: subvolume type, indicating whether it is ``clone`` or ``subvolume``
* ``pool_namespace``: RADOS namespace of the subvolume
* ``features``: features supported by the subvolume
* ``state``: current state of the subvolume
* ``earmark``: earmark of the subvolume

If a subvolume has been removed but its snapshots have been retained, the
output contains only the following fields.

* ``type``: subvolume type indicating whether it is ``clone`` or ``subvolume``
* ``features``: features supported by the subvolume
* ``state``: current state of the subvolume

A subvolume's ``features`` are based on the internal version of the subvolume
and are a subset of the following:

* ``snapshot-clone``: supports cloning using a subvolume's snapshot as the
  source
* ``snapshot-autoprotect``: supports automatically protecting snapshots from
  deletion if they are active clone sources 
* ``snapshot-retention``: supports removing subvolume contents, retaining any
  existing snapshots

A subvolume's ``state`` is based on the current state of the subvolume and
contains one of the following values.

* ``complete``: subvolume is ready for all operations
* ``snapshot-retained``: subvolume is removed but its snapshots are retained

Listing Subvolumes
~~~~~~~~~~~~~~~~~~

Use a command of the following form to list subvolumes:

.. prompt:: bash #

   ceph fs subvolume ls <vol_name> [--group_name <subvol_group_name>]

.. note:: Subvolumes that have been removed but have snapshots retained, are
   also listed.

Checking for the Presence of a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to check for the presence of a given
subvolume:

.. prompt:: bash #

   ceph fs subvolume exist <vol_name> [--group_name <subvol_group_name>]

These are the possible results of the ``exist`` command:

* ``subvolume exists``: if any subvolume of given ``group_name`` is present
* ``no subvolume exists``: if no subvolume of given ``group_name`` is present

Setting Custom Metadata On a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to set custom metadata on the subvolume as
a key-value pair:

.. prompt:: bash #

   ceph fs subvolume metadata set <vol_name> <subvol_name> <key_name> <value> [--group_name <subvol_group_name>]

.. note:: If the key_name already exists then the old value will get replaced by the new value.

.. note:: ``key_name`` and ``value`` should be a string of ASCII characters (as
   specified in Python's ``string.printable``). ``key_name`` is
   case-insensitive and always stored in lower case.

.. note:: Custom metadata on a subvolume is not preserved when snapshotting the
   subvolume, and is therefore also not preserved when cloning the subvolume
   snapshot.

Getting The Custom Metadata Set of a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to get the custom metadata set on the
subvolume using the metadata key:

.. prompt:: bash #

   ceph fs subvolume metadata get <vol_name> <subvol_name> <key_name> [--group_name <subvol_group_name>]

Listing The Custom Metadata Set of a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to list custom metadata (key-value pairs)
set on the subvolume:

.. prompt:: bash #

   ceph fs subvolume metadata ls <vol_name> <subvol_name> [--group_name <subvol_group_name>]

Removing a Custom Metadata Set from a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to remove custom metadata set on the
subvolume using the metadata key:

.. prompt:: bash #

   ceph fs subvolume metadata rm <vol_name> <subvol_name> <key_name> [--group_name <subvol_group_name>] [--force]

Using the ``--force`` flag allows the command to succeed when it would
otherwise fail (if the metadata key did not exist).

Getting earmark of a subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to get the earmark of a subvolume:

.. prompt:: bash #

   ceph fs subvolume earmark get <vol_name> <subvol_name> [--group_name <subvol_group_name>]

Setting earmark of a subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to set the earmark of a subvolume:

.. prompt:: bash #

   ceph fs subvolume earmark set <vol_name> <subvol_name> [--group_name <subvol_group_name>] <earmark>

Removing earmark of a subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to remove the earmark of a subvolume:

.. prompt:: bash #

   ceph fs subvolume earmark rm <vol_name> <subvol_name> [--group_name <subvol_group_name>]

Creating a Snapshot of a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to create a snapshot of a subvolume:

.. prompt:: bash #

   ceph fs subvolume snapshot create <vol_name> <subvol_name> <snap_name> [--group_name <subvol_group_name>]


Removing a Snapshot of a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to remove a snapshot of a subvolume:

.. prompt:: bash #

   ceph fs subvolume snapshot rm <vol_name> <subvol_name> <snap_name> [--group_name <subvol_group_name>] [--force]

Using the ``--force`` flag allows the command to succeed when it would
otherwise fail (if the snapshot did not exist).

.. note:: if the last snapshot within a snapshot retained subvolume is removed, the subvolume is also removed

Listing the Snapshots of a Subvolume
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following from to list the snapshots of a subvolume:

.. prompt:: bash #

   ceph fs subvolume snapshot ls <vol_name> <subvol_name> [--group_name <subvol_group_name>]

Fetching a Snapshot's Information
