Notice

This document is for a development version of Ceph.

CephFS Mirroring

CephFS supports asynchronous replication of snapshots to a remote CephFS file system via cephfs-mirror tool. Snapshots are synchronized by mirroring snapshot data followed by creating a snapshot with the same name (for a given directory on the remote file system) as the snapshot being synchronized.

Requirements

The primary (local) and secondary (remote) Ceph clusters version should be Pacific or later.

Key Idea

For a given snapshot pair in a directory, cephfs-mirror daemon will rely on readdir diff to identify changes in a directory tree. The diffs are applied to directory in the remote file system thereby only synchronizing files that have changed between two snapshots.

This feature is tracked here: https://tracker.ceph.com/issues/47034.

Currently, snapshot data is synchronized by bulk copying to the remote filesystem.

Note

Synchronizing hardlinks is not supported – hardlinked files get synchronized as separate files.

Creating Users

Start by creating a user (on the primary/local cluster) for the mirror daemon. This user requires write capability on the metadata pool to create RADOS objects (index objects) for watch/notify operation and read capability on the data pool(s).

$ ceph auth get-or-create client.mirror mon ‘profile cephfs-mirror’ mds ‘allow r’ osd ‘allow rw tag cephfs metadata=*, allow r tag cephfs data=*’ mgr ‘allow r’

Create a user for each file system peer (on the secondary/remote cluster). This user needs to have full capabilities on the MDS (to take snapshots) and the OSDs:

$ ceph fs authorize <fs_name> client.mirror_remote / rwps

This user should be used (as part of peer specification) when adding a peer.

Starting Mirror Daemon

Mirror daemon should be spawned using systemctl(1) unit files:

$ systemctl enable cephfs-mirror@mirror
$ systemctl start cephfs-mirror@mirror

cephfs-mirror daemon can be run in foreground using:

$ cephfs-mirror --id mirror --cluster site-a -f

Note

User used here is mirror as created in the Creating Users section.

Mirroring Design

CephFS supports asynchronous replication of snapshots to a remote CephFS file system via cephfs-mirror tool. For a given directory, snapshots are synchronized by transferring snapshot data to the remote file system and creating a snapshot with the same name as the snapshot being synchronized.

Snapshot Synchronization Order

Although the order in which snapshots get chosen for synchronization does not matter, snapshots are picked based on creation order (using snap-id).

Snapshot Incarnation

A snapshot may be deleted and recreated (with the same name) with different contents. An “old” snapshot could have been synchronized (earlier) and the recreation of the snapshot could have been done when mirroring was disabled. Using snapshot names to infer the point-of-continuation would result in the “new” snapshot (incarnation) never getting picked up for synchronization.

Snapshots on the secondary file system stores the snap-id of the snapshot it was synchronized from. This metadata is stored in SnapInfo structure on the MDS.

Interfaces

Mirroring module (manager plugin) provides interfaces for managing directory snapshot mirroring. Manager interfaces are (mostly) wrappers around monitor commands for managing file system mirroring and is the recommended control interface.

Mirroring Module and Interface

Mirroring module provides interface for managing directory snapshot mirroring. The module is implemented as a Ceph Manager plugin. Mirroring module does not manage spawning (and terminating) the mirror daemons. Right now the preferred way would be to start/stop mirror daemons via systemctl(1). Going forward, deploying mirror daemons would be managed by cephadm (Tracker: http://tracker.ceph.com/issues/47261).

The manager module is responsible for assigning directories to mirror daemons for synchronization. Multiple mirror daemons can be spawned to achieve concurrency in directory snapshot synchronization. When mirror daemons are spawned (or terminated) , the mirroring module discovers the modified set of mirror daemons and rebalances the directory assignment amongst the new set thus providing high-availability.

Note

Multiple mirror daemons is currently untested. Only a single mirror daemon is recommended.

Mirroring module is disabled by default. To enable mirroring use:

$ ceph mgr module enable mirroring

Mirroring module provides a family of commands to control mirroring of directory snapshots. To add or remove directories, mirroring needs to be enabled for a given file system. To enable mirroring use:

$ ceph fs snapshot mirror enable <fs_name>

Note

Mirroring module commands use fs snapshot mirror prefix as compared to the monitor commands which fs mirror prefix. Make sure to use module commands.

To disable mirroring, use:

$ ceph fs snapshot mirror disable <fs_name>

Once mirroring is enabled, add a peer to which directory snapshots are to be mirrored. Peers follow <client>@<cluster> specification and get assigned a unique-id (UUID) when added. See Creating Users section on how to create Ceph users for mirroring.

To add a peer use:

$ ceph fs snapshot mirror peer_add <fs_name> <remote_cluster_spec> [<remote_fs_name>] [<remote_mon_host>] [<cephx_key>]

<remote_fs_name> is optional, and default to <fs_name> (on the remote cluster).

This requires the remote cluster ceph configuration and user keyring to be available in the primary cluster. See Bootstrap Peers section to avoid this. peer_add additionally supports passing the remote cluster monitor address and the user key. However, bootstrapping a peer is the recommended way to add a peer.

