mirror of https://github.com/ceph/ceph
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15 KiB
ReStructuredText
404 lines
15 KiB
ReStructuredText
================
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CephFS Mirroring
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================
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CephFS supports asynchronous replication of snapshots to a remote CephFS file system via
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`cephfs-mirror` tool. Snapshots are synchronized by mirroring snapshot data followed by
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creating a snapshot with the same name (for a given directory on the remote file system) as
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the snapshot being synchronized.
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Requirements
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------------
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The primary (local) and secondary (remote) Ceph clusters version should be Pacific or later.
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Key Idea
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--------
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For a given snapshot pair in a directory, `cephfs-mirror` daemon will rely on readdir diff
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to identify changes in a directory tree. The diffs are applied to directory in the remote
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file system thereby only synchronizing files that have changed between two snapshots.
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This feature is tracked here: https://tracker.ceph.com/issues/47034.
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Currently, snapshot data is synchronized by bulk copying to the remote filesystem.
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.. note:: Synchronizing hardlinks is not supported -- hardlinked files get synchronized
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as separate files.
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Creating Users
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--------------
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Start by creating a user (on the primary/local cluster) for the mirror daemon. This user
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requires write capability on the metadata pool to create RADOS objects (index objects)
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for watch/notify operation and read capability on the data pool(s).
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$ 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'
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Create a user for each file system peer (on the secondary/remote cluster). This user needs
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to have full capabilities on the MDS (to take snapshots) and the OSDs::
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$ ceph fs authorize <fs_name> client.mirror_remote / rwps
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This user should be used (as part of peer specification) when adding a peer.
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Starting Mirror Daemon
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----------------------
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Mirror daemon should be spawned using `systemctl(1)` unit files::
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$ systemctl enable cephfs-mirror@mirror
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$ systemctl start cephfs-mirror@mirror
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`cephfs-mirror` daemon can be run in foreground using::
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$ cephfs-mirror --id mirror --cluster site-a -f
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.. note:: User used here is `mirror` as created in the `Creating Users` section.
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Mirroring Design
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----------------
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CephFS supports asynchronous replication of snapshots to a remote CephFS file system
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via `cephfs-mirror` tool. For a given directory, snapshots are synchronized by transferring
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snapshot data to the remote file system and creating a snapshot with the same name as the
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snapshot being synchronized.
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Snapshot Synchronization Order
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------------------------------
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Although the order in which snapshots get chosen for synchronization does not matter,
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snapshots are picked based on creation order (using snap-id).
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Snapshot Incarnation
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--------------------
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A snapshot may be deleted and recreated (with the same name) with different contents.
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An "old" snapshot could have been synchronized (earlier) and the recreation of the
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snapshot could have been done when mirroring was disabled. Using snapshot names to
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infer the point-of-continuation would result in the "new" snapshot (incarnation)
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never getting picked up for synchronization.
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Snapshots on the secondary file system stores the snap-id of the snapshot it was
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synchronized from. This metadata is stored in `SnapInfo` structure on the MDS.
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Interfaces
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----------
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`Mirroring` module (manager plugin) provides interfaces for managing directory snapshot
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mirroring. Manager interfaces are (mostly) wrappers around monitor commands for managing
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file system mirroring and is the recommended control interface.
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Mirroring Module and Interface
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------------------------------
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Mirroring module provides interface for managing directory snapshot mirroring. The module
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is implemented as a Ceph Manager plugin. Mirroring module does not manage spawning (and
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terminating) the mirror daemons. Right now the preferred way would be to start/stop
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mirror daemons via `systemctl(1)`. Going forward, deploying mirror daemons would be
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managed by `cephadm` (Tracker: http://tracker.ceph.com/issues/47261).
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The manager module is responsible for assigning directories to mirror daemons for
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synchronization. Multiple mirror daemons can be spawned to achieve concurrency in
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directory snapshot synchronization. When mirror daemons are spawned (or terminated)
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, the mirroring module discovers the modified set of mirror daemons and rebalances
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the directory assignment amongst the new set thus providing high-availability.