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to fetch a snapshot's information:

.. prompt:: bash #

   ceph fs subvolume snapshot info <vol_name> <subvol_name> <snap_name> [--group_name <subvol_group_name>]

The output format is JSON and contains the following fields.

* ``created_at``: creation time of the snapshot in the format ``YYYY-MM-DD
  HH:MM:SS:ffffff``
* ``data_pool``: data pool to which the snapshot belongs
* ``has_pending_clones``: ``yes`` if snapshot clone is in progress, otherwise
  ``no``
* ``pending_clones``: list of in-progress or pending clones and their target
  groups if any exist; otherwise this field is not shown
* ``orphan_clones_count``: count of orphan clones if the snapshot has orphan
  clones, otherwise this field is not shown

Sample output when snapshot clones are in progress or pending:

.. prompt:: bash #

   ceph fs subvolume snapshot info cephfs subvol snap

::

    {
        "created_at": "2022-06-14 13:54:58.618769",
        "data_pool": "cephfs.cephfs.data",
        "has_pending_clones": "yes",
        "pending_clones": [
            {
                "name": "clone_1",
                "target_group": "target_subvol_group"
            },
            {
                "name": "clone_2"
            },
            {
                "name": "clone_3",
                "target_group": "target_subvol_group"
            }
        ]
    }

Sample output when no snapshot clone is in progress or pending:

.. prompt:: bash #

   ceph fs subvolume snapshot info cephfs subvol snap

::

    {
        "created_at": "2022-06-14 13:54:58.618769",
        "data_pool": "cephfs.cephfs.data",
        "has_pending_clones": "no"
    }

Setting Custom Key-Value Pair Metadata on a Snapshot
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to set custom key-value metadata on the
snapshot:

.. prompt:: bash #

   ceph fs subvolume snapshot metadata set <vol_name> <subvol_name> <snap_name> <key_name> <value> [--group_name <subvol_group_name>]

.. note:: If the ``key_name`` already exists then the old value will get replaced
   by the new value.

.. note:: The ``key_name`` and value should be a strings of ASCII characters
   (as specified in Python's ``string.printable``). The ``key_name`` is
   case-insensitive and always stored in lowercase.

.. note:: Custom metadata on a snapshot is not preserved when snapshotting the
   subvolume, and is therefore not preserved when cloning the subvolume
   snapshot.

Getting Custom Metadata That Has Been Set on a Snapshot
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to get custom metadata that has been set on
the snapshot using the metadata key:

.. prompt:: bash #

   ceph fs subvolume snapshot metadata get <vol_name> <subvol_name> <snap_name> <key_name> [--group_name <subvol_group_name>]

Listing Custom Metadata that has been Set on a Snapshot
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following from to list custom metadata (key-value pairs)
set on the snapshot:

.. prompt:: bash #

   ceph fs subvolume snapshot metadata ls <vol_name> <subvol_name> <snap_name> [--group_name <subvol_group_name>]

Removing Custom Metadata from a Snapshot
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use a command of the following form to remove custom metadata set on the
snapshot using the metadata key:

.. prompt:: bash #

   ceph fs subvolume snapshot metadata rm <vol_name> <subvol_name> <snap_name> <key_name> [--group_name <subvol_group_name>] [--force]

Using the ``--force`` flag allows the command to succeed when it would otherwise
fail (if the metadata key did not exist).

Cloning Snapshots
-----------------

Subvolumes can be created by cloning subvolume snapshots. Cloning is an
asynchronous operation that copies data from a snapshot to a subvolume. Because
cloning is an operation that involves bulk copying, it is slow for
very large data sets.

.. note:: Removing a snapshot (source subvolume) fails when there are
   pending or in-progress clone operations.

Protecting snapshots prior to cloning was a prerequisite in the Nautilus
release. Commands that made possible the protection and unprotection of
snapshots were introduced for this purpose. This prerequisite is being
deprecated and may be removed from a future release.

The commands being deprecated are:

.. prompt:: bash #

   ceph fs subvolume snapshot protect <vol_name> <subvol_name> <snap_name> [--group_name <subvol_group_name>]
   ceph fs subvolume snapshot unprotect <vol_name> <subvol_name> <snap_name> [--group_name <subvol_group_name>]

.. note:: Using the above commands will not result in an error, but they have no useful purpose.

.. note:: Use the ``subvolume info`` command to fetch subvolume metadata regarding supported ``features`` to help decide if protect/unprotect of snapshots is required, based on the availability of the ``snapshot-autoprotect`` feature.

Run a command of the following form to initiate a clone operation:

.. prompt:: bash #

   ceph fs subvolume snapshot clone <vol_name> <subvol_name> <snap_name> <target_subvol_name>

.. note:: ``subvolume snapshot clone`` command depends upon the above mentioned config option ``snapshot_clone_no_wait``

Run a command of the following form when a snapshot (source subvolume) is a
part of non-default group. Note that the group name needs to be specified:

.. prompt:: bash #

   ceph fs subvolume snapshot clone <vol_name> <subvol_name> <snap_name> <target_subvol_name> --group_name <subvol_group_name>

Cloned subvolumes can be a part of a different group than the source snapshot
(by default, cloned subvolumes are created in default group). Run a command of
the following form to clone to a particular group use:

.. prompt:: bash #

   ceph fs subvolume snapshot clone <vol_name> <subvol_name> <snap_name> <target_subvol_name> --target_group_name <subvol_group_name>

Pool layout can be specified when creating a cloned subvolume in a way that is
similar to specifying a pool layout when creating a subvolume. Run a command of
the following form to create a cloned subvolume with a specific pool layout:

.. prompt:: bash #

   ceph fs subvolume snapshot clone <vol_name> <subvol_name> <snap_name> <target_subvol_name> --pool_layout <pool_layout>

Run a command of the following form to check the status of a clone operation:

.. prompt:: bash #

   ceph fs clone status <vol_name> <clone_name> [--group_name <group_name>]

A clone can be in one of the following states:

#. ``pending``     : Clone operation has not started
#. ``in-progress`` : Clone operation is in progress
#. ``complete``    : Clone operation has successfully finished
#. ``failed``      : Clone operation has failed
#. ``canceled``    : Clone operation is cancelled by user

The reason for a clone failure is shown as below:

#. ``errno``     : error number
#. ``error_msg`` : failure error string

Here is an example of an ``in-progress`` clone:

.. prompt:: bash #

   ceph fs subvolume snapshot clone cephfs subvol1 snap1 clone1
   ceph fs clone status cephfs clone1

::

    {
      "status": {
        "state": "in-progress",
        "source": {
          "volume": "cephfs",
          "subvolume": "subvol1",
          "snapshot": "snap1"
        },
        "progress_report": {
          "percentage cloned": "12.24%",
          "amount cloned": "376M/3.0G",
          "files cloned": "4/6"
        }
      }
    }

A progress report is also printed in the output when clone is ``in-progress``.
Here the progress is reported only for the specific clone. For collective
progress made by all ongoing clones, a progress bar is printed at the bottom
in ouput of ``ceph status`` command::

  progress:
    3 ongoing clones - average progress is 47.569% (10s)
      [=============...............] (remaining: 11s)

If the number of clone jobs are more than cloner threads, two progress bars
are printed, one for ongoing clones (same as above) and other for all
(ongoing+pending) clones::

  progress:
    4 ongoing clones - average progress is 27.669% (15s)
      [=======.....................] (remaining: 41s)
    Total 5 clones - average progress is 41.667% (3s)
      [===========.................] (remaining: 4s)

.. note:: The ``failure`` section will be shown only if the clone's state is ``failed`` or ``cancelled``

Here is an example of a ``failed`` clone:

.. prompt:: bash #

   ceph fs subvolume snapshot clone cephfs subvol1 snap1 clone1
   ceph fs clone status cephfs clone1

::

    {
        "status": {
            "state": "failed",
            "source": {
                "volume": "cephfs",
                "subvolume": "subvol1",
                "snapshot": "snap1"
                "size": "104857600"
            },
            "failure": {
                "errno": "122",
                "errstr": "Disk quota exceeded"
            }
        }
    }

.. note::  Because ``subvol1`` is in the default group, the ``source`` object's
   ``clone status`` does not include the group name)

.. note:: Cloned subvolumes are accessible only after the clone operation has
   successfully completed.

After a successful clone operation, ``clone status`` will look like the
following:

.. prompt:: bash #

   ceph fs clone status cephfs clone1

::

    {
        "status": {
            "state": "complete"
        }
    }

If a clone operation is unsuccessful, the ``state`` value will be  ``failed``.

To retry a failed clone operation, the incomplete clone must be deleted and the
clone operation must be issued again.

Run a command of the following form to delete a partial clone:

.. prompt:: bash #

   ceph fs subvolume rm <vol_name> <clone_name> [--group_name <group_name>] --force

.. note:: Cloning synchronizes only directories, regular files and symbolic
   links. inode timestamps (access and modification times) are synchronized up
   to a second's granularity.

An ``in-progress`` or a ``pending`` clone operation may be canceled. To cancel
a clone operation use the ``clone cancel`` command:

.. prompt:: bash #

   ceph fs clone cancel <vol_name> <clone_name> [--group_name <group_name>]

On successful cancellation, the cloned subvolume is moved to the ``canceled``
state:

.. prompt:: bash #

   ceph fs subvolume snapshot clone cephfs subvol1 snap1 clone1
   ceph fs clone cancel cephfs clone1
   ceph fs clone status cephfs clone1

::

    {
        "status": {
            "state": "canceled",
            "source": {
                "volume": "cephfs",
                "subvolume": "subvol1",
                "snapshot": "snap1"
            }
        }
    }

.. note:: Delete the canceled cloned by supplying the ``--force`` option to the
   ``fs subvolume rm`` command.

Configurables
~~~~~~~~~~~~~