Note

Only a single peer is supported right now.

To remove a peer use:

$ ceph fs snapshot mirror peer_remove <fs_name> <peer_uuid>

Note

See Mirror Daemon Status section on how to figure out Peer UUID.

To list file system mirror peers use:

$ ceph fs snapshot mirror peer_list <fs_name>

To configure a directory for mirroring, use:

$ ceph fs snapshot mirror add <fs_name> <path>

To stop a mirroring directory snapshots use:

$ ceph fs snapshot mirror remove <fs_name> <path>

Only absolute directory paths are allowed. Also, paths are normalized by the mirroring module, therfore, /a/b/../b is equivalent to /a/b.

$ mkdir -p /d0/d1/d2 $ ceph fs snapshot mirror add cephfs /d0/d1/d2 {} $ ceph fs snapshot mirror add cephfs /d0/d1/../d1/d2 Error EEXIST: directory /d0/d1/d2 is already tracked

Once a directory is added for mirroring, its subdirectory or ancestor directories are disallowed to be added for mirorring:

$ ceph fs snapshot mirror add cephfs /d0/d1
Error EINVAL: /d0/d1 is a ancestor of tracked path /d0/d1/d2
$ ceph fs snapshot mirror add cephfs /d0/d1/d2/d3
Error EINVAL: /d0/d1/d2/d3 is a subtree of tracked path /d0/d1/d2

Commands to check directory mapping (to mirror daemons) and directory distribution are detailed in Mirror Daemon Status section.

Bootstrap Peers

Adding a peer (via peer_add) requires the peer cluster configuration and user keyring to be available in the primary cluster (manager host and hosts running the mirror daemon). This can be avoided by bootstrapping and importing a peer token. Peer bootstrap involves creating a bootstrap token on the peer cluster via:

$ ceph fs snapshot mirror peer_bootstrap create <fs_name> <client_entity> <site-name>

e.g.:

$ ceph fs snapshot mirror peer_bootstrap create backup_fs client.mirror_remote site-remote
{"token": "eyJmc2lkIjogIjBkZjE3MjE3LWRmY2QtNDAzMC05MDc5LTM2Nzk4NTVkNDJlZiIsICJmaWxlc3lzdGVtIjogImJhY2t1cF9mcyIsICJ1c2VyIjogImNsaWVudC5taXJyb3JfcGVlcl9ib290c3RyYXAiLCAic2l0ZV9uYW1lIjogInNpdGUtcmVtb3RlIiwgImtleSI6ICJBUUFhcDBCZ0xtRmpOeEFBVnNyZXozai9YYUV0T2UrbUJEZlJDZz09IiwgIm1vbl9ob3N0IjogIlt2MjoxOTIuMTY4LjAuNTo0MDkxOCx2MToxOTIuMTY4LjAuNTo0MDkxOV0ifQ=="}

site-name refers to a user-defined string to identify the remote filesystem. In context of peer_add interface, site-name is the passed in cluster name from remote_cluster_spec.

Import the bootstrap token in the primary cluster via:

$ ceph fs snapshot mirror peer_bootstrap import <fs_name> <token>

e.g.:

$ ceph fs snapshot mirror peer_bootstrap import cephfs eyJmc2lkIjogIjBkZjE3MjE3LWRmY2QtNDAzMC05MDc5LTM2Nzk4NTVkNDJlZiIsICJmaWxlc3lzdGVtIjogImJhY2t1cF9mcyIsICJ1c2VyIjogImNsaWVudC5taXJyb3JfcGVlcl9ib290c3RyYXAiLCAic2l0ZV9uYW1lIjogInNpdGUtcmVtb3RlIiwgImtleSI6ICJBUUFhcDBCZ0xtRmpOeEFBVnNyZXozai9YYUV0T2UrbUJEZlJDZz09IiwgIm1vbl9ob3N0IjogIlt2MjoxOTIuMTY4LjAuNTo0MDkxOCx2MToxOTIuMTY4LjAuNTo0MDkxOV0ifQ==

Mirror Daemon Status

Mirror daemons get asynchronously notified about changes in file system mirroring status and/or peer updates.

CephFS mirroring module provides mirror daemon status interface to check mirror daemon status:

$ ceph fs snapshot mirror daemon status

E.g:

$ ceph fs snapshot mirror daemon status | jq
[
  {
    "daemon_id": 284167,
    "filesystems": [
      {
        "filesystem_id": 1,
        "name": "a",
        "directory_count": 1,
        "peers": [
          {
            "uuid": "02117353-8cd1-44db-976b-eb20609aa160",
            "remote": {
              "client_name": "client.mirror_remote",
              "cluster_name": "ceph",
              "fs_name": "backup_fs"
            },
            "stats": {
              "failure_count": 1,
              "recovery_count": 0
            }
          }
        ]
      }
    ]
  }
]

An entry per mirror daemon instance is displayed along with information such as configured peers and basic stats. For more detailed stats, use the admin socket interface as detailed below.