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.. note:: Multiple mirror daemons is currently untested. Only a single mirror daemon
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is recommended.
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Mirroring module is disabled by default. To enable mirroring use::
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$ ceph mgr module enable mirroring
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Mirroring module provides a family of commands to control mirroring of directory
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snapshots. To add or remove directories, mirroring needs to be enabled for a given
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file system. To enable mirroring use::
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$ ceph fs snapshot mirror enable <fs_name>
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.. note:: Mirroring module commands use `fs snapshot mirror` prefix as compared to
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the monitor commands which `fs mirror` prefix. Make sure to use module
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commands.
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To disable mirroring, use::
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$ ceph fs snapshot mirror disable <fs_name>
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Once mirroring is enabled, add a peer to which directory snapshots are to be mirrored.
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Peers follow `<client>@<cluster>` specification and get assigned a unique-id (UUID)
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when added. See `Creating Users` section on how to create Ceph users for mirroring.
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To add a peer use::
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$ ceph fs snapshot mirror peer_add <fs_name> <remote_cluster_spec> [<remote_fs_name>] [<remote_mon_host>] [<cephx_key>]
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`<remote_fs_name>` is optional, and default to `<fs_name>` (on the remote cluster).
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This requires the remote cluster ceph configuration and user keyring to be available in
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the primary cluster. See `Bootstrap Peers` section to avoid this. `peer_add` additionally
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supports passing the remote cluster monitor address and the user key. However, bootstrapping
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a peer is the recommended way to add a peer.
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.. note:: Only a single peer is supported right now.
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To remove a peer use::
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$ ceph fs snapshot mirror peer_remove <fs_name> <peer_uuid>
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.. note:: See `Mirror Daemon Status` section on how to figure out Peer UUID.
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To list file system mirror peers use::
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$ ceph fs snapshot mirror peer_list <fs_name>
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To configure a directory for mirroring, use::
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$ ceph fs snapshot mirror add <fs_name> <path>
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To stop a mirroring directory snapshots use::
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$ ceph fs snapshot mirror remove <fs_name> <path>
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Only absolute directory paths are allowed. Also, paths are normalized by the mirroring
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module, therefore, `/a/b/../b` is equivalent to `/a/b`.
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$ mkdir -p /d0/d1/d2
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$ ceph fs snapshot mirror add cephfs /d0/d1/d2
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{}
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$ ceph fs snapshot mirror add cephfs /d0/d1/../d1/d2
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Error EEXIST: directory /d0/d1/d2 is already tracked
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Once a directory is added for mirroring, its subdirectory or ancestor directories are
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disallowed to be added for mirroring::
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$ ceph fs snapshot mirror add cephfs /d0/d1
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Error EINVAL: /d0/d1 is a ancestor of tracked path /d0/d1/d2
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$ ceph fs snapshot mirror add cephfs /d0/d1/d2/d3
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Error EINVAL: /d0/d1/d2/d3 is a subtree of tracked path /d0/d1/d2
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Commands to check directory mapping (to mirror daemons) and directory distribution are
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detailed in `Mirror Daemon Status` section.
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Bootstrap Peers
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---------------
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Adding a peer (via `peer_add`) requires the peer cluster configuration and user keyring
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to be available in the primary cluster (manager host and hosts running the mirror daemon).
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This can be avoided by bootstrapping and importing a peer token. Peer bootstrap involves
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creating a bootstrap token on the peer cluster via::
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$ ceph fs snapshot mirror peer_bootstrap create <fs_name> <client_entity> <site-name>
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e.g.::
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$ ceph fs snapshot mirror peer_bootstrap create backup_fs client.mirror_remote site-remote
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{"token": "eyJmc2lkIjogIjBkZjE3MjE3LWRmY2QtNDAzMC05MDc5LTM2Nzk4NTVkNDJlZiIsICJmaWxlc3lzdGVtIjogImJhY2t1cF9mcyIsICJ1c2VyIjogImNsaWVudC5taXJyb3JfcGVlcl9ib290c3RyYXAiLCAic2l0ZV9uYW1lIjogInNpdGUtcmVtb3RlIiwgImtleSI6ICJBUUFhcDBCZ0xtRmpOeEFBVnNyZXozai9YYUV0T2UrbUJEZlJDZz09IiwgIm1vbl9ob3N0IjogIlt2MjoxOTIuMTY4LjAuNTo0MDkxOCx2MToxOTIuMTY4LjAuNTo0MDkxOV0ifQ=="}
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`site-name` refers to a user-defined string to identify the remote filesystem. In context
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of `peer_add` interface, `site-name` is the passed in `cluster` name from `remote_cluster_spec`.