Configure the maximum number of concurrent clone operations. The default is 4:

.. prompt:: bash #

   ceph config set mgr mgr/volumes/max_concurrent_clones <value>

Configure the ``snapshot_clone_no_wait`` option:

The ``snapshot_clone_no_wait`` config option is used to reject clone-creation
requests when cloner threads (which can be configured using the above options,
for example, ``max_concurrent_clones``) are not available. It is enabled by
default. This means that the value is set to ``True``, but it can be configured
by using the following command:

.. prompt:: bash #

   ceph config set mgr mgr/volumes/snapshot_clone_no_wait <bool>

The current value of ``snapshot_clone_no_wait`` can be fetched by running the
following command.

.. prompt:: bash #
    
   ceph config get mgr mgr/volumes/snapshot_clone_no_wait


.. _subvol-pinning:

Pinning Subvolumes and Subvolume Groups
---------------------------------------

Subvolumes and subvolume groups may be automatically pinned to ranks according
to policies. This can distribute load across MDS ranks in predictable and
stable ways.  Review :ref:`cephfs-pinning` and :ref:`cephfs-ephemeral-pinning`
for details on how pinning works.

Run a command of the following form to configure pinning for subvolume groups:

.. prompt:: bash #

   ceph fs subvolumegroup pin <vol_name> <group_name> <pin_type> <pin_setting>

Run a command of the following form to configure pinning for subvolumes:

.. prompt:: bash #

   ceph fs subvolume pin <vol_name> <group_name> <pin_type> <pin_setting>

Under most circumstances, you will want to set subvolume group pins. The
``pin_type`` may be ``export``, ``distributed``, or ``random``. The
``pin_setting`` corresponds to the extended attributed "value" as in the
pinning documentation referenced above.

Here is an example of setting a distributed pinning strategy on a subvolume
group:

.. prompt:: bash #

   ceph fs subvolumegroup pin cephfilesystem-a csi distributed 1

This enables distributed subtree partitioning policy for the "csi" subvolume
group. This will cause every subvolume within the group to be automatically
pinned to one of the available ranks on the file system.

Subvolume quiesce
-----------------

.. note:: The information in this section applies only to Squid and later
   releases of Ceph.

CephFS snapshots do not provide strong-consistency guarantees in cases involving writes
performed by multiple clients, which makes consistent backups and disaster recovery a serious
challenge for distributed applications. Even in a case where an application uses
file system flushes to synchronize checkpoints across its distributed components, there is
no guarantee that all acknowledged writes will be part of a given snapshot.

