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115 lines
5.4 KiB
ReStructuredText
115 lines
5.4 KiB
ReStructuredText
CephFS Snapshots
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================
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CephFS supports snapshots, generally created by invoking mkdir against the
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(hidden, special) .snap directory.
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Overview
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-----------
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Generally, snapshots do what they sound like: they create an immutable view
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of the filesystem at the point in time they're taken. There are some headline
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features that make CephFS snapshots different from what you might expect:
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* Arbitrary subtrees. Snapshots are created within any directory you choose,
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and cover all data in the filesystem under that directory.
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* Asynchronous. If you create a snapshot, buffered data is flushed out lazily,
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including from other clients. As a result, "creating" the snapshot is
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very fast.
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Important Data Structures
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-------------------------
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* SnapRealm: A `SnapRealm` is created whenever you create a snapshot at a new
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point in the hierarchy (or, when a snapshotted inode is moved outside of its
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parent snapshot). SnapRealms contain an `sr_t srnode`, links to `past_parents`
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and `past_children`, and all `inodes_with_caps` that are part of the snapshot.
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Clients also have a SnapRealm concept that maintains less data but is used to
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associate a `SnapContext` with each open file for writing.
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* sr_t: An `sr_t` is the on-disk snapshot metadata. It is part of the containing
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directory and contains sequence counters, timestamps, the list of associated
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snapshot IDs, and `past_parents`.
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* snaplink_t: `past_parents` et al are stored on-disk as a `snaplink_t`, holding
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the inode number and first `snapid` of the inode/snapshot referenced.
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Creating a snapshot
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-------------------
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To make a snapshot on directory "/1/2/3/foo", the client invokes "mkdir" on
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"/1/2/3/foo/.snaps" directory. This is transmitted to the MDS Server as a
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CEPH_MDS_OP_MKSNAP-tagged `MClientRequest`, and initially handled in
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Server::handle_client_mksnap(). It allocates a `snapid` from the `SnapServer`,
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projects a new inode with the new SnapRealm, and commits it to the MDLog as
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usual. When committed, it invokes
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`MDCache::do_realm_invalidate_and_update_notify()`, which triggers most of the
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real work of the snapshot.
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If there were already snapshots above directory "foo" (rooted at "/1", say),
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the new SnapRealm adds its most immediate ancestor as a `past_parent` on
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creation. After committing to the MDLog, all clients with caps on files in
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"/1/2/3/foo/" are notified (MDCache::send_snaps()) of the new SnapRealm, and
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update the `SnapContext` they are using with that data. Note that this
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*is not* a synchronous part of the snapshot creation!
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Updating a snapshot
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-------------------
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If you delete a snapshot, or move data out of the parent snapshot's hierarchy,
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a similar process is followed. Extra code paths check to see if we can break
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the `past_parent` links between SnapRealms, or eliminate them entirely.
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Generating a SnapContext
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------------------------
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A RADOS `SnapContext` consists of a snapshot sequence ID (`snapid`) and all
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the snapshot IDs that an object is already part of. To generate that list, we
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generate a list of all `snapids` associated with the SnapRealm and all its
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`past_parents`.
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Storing snapshot data
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---------------------
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File data is stored in RADOS "self-managed" snapshots. Clients are careful to
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use the correct `SnapContext` when writing file data to the OSDs.
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Storing snapshot metadata
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-------------------------
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Snapshotted dentries (and their inodes) are stored in-line as part of the
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directory they were in at the time of the snapshot. *All dentries* include a
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`first` and `last` snapid for which they are valid. (Non-snapshotted dentries
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will have their `last` set to CEPH_NOSNAP).
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Snapshot writeback
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------------------
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There is a great deal of code to handle writeback efficiently. When a Client
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receives an `MClientSnap` message, it updates the local `SnapRealm`
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representation and its links to specific `Inodes`, and generates a `CapSnap`
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for the `Inode`. The `CapSnap` is flushed out as part of capability writeback,
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and if there is dirty data the `CapSnap` is used to block fresh data writes
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until the snapshot is completely flushed to the OSDs.
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In the MDS, we generate snapshot-representing dentries as part of the regular
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process for flushing them. Dentries with outstanding `CapSnap` data is kept
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pinned and in the journal.
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Deleting snapshots
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------------------
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Snapshots are deleted by invoking "rmdir" on the ".snaps" directory they are
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rooted in. (Attempts to delete a directory which roots snapshots *will fail*;
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you must delete the snapshots first.) Once deleted, they are entered into the
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`OSDMap` list of deleted snapshots and the file data is removed by the OSDs.
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Metadata is cleaned up as the directory objects are read in and written back
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out again.
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Hard links
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----------
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Hard links do not interact well with snapshots. A file is snapshotted when its
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primary link is part of a SnapRealm; other links *will not* preserve data.
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Generally the location where a file was first created will be its primary link,
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but if the original link has been deleted it is not easy (nor always
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determnistic) to find which link is now the primary.
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Multi-FS
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---------
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Snapshots and multiiple filesystems don't interact well. Specifically, each
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MDS cluster allocates `snapids` independently; if you have multiple filesystems
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sharing a single pool (via namespaces), their snapshots *will* collide and
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deleting one will result in missing file data for others. (This may even be
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invisible, not throwing errors to the user.) If each FS gets its own
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pool things probably work, but this isn't tested and may not be true.
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