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Import the bootstrap token in the primary cluster via::
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$ ceph fs snapshot mirror peer_bootstrap import <fs_name> <token>
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e.g.::
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$ ceph fs snapshot mirror peer_bootstrap import cephfs eyJmc2lkIjogIjBkZjE3MjE3LWRmY2QtNDAzMC05MDc5LTM2Nzk4NTVkNDJlZiIsICJmaWxlc3lzdGVtIjogImJhY2t1cF9mcyIsICJ1c2VyIjogImNsaWVudC5taXJyb3JfcGVlcl9ib290c3RyYXAiLCAic2l0ZV9uYW1lIjogInNpdGUtcmVtb3RlIiwgImtleSI6ICJBUUFhcDBCZ0xtRmpOeEFBVnNyZXozai9YYUV0T2UrbUJEZlJDZz09IiwgIm1vbl9ob3N0IjogIlt2MjoxOTIuMTY4LjAuNTo0MDkxOCx2MToxOTIuMTY4LjAuNTo0MDkxOV0ifQ==
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Mirror Daemon Status
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--------------------
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Mirror daemons get asynchronously notified about changes in file system mirroring status
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and/or peer updates.
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CephFS mirroring module provides `mirror daemon status` interface to check mirror daemon
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status::
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$ ceph fs snapshot mirror daemon status
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E.g::
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$ ceph fs snapshot mirror daemon status | jq
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[
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{
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"daemon_id": 284167,
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"filesystems": [
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{
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"filesystem_id": 1,
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"name": "a",
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"directory_count": 1,
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"peers": [
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{
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"uuid": "02117353-8cd1-44db-976b-eb20609aa160",
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"remote": {
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"client_name": "client.mirror_remote",
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"cluster_name": "ceph",
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"fs_name": "backup_fs"
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},
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"stats": {
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"failure_count": 1,
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"recovery_count": 0
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}
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}
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]
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}
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]
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}
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]
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An entry per mirror daemon instance is displayed along with information such as configured
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peers and basic stats. For more detailed stats, use the admin socket interface as detailed
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below.
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CephFS mirror daemons provide admin socket commands for querying mirror status. To check
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available commands for mirror status use::
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$ ceph --admin-daemon /path/to/mirror/daemon/admin/socket help
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{
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....
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....
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"fs mirror status cephfs@360": "get filesystem mirror status",
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....
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....
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}
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Commands with `fs mirror status` prefix provide mirror status for mirror enabled
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file systems. Note that `cephfs@360` is of format `filesystem-name@filesystem-id`.
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This format is required since mirror daemons get asynchronously notified regarding
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file system mirror status (A file system can be deleted and recreated with the same
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name).
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Right now, the command provides minimal information regarding mirror status::
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$ ceph --admin-daemon /var/run/ceph/cephfs-mirror.asok fs mirror status cephfs@360
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{
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"rados_inst": "192.168.0.5:0/1476644347",
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"peers": {
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"a2dc7784-e7a1-4723-b103-03ee8d8768f8": {
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"remote": {
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"client_name": "client.mirror_remote",
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"cluster_name": "site-a",
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"fs_name": "backup_fs"
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}
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}
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},
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"snap_dirs": {
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"dir_count": 1
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}
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}
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`Peers` section in the command output above shows the peer information such as unique
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peer-id (UUID) and specification. The peer-id is required to remove an existing peer
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as mentioned in the `Mirror Module and Interface` section.
