Notice

This document is for a development version of Ceph.

FS volumes and subvolumes

The volumes module of the 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 subvolumes, an abstraction for independent CephFS directory trees

  • FS subvolume groups, an abstraction for a directory level higher than FS subvolumes. Used to effect policies (e.g., File layouts) across a set of subvolumes

Some 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 User Management) with at least the following capabilities:

    mon 'allow r'
    mgr 'allow rw'
    

FS Volumes

Create a volume by running the following command:

ceph fs volume create <vol_name> [placement]

This creates a CephFS file system and its data and metadata pools. It can also deploy MDS daemons for the filesystem using a ceph-mgr orchestrator module (for example Rook). See Orchestrator CLI.

<vol_name> is the volume name (an arbitrary string). [placement] is an optional string that specifies the Daemon Placement for the MDS. See also Deploy 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:

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

This removes a file system and its data and metadata pools. It also tries to remove MDS daemons using the enabled ceph-mgr orchestrator module.

Note

After volume deletion, it is recommended to restart ceph-mgr if a new file system is created on the same cluster and subvolume interface is being used. Please see https://tracker.ceph.com/issues/49605#note-5 for more details.

List volumes by running the following command:

ceph fs volume ls

Rename a volume by running the following command:

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 matching <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 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:

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:

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:

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 creating a subvolume group you can specify its data pool layout (see 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 CephFS Quotas). 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:

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 non-existent “subvolume group remove command” to succeed.

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

ceph fs subvolumegroup getpath <vol_name> <group_name>

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

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:

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: modification time of the subvolume group path in the format “YYYY-MM-DD HH:MM:SS”

  • ctime: change time 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 the presence of any subvolume group by running a command of the following form:

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. To validate the emptiness of the volume, a subvolumegroup existence check alone is not sufficient. Subvolume existence also needs to be checked as there might be subvolumes in the default group.

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

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

The 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:

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:

ceph fs subvolumegroup snapshot ls <vol_name> <group_name>

FS Subvolumes

Create a subvolume using:

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]

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 CephFS Quotas). The subvolume can be created in a separate RADOS namespace by specifying --namespace-isolated option. By default a subvolume is created within the default subvolume group, and with an octal file mode ‘755’, uid of its subvolume group, gid of its subvolume group, data pool layout of its parent directory and no size limit.

Remove a subvolume using:

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

The command removes the subvolume and its contents. It does this in two steps. First, it moves the subvolume to a trash folder, and then asynchronously purges its contents.

The removal of a subvolume fails if it has snapshots, or is non-existent. ‘--force’ flag allows the non-existent subvolume remove command to succeed.

A subvolume can be removed retaining existing snapshots of the subvolume using the ‘--retain-snapshots’ option. If snapshots are retained, the subvolume is considered empty for all operations not involving the retained snapshots.

Note

Snapshot retained subvolumes can be recreated using ‘ceph fs subvolume create’

Note

Retained snapshots can be used as a clone source to recreate the subvolume, or clone to a newer subvolume.

Resize a subvolume using:

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

The 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.

Authorize cephx auth IDs, the read/read-write access to fs subvolumes:

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

The access_level takes r or rw as value.

Deauthorize cephx auth IDs, the read/read-write access to fs subvolumes:

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

List cephx auth IDs authorized to access fs subvolume:

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

Evict fs clients based on auth ID and subvolume mounted:

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

Fetch the absolute path of a subvolume using:

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

Fetch the information of a subvolume using:

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

The output format is JSON and contains fields as follows.

  • 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’s clone or subvolume

  • pool_namespace: RADOS namespace of the subvolume

  • features: features supported by the subvolume

  • state: current state of the subvolume

If a subvolume has been removed retaining its snapshots, the output contains only fields as follows.

  • type: subvolume type indicating whether it’s 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 subvolumes snapshot as the source

  • snapshot-autoprotect: supports automatically protecting snapshots, that are active clone sources, from deletion

  • 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

List subvolumes using:

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

Note

subvolumes that are removed but have snapshots retained, are also listed.

Check the presence of any subvolume using:

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

Set custom metadata on the subvolume as a key-value pair using:

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 hence, is also not preserved when cloning the subvolume snapshot.