The subvolume quiesce feature has been developed to provide enterprise-level consistency guarantees
for multi-client applications that work with one or more subvolumes. The feature makes it possible to pause IO
to a set of subvolumes of a given volume (file system). Enforcing such a pause across all clients makes
it possible to guarantee that any persistent checkpoints reached by the application before the pause
will be recoverable from the snapshots made during the pause.

The `volumes` plugin provides a CLI to initiate and await the pause for a set of subvolumes.
This pause is called a `quiesce`, which is also used as the command name:

.. prompt:: bash $ auto

  $ ceph fs quiesce <vol_name> --set-id myset1 <[group_name/]sub_name...> --await
  # perform actions while the IO pause is active, like taking snapshots
  $ ceph fs quiesce <vol_name> --set-id myset1 --release --await
  # if successful, all members of the set were confirmed as still paused and released

The ``fs quiesce`` functionality is based on a lower level ``quiesce db`` service provided by the MDS
daemons, which operates at a file system path granularity. 
The `volumes` plugin merely maps the subvolume names to their corresponding paths on the given file system
and then issues the corresponding ``quiesce db`` command to the MDS. You can learn more about the low-level service
in the developer guides.

Operations
~~~~~~~~~~

The quiesce can be requested for a set of one or more subvolumes (i.e. paths in a filesystem).
This set is referred to as `quiesce set`. Every quiesce set is identified by a unique `set id`.
A quiesce set can be manipulated in the following ways:

* **include** one or more subvolumes - quiesce set members
* **exclude** one or more members
* **cancel** the set, asynchronously aborting the pause on all its current members
* **release** the set, requesting the end of the pause from all members and expecting an ack from all clients
* **query** the current state of a set by id or all active sets or all known sets
* **cancel all** active sets in case an immediate resume of IO is required.

The operations listed above are non-blocking: they attempt the intended modification 
and return with an up to date version of the target set, whether the operation was successful or not. 
The set may change states as a result of the modification, and the version that's returned in the response 
is guaranteed to be in a state consistent with this and potentialy other successful operations from 
the same control loop batch.

Some set states are `awaitable`. We will discuss those below, but for now it's important to mention that
any of the commands above can be amended with an **await** modifier, which will cause them to block
on the set after applying their intended modification, as long as the resulting set state is `awaitable`.
Such a command will block until the set reaches the awaited state, gets modified by another command,
or transitions into another state. The return code will unambiguously identify the exit condition, and
the contents of the response will always carry the latest known set state.

.. image:: quiesce-set-states.svg

`Awaitable` states on the diagram are marked with ``(a)`` or ``(A)``. Blocking versions of the operations
will pend while the set is in an ``(a)`` state and will complete with success if it reaches an ``(A)`` state.
If the set is already at an ``(A)`` state, the operation completes immediately with a success. 

Most of the operations require a set-id. The exceptions are:

* creation of a new set without specifying a set id,
* query of active or all known sets, and
* the cancel all

Creating a new set is achieved by including member(s) via the `include` or `reset` commands.
It's possible to specify a set id, and if it's a new id then the set will be created
with the specified member(s) in the `QUIESCING` state. When no set id is specified while including
or resetting members, then a new set with a unique set id is created. The set id will be known
to the caller by inspecting the output

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 sub1 --set-id=unique-id
  {
      "epoch": 3,
      "set_version": 1,
      "sets": {
          "unique-id": {
              "version": 1,
              "age_ref": 0.0,
              "state": {
                  "name": "TIMEDOUT",
                  "age": 0.0
              },
              "timeout": 0.0,
              "expiration": 0.0,
              "members": {
                  "file:/volumes/_nogroup/sub1/b1fcce76-3418-42dd-aa76-f9076d047dd3": {
                      "excluded": false,
                      "state": {
                          "name": "QUIESCING",
                          "age": 0.0
                      }
                  }
              }
          }
      }
  }

The output contains the set we just created successfully, however it's already `TIMEDOUT`. 
This is expected, since we have not specified the timeout for this quiesce,
and we can see in the output that it was initialized to 0 by default, along with the expiration.

Timeouts
~~~~~~~~

The two timeout parameters, `timeout` and `expiration`, are the main guards against 
accidentally causing a DOS condition for our application. Any command to an active set
may carry the ``--timeout`` or ``--expiration`` arguments to update these values for the set.
If present, the values will be applied before the action this command requests.