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Command with `fs mirror peer status` prefix provide peer synchronization status. This
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command is of format `filesystem-name@filesystem-id peer-uuid`::
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$ ceph --admin-daemon /var/run/ceph/cephfs-mirror.asok fs mirror peer status cephfs@360 a2dc7784-e7a1-4723-b103-03ee8d8768f8
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{
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"/d0": {
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"state": "idle",
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"last_synced_snap": {
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"id": 120,
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"name": "snap1",
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"sync_duration": 0.079997898999999997,
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"sync_time_stamp": "274900.558797s"
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},
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"snaps_synced": 2,
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"snaps_deleted": 0,
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"snaps_renamed": 0
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}
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}
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Synchronization stats such as `snaps_synced`, `snaps_deleted` and `snaps_renamed` are reset
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on daemon restart and/or when a directory is reassigned to another mirror daemon (when
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multiple mirror daemons are deployed).
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A directory can be in one of the following states::
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- `idle`: The directory is currently not being synchronized
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- `syncing`: The directory is currently being synchronized
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- `failed`: The directory has hit upper limit of consecutive failures
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When a directory hits a configured number of consecutive synchronization failures, the
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mirror daemon marks it as `failed`. Synchronization for these directories are retried.
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By default, the number of consecutive failures before a directory is marked as failed
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is controlled by `cephfs_mirror_max_consecutive_failures_per_directory` configuration
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option (default: 10) and the retry interval for failed directories is controlled via
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`cephfs_mirror_retry_failed_directories_interval` configuration option (default: 60s).
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E.g., adding a regular file for synchronization would result in failed status::
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$ ceph fs snapshot mirror add cephfs /f0
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$ ceph --admin-daemon /var/run/ceph/cephfs-mirror.asok fs mirror peer status cephfs@360 a2dc7784-e7a1-4723-b103-03ee8d8768f8
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{
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"/d0": {
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"state": "idle",
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"last_synced_snap": {
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"id": 120,
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"name": "snap1",
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"sync_duration": 0.079997898999999997,
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"sync_time_stamp": "274900.558797s"
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},
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"snaps_synced": 2,
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"snaps_deleted": 0,
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"snaps_renamed": 0
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},
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"/f0": {
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"state": "failed",
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"snaps_synced": 0,
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"snaps_deleted": 0,
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"snaps_renamed": 0
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}
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}
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This allows a user to add a non-existent directory for synchronization. The mirror daemon
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would mark the directory as failed and retry (less frequently). When the directory comes
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to existence, the mirror daemons would unmark the failed state upon successful snapshot
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synchronization.
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When mirroring is disabled, the respective `fs mirror status` command for the file system
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will not show up in command help.
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Mirroring module provides a couple of commands to display directory mapping and distribution
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information. To check which mirror daemon a directory has been mapped to use::
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$ ceph fs snapshot mirror dirmap cephfs /d0/d1/d2
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{
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"instance_id": "404148",
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"last_shuffled": 1601284516.10986,
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"state": "mapped"
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}
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.. note:: `instance_id` is the RAODS instance-id associated with a mirror daemon.
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Other information such as `state` and `last_shuffled` are interesting when running
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multiple mirror daemons.
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When no mirror daemons are running the above command shows::
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$ ceph fs snapshot mirror dirmap cephfs /d0/d1/d2
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{
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"reason": "no mirror daemons running",
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"state": "stalled"
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}
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Signifying that no mirror daemons are running and mirroring is stalled.
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Re-adding Peers
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---------------
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When re-adding (reassigning) a peer to a file system in another cluster, ensure that
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all mirror daemons have stopped synchronization to the peer. This can be checked
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via `fs mirror status` admin socket command (the `Peer UUID` should not show up
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in the command output). Also, it is recommended to purge synchronized directories
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from the peer before re-adding it to another file system (especially those directories
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which might exist in the new primary file system). This is not required if re-adding
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a peer to the same primary file system it was earlier synchronized from.
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Feature Status
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--------------
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`cephfs-mirror` daemon is built by default (follows `WITH_CEPHFS` CMake rule).
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