Get custom metadata set on the subvolume using the metadata key:

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

List custom metadata (key-value pairs) set on the subvolume using:

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

Remove custom metadata set on the subvolume using the metadata key:

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 that would otherwise fail if the metadata key did not exist.

Create a snapshot of a subvolume using:

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

Remove a snapshot of a subvolume using:

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 that 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

List snapshots of a subvolume using:

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

Fetch the information of a snapshot using:

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

The output format is JSON and contains fields as follows.

  • 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 group 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:

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:

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"
}

Set custom key-value metadata on the snapshot by running:

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 hence is also not preserved when cloning the subvolume snapshot.

Get custom metadata set on the snapshot using the metadata key:

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

List custom metadata (key-value pairs) set on the snapshot using:

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

Remove custom metadata set on the snapshot using the metadata key:

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 that 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. Due to this bulk copying, cloning is inefficient for very large data sets.

Note

Removing a snapshot (source subvolume) would fail if there are pending or in progress clone operations.

Protecting snapshots prior to cloning was a prerequisite in the Nautilus release, and the commands to protect/unprotect snapshots were introduced for this purpose. This prerequisite, and hence the commands to protect/unprotect, is being deprecated and may be removed from a future release.

The commands being deprecated are:

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.

To initiate a clone operation use:

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

If a snapshot (source subvolume) is a part of non-default group, the group name needs to be specified:

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). To clone to a particular group use:

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

Similar to specifying a pool layout when creating a subvolume, pool layout can be specified when creating a cloned subvolume. To create a cloned subvolume with a specific pool layout use:

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

To check the status of a clone operation use:

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

A clone can be in one of the following states:

  1. pending : Clone operation has not started

  2. in-progress : Clone operation is in progress

  3. complete : Clone operation has successfully finished

  4. failed : Clone operation has failed

  5. canceled : Clone operation is cancelled by user

The reason for a clone failure is shown as below:

  1. errno : error number

  2. error_msg : failure error string

Here is an example of an in-progress clone:

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"
        }
    }
}

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:

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: since 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 below:

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. To delete a partial clone use:

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 seconds granularity.

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

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

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

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

The canceled cloned may be deleted by supplying the --force option to the fs subvolume rm command.

Configurables

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

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

Configure the snapshot_clone_no_wait option :

``snapshot_clone_no_wait`` config option is used to reject the clone creation request when the cloner threads
( which can be configured using above option i.e. ``max_concurrent_clones``) are not available.
It is enabled by default i.e. the value set is True, whereas it can be configured by using below command.

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

The current value of ``snapshot_clone_no_wait`` can be fetched by using below command.

ceph config get mgr mgr/volumes/snapshot_clone_no_wait

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 Manually pinning directory trees to a particular rank and Setting subtree partitioning policies for details on how pinning works.

Pinning is configured by:

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

or for subvolumes:

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

Typically you will want to set subvolume group pins. The pin_type may be one of export, distributed, or random. The pin_setting corresponds to the extended attributed “value” as in the pinning documentation referenced above.

So, for example, setting a distributed pinning strategy on a subvolume group:

ceph fs subvolumegroup pin cephfilesystem-a csi distributed 1

Will enable 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

It may be needed to pause IO to a set of subvolumes of a given volume (file system). A good example of such case is a consistent snapshot spanning multiple subvolumes. The task arises often in an orchestrated environment such as Kubernetes, where a single deployed application can work with many mounted subvolumes across several hosts. When a snapshot of such a system is needed, the application may not find the result consistent unless the snapshots were taken during an active write pause.

The volumes plugin provides a tool to initiate and await such a pause across a set of subvolumes:

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 in pause and released from such

The quiesce functionality is itself based on a lower level QuiesceDb 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 appropriate quiesce command to the MDS. You can learn more about the feature in the developer guides.

Operations

The IO pause (referred to as quiesce) is requested for a group of one or more subvolumes (i.e. paths in a filesystem). The group is referred to as “quiesce set”, and every quiesce set must have a unique string id to interact with. 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.

../../_images/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

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.

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:

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

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.

Technically, 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 already RELEASING or RELEASED. 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.

When awaiting, one may also specify a maximum duration that they would like this await request to block for, not affecting the two intrinsic 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.

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 --awaiting an already QUIESCED set. Consider the following example script:

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:

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

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

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

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

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