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 --set-id=unique-id --timeout=10 > /dev/null
  Error EPERM:  

It's too late for our ``unique-id`` set, as it's in a terminal state. No changes are allowed
to sets that are in their terminal states, i.e. inactive. Let's create a new set:

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 sub1 --timeout 60
  {
      "epoch": 3,
      "set_version": 2,
      "sets": {
          "8988b419": {
              "version": 2,
              "age_ref": 0.0,
              "state": {
                  "name": "QUIESCING",
                  "age": 0.0
              },
              "timeout": 60.0,
              "expiration": 0.0,
              "members": {
                  "file:/volumes/_nogroup/sub1/b1fcce76-3418-42dd-aa76-f9076d047dd3": {
                      "excluded": false,
                      "state": {
                          "name": "QUIESCING",
                          "age": 0.0
                      }
                  }
              }
          }
      }
  }

This time, we haven't specified a set id, so the system created a new one. We see its id
in the output, it's ``8988b419``. The command was a success and we see that 
this time the set is `QUIESCING`. At this point, we can add more members to the set

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 --set-id 8988b419 --include sub2 sub3
  {
      "epoch": 3,
      "set_version": 3,
      "sets": {
          "8988b419": {
              "version": 3,
              "age_ref": 0.0,
              "state": {
                  "name": "QUIESCING",
                  "age": 30.7
              },
              "timeout": 60.0,
              "expiration": 0.0,
              "members": {
                  "file:/volumes/_nogroup/sub1/b1fcce76-3418-42dd-aa76-f9076d047dd3": {
                      "excluded": false,
                      "state": {
                          "name": "QUIESCING",
                          "age": 30.7
                      }
                  },
                  "file:/volumes/_nogroup/sub2/bc8f770e-7a43-48f3-aa26-d6d76ef98d3e": {
                      "excluded": false,
                      "state": {
                          "name": "QUIESCING",
                          "age": 0.0
                      }
                  },
                  "file:/volumes/_nogroup/sub3/24c4b57b-e249-4b89-b4fa-7a810edcd35b": {
                      "excluded": false,
                      "state": {
                          "name": "QUIESCING",
                          "age": 0.0
                      }
                  }
              }
          }
      }
  }

The ``--include`` bit is optional, as if no operation is given while members are provided, 
then "include" is assumed.

As we have seen, the timeout argument specifies how much time we are ready to give the system
to reach the `QUIESCED` state on the set. However, since new members can be added to an
active set at any time, it wouldn't be fair to measure the timeout from the set creation time.
Hence, the timeout is tracked per member: every member has `timeout` seconds to quiesce,
and if any one takes longer than that, the whole set is marked as `TIMEDOUT` and the pause is released.

Once the set is in the `QUIESCED` state, it will begin its expiration timer. This timer is tracked
per set as a whole, not per members. Once the `expiration` seconds elapse, the set will transition
into an `EXPIRED` state, unless it was actively released or canceled by a dedicated operation.

It's possible to add new members to a `QUIESCED` set. In this case, it will transition back to `QUIESCING`,
and the new member(s) will have their own timeout to quiesce. If they succeed, then the set will
again be `QUIESCED` and the expiration timer will restart. 

.. warning:: 
  * The `expiration timer` doesn't apply when a set is `QUIESCING`; it is reset to the
    value of the `expiration` property when the **set** becomes `QUIESCED`
  * The `timeout` doesn't apply to **members** that are `QUIESCED`

