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74 lines
2.8 KiB
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================
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CephFS Mirroring
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================
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.. note:: CephFS mirroring feature is currently under development. Interfaces detailed in
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this document might change during development cycle.
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CephFS will support asynchronous replication of snapshots to a remote CephFS filesystem 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 filesystem) as
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the snapshot being synchronized.
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Key Idea
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--------
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For a given snapshot in a directory, `cephfs-mirror` daemon uses recursive stats to identify
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changes in a directory tree. For a given directory, local rctime being larger than its
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corresponding remote timestamp implies changes exist somewhere in the subtree. The subtree is
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scanned further using the same rctime comparison principle to collect a set of changes for
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propagation to remote filesystem.
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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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filesystem mirroring and is the recommended control interface. See `Internal Interfaces`
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(monitor commands) are detailed below.
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.. note:: `Mirroring` module is work in progress and will be detailed when ready for use.
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Internal Interfaces
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-------------------
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Mirroring needs be enabled for a Ceph filesystem for `cephfs-mirror` daemons to start
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mirroring directory snapshots::
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$ ceph fs mirror enable <fsname>
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Once mirroring is enabled, mirror peers can be added. `cephfs-mirror` daemon mirrors
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directory snapshots to mirror peers. A mirror peer is represented as a client name
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and cluster name tuple. To add a mirror peer::
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$ ceph fs mirror peer_add <fsname> <client@cluster> <remote_fsname>
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Each peer is assigned a unique identifier which can be fetched via `ceph fs dump` or
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`ceph fs get` commands as below::
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$ ceph fs get <fsname>
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...
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...
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[peers={uuid=e3739ebf-dbce-460a-bf9c-c66b57697c9a, remote_cluster={client_name=client.site-a, cluster_name=site-a, fs_name=backup}}]
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To remove a mirror peer use the following::
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$ ceph fs mirror peer_remove <uuid>
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Mirroring can be disabled for a Ceph filesystem with::
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$ ceph fs mirror disable <fsname>
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Mirror status (enabled/disabled) and filesystem mirror peers are persisted in `FSMap`.
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This enables any entity in a Ceph cluster to subscribe to `FSMap` updates and get
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notified about changes in mirror status and/or peers. `cephfs-mirror` daemon subscribes
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to `FSMap` and gets notified on mirror status and/or peer updates. Peer changes are
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handled by starting or stopping mirroring to when a new peer is added or an existing peer
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is removed.
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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). However, the
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feature is in development phase.
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