CephFS mirror daemons provide admin socket commands for querying mirror status. To check available commands for mirror status use:

$ ceph --admin-daemon /path/to/mirror/daemon/admin/socket help
{
    ....
    ....
    "fs mirror status cephfs@360": "get filesystem mirror status",
    ....
    ....
}

Commands with fs mirror status prefix provide mirror status for mirror enabled file systems. Note that cephfs@360 is of format filesystem-name@filesystem-id. This format is required since mirror daemons get asynchronously notified regarding file system mirror status (A file system can be deleted and recreated with the same name).

Right now, the command provides minimal information regarding mirror status:

$ ceph --admin-daemon /var/run/ceph/cephfs-mirror.asok fs mirror status cephfs@360
{
  "rados_inst": "192.168.0.5:0/1476644347",
  "peers": {
      "a2dc7784-e7a1-4723-b103-03ee8d8768f8": {
          "remote": {
              "client_name": "client.mirror_remote",
              "cluster_name": "site-a",
              "fs_name": "backup_fs"
          }
      }
  },
  "snap_dirs": {
      "dir_count": 1
  }
}

Peers section in the command output above shows the peer information such as unique peer-id (UUID) and specification. The peer-id is required to remove an existing peer as mentioned in the Mirror Module and Interface section.

Command with fs mirror peer status prefix provide peer synchronization status. This command is of format filesystem-name@filesystem-id peer-uuid:

$ ceph --admin-daemon /var/run/ceph/cephfs-mirror.asok fs mirror peer status cephfs@360 a2dc7784-e7a1-4723-b103-03ee8d8768f8
{
  "/d0": {
      "state": "idle",
      "last_synced_snap": {
          "id": 120,
          "name": "snap1",
          "sync_duration": 0.079997898999999997,
          "sync_time_stamp": "274900.558797s"
      },
      "snaps_synced": 2,
      "snaps_deleted": 0,
      "snaps_renamed": 0
  }
}

Synchronization stats such as snaps_synced, snaps_deleted and snaps_renamed are reset on daemon restart and/or when a directory is reassigned to another mirror daemon (when multiple mirror daemons are deployed).

A directory can be in one of the following states:

- `idle`: The directory is currently not being synchronized
- `syncing`: The directory is currently being synchronized
- `failed`: The directory has hit upper limit of consecutive failures

When a directory hits a configured number of consecutive synchronization failures, the mirror daemon marks it as failed. Synchronization for these directories are retried. By default, the number of consecutive failures before a directory is marked as failed is controlled by cephfs_mirror_max_consecutive_failures_per_directory configuration option (default: 10) and the retry interval for failed directories is controlled via cephfs_mirror_retry_failed_directories_interval configuration option (default: 60s).

E.g., adding a regular file for synchronization would result in failed status:

$ ceph fs snapshot mirror add cephfs /f0
$ ceph --admin-daemon /var/run/ceph/cephfs-mirror.asok fs mirror peer status cephfs@360 a2dc7784-e7a1-4723-b103-03ee8d8768f8
{
  "/d0": {
      "state": "idle",
      "last_synced_snap": {
          "id": 120,
          "name": "snap1",
          "sync_duration": 0.079997898999999997,
          "sync_time_stamp": "274900.558797s"
      },
      "snaps_synced": 2,
      "snaps_deleted": 0,
      "snaps_renamed": 0
  },
  "/f0": {
      "state": "failed",
      "snaps_synced": 0,
      "snaps_deleted": 0,
      "snaps_renamed": 0
  }
}

This allows a user to add a non-existent directory for synchronization. The mirror daemon would mark the directory as failed and retry (less frequently). When the directory comes to existence, the mirror daemons would unmark the failed state upon successfull snapshot synchronization.

When mirroring is disabled, the respective fs mirror status command for the file system will not show up in command help.

Mirroring module provides a couple of commands to display directory mapping and distribution information. To check which mirror daemon a directory has been mapped to use:

$ ceph fs snapshot mirror dirmap cephfs /d0/d1/d2
{
  "instance_id": "404148",
  "last_shuffled": 1601284516.10986,
  "state": "mapped"
}

Note

instance_id is the RAODS instance-id associated with a mirror daemon.

Other information such as state and last_shuffled are interesting when running multiple mirror daemons.

When no mirror daemons are running the above command shows:

$ ceph fs snapshot mirror dirmap cephfs /d0/d1/d2
{
  "reason": "no mirror daemons running",
  "state": "stalled"
}

Signifying that no mirror daemons are running and mirroring is stalled.

Re-adding Peers

When re-adding (reassigning) a peer to a file system in another cluster, ensure that all mirror daemons have stopped synchronization to the peer. This can be checked via fs mirror status admin socket command (the Peer UUID should not show up in the command output). Also, it is recommended to purge synchronized directories from the peer before re-adding it to another file system (especially those directories which might exist in the new primary file system). This is not required if re-adding a peer to the same primary file system it was earlier synchronized from.

Feature Status

cephfs-mirror daemon is built by default (follows WITH_CEPHFS CMake rule).