Awaiting
~~~~~~~~

Note that the commands above are all non-blocking. If we want to wait for the quiesce set
to reach the `QUIESCED` state, we should await it at some point. ``--await`` can be given
along with other arguments to let the system know our intention.

There are two types of await: `quiesce await` and `release await`. The former is the default,
and the latter can only be achieved with ``--release`` present in the argument list.
To avoid confision, it is not permitted to issue a `quiesce await` when the set is not `QUIESCING`.
Trying to ``--release`` a set that is not `QUIESCED` is an ``EPERM`` error as well, regardless
of whether await is requested alongside. However, it's not an error to `release await`
an already released set, or to `quiesce await` a `QUIESCED` one - those are successful no-ops.

Since a set is awaited after the application of the ``--await``-augmented command, the await operation
may mask a successful result with its own error. A good example is trying to cancel-await a set:

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 --set-id set1 --cancel --await
  {
      // ...
      "sets": {
          "set1": {
              // ...
              "state": {
                  "name": "CANCELED",
                  "age": 0
              },
              // ...
          }
      }
  }
  Error EPERM: 

Although ``--cancel`` will succeed syncrhonously for a set in an active state, awaiting a canceled
set is not permitted, hence this call will result in an ``EPERM``. This is deliberately different from 
returning a ``EINVAL`` error, denoting an error on the user's side, to simplify the system's behavior
when ``--await`` is requested. As a result, it's also a simpler model for the user to work with.

When awaiting, one may specify a maximum duration that they would like this await request to block for,
orthogonally to the two intrinsic set timeouts discussed above. If the target awaited state isn't reached
within the specified duration, then ``EINPROGRESS`` is returned. For that, one should use the argument
``--await-for=<seconds>``. One could think of ``--await`` as equivalent to ``--await-for=Infinity``.
While it doesn't make sense to specify both arguments, it is not considered an error. If
both ``--await`` and ``--await-for`` are present, then the former is ignored, and the time limit
from ``--await-for`` is honored.

.. prompt:: bash $ auto

  $ time ceph fs quiesce fs1 sub1 --timeout=10 --await-for=2
  {
      "epoch": 6,
      "set_version": 3,
      "sets": {
          "c3c1d8de": {
              "version": 3,
              "age_ref": 0.0,
              "state": {
                  "name": "QUIESCING",
                  "age": 2.0
              },
              "timeout": 10.0,
              "expiration": 0.0,
              "members": {
                  "file:/volumes/_nogroup/sub1/b1fcce76-3418-42dd-aa76-f9076d047dd3": {
                      "excluded": false,
                      "state": {
                          "name": "QUIESCING",
                          "age": 2.0
                      }
                  }
              }
          }
      }
  }
  Error EINPROGRESS: 
  ceph fs quiesce fs1 sub1 --timeout=10 --await-for=2  0.41s user 0.04s system 17% cpu 2.563 total

(there is a ~0.5 sec overhead that the ceph client adds, at least in a local debug setup)

Quiesce-Await and Expiration
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Quiesce await has a side effect: it resets the internal expiration timer. This allows for a watchdog
approach to a long running multistep process under the IO pause by repeatedly ``--await``\ ing an already
`QUIESCED` set. Consider the following example script:

.. prompt:: bash $ auto

  $ set -e   # (1)
  $ ceph fs quiesce fs1 sub1 sub2 sub3 --timeout=30 --expiration=10 --set-id="snapshots" --await # (2)
  $ ceph fs subvolume snapshot create a sub1 snap1-sub1  # (3)
  $ ceph fs quiesce fs1 --set-id="snapshots" --await  # (4)
  $ ceph fs subvolume snapshot create a sub2 snap1-sub2  # (3)
  $ ceph fs quiesce fs1 --set-id="snapshots" --await  # (4)
  $ ceph fs subvolume snapshot create a sub3 snap1-sub3  # (3)
  $ ceph fs quiesce fs1 --set-id="snapshots" --release --await  # (5)

.. warning:: This example uses arbitrary timeouts to convey the concept. In real life, the values must be carefully
  chosen in accordance with the actual system requirements and specifications.

The goal of the script is to take consistent snapshots of 3 subvolumes. 
We begin by setting the bash ``-e`` option `(1)` to exit this script if any or the following commands 
returns with a non-zero status.

We go on requesting an IO pause for the three subvolumes `(2)`. We set our timeouts allowing 
the system to spend up to 30 seconds reaching the quiesced state across all members
and stay quiesced for up to 10 seconds before the quiesce expires and the IO
is resumed. We also specify ``--await`` to only proceed once the quiesce is reached.

We then proceed with a set of command pairs that take the next snapshot and call ``--await`` on our set
to extend the expiration timeout for 10 more seconds `(3,4)`. This approach gives us up to 10 seconds
for every snapshot, but also allows taking as many snapshots as we need without losing the IO pause,
and with it - consistency. If we wanted, we could update the `expiration` every time we called for await.

If any of the snapshots gets stuck and takes longer than 10 seconds to complete, then the next call
to ``--await`` will return an error since the set will be `EXPIRED` which is not an awaitable state.
This limits the impact on the applications in the bad case scenarios.

We could have set the `expiration` timeout to 30 at the beginning `(2)`, but that would mean that
a single stuck snapshot would keep the applications pending for all this time.

If Version
~~~~~~~~~~

