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diff --git a/doc/dev/cephfs-mirroring.rst b/doc/dev/cephfs-mirroring.rst new file mode 100644 index 000000000..3ca487c03 --- /dev/null +++ b/doc/dev/cephfs-mirroring.rst @@ -0,0 +1,403 @@ +================ +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). |