Sometimes, it's not enough to just observe the successful quiesce or release. The reason could be
a concurrent change of the set by another client. Consider this example:

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 sub1 sub2 sub3 --timeout=30 --expiration=60 --set-id="snapshots" --await  # (1)
  $ ceph fs subvolume snapshot create a sub1 snap1-sub1  # (2)
  $ ceph fs subvolume snapshot create a sub2 snap1-sub2  # (3)
  $ ceph fs subvolume snapshot create a sub3 snap1-sub3  # (4)
  $ ceph fs quiesce fs1 --set-id="snapshots" --release --await  # (5)

The sequence looks good, and the release `(5)` completes successfully. However, it could be that
before snap for sub3 `(4)` is taken, another session excludes sub3 from the set, resuming its IOs

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 --set-id="snapshots" --exclude sub3

Since removing a member from a set doesn't affect its `QUIESCED` state, the release command `(5)`
has no reason to fail. It will ack the two unexcluded members sub1 and sub2 and report success.

In order to address this or similar problems, the quiesce command supports an optimistic concurrency
mode. To activate it, one needs to pass an ``--if-version=<version>`` that will be compared
to the set's db version and the operation will only proceed if the values match. Otherwise, the command
will not be executed and the return status will be ``ESTALE``.

It's easy to know which version to expect of a set, since every command that modifies a set will return
this set on the stdout, regarldess of the exit status. In the examples above one can notice that every
set carries a ``"version"`` property which gets updated whenever this set is modified, explicitly
by the user or implicitly during 

In the example at the beginning of this subsection, the initial quiesce command `(1)` would have returned
the newly created set with id ``"snapshots"`` and some version, let's say ``13``. Since we don't expect any other
changes to the set while we are making snapshots with the commands `(2,3,4)`, the release command `(5)`
could have looked like

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 --set-id="snapshots" --release --await --if-version=13 # (5)

This way, the result of the release command would have been ``ESTALE`` instead of 0, and we would
know that something wasn't right with the quiesce set and our snapshots might not be consistent.

.. tip:: When ``--if-version`` and the command returns ``ESTALE``, the requested action is **not** executed.
  It means that the script may want to execute some unconditional command on the set to adjust its state
  according to the requirements

There is another use of the ``--if-version`` argument which could come handy for automation software.
As we have discussed earlier, it is possible to create a new quiesce set with a given set id. Drivers like
the CSI for Kubernetes could use their internal request id to eliminate the need to keep an additional mapping
to the quiesce set id. However, to guarantee uniqueness, the driver may want to verify that the set is
indeed new. For that, ``if-version=0`` may be used, and it will only create the new set if no other
set with this id was present in the database

.. prompt:: bash $ auto

  $ ceph fs quiesce fs1 sub1 sub2 sub3 --set-id="external-id" --if-version=0


.. _disabling-volumes-plugin:

Disabling Volumes Plugin
------------------------
By default the volumes plugin is enabled and set to ``always on``. However, in
certain cases it might be appropriate to disable it. For example, when a CephFS
is in a degraded state, the volumes plugin commands may accumulate in MGR
instead of getting served. Which eventually causes policy throttles to kick in
and the MGR becomes unresponsive.

In this event, volumes plugin can be disabled even though it is an
``always on`` module in MGR. To do so, run ``ceph mgr module disable volumes
--yes-i-really-mean-it``. Do note that this command will disable operations
and remove commands of volumes plugin since it will disable all CephFS
services on the Ceph cluster accessed through this plugin.

Before resorting to a measure as drastic as this, it is a good idea to try less
drastic measures and then assess if the file system experience has improved due
to it. One example of such less drastic measure is to disable asynchronous
threads launched by volumes plugins for cloning and purging trash.


.. _manila: https://github.com/openstack/manila
.. _CSI: https://github.com/ceph/ceph-csi