ceph/doc/dev/cephfs-snapshots.rst

   1 CephFS Snapshots
   2 ================
   3
   4 CephFS supports snapshots, generally created by invoking mkdir within the
   5 ``.snap`` directory. Note this is a hidden, special directory, not visible
   6 during a directory listing.
   7
   8 Overview
   9 -----------
  10
  11 Generally, snapshots do what they sound like: they create an immutable view
  12 of the file system at the point in time they're taken. There are some headline
  13 features that make CephFS snapshots different from what you might expect:
  14
  15 * Arbitrary subtrees. Snapshots are created within any directory you choose,
  16   and cover all data in the file system under that directory.
  17 * Asynchronous. If you create a snapshot, buffered data is flushed out lazily,
  18   including from other clients. As a result, "creating" the snapshot is
  19   very fast.
  20
  21 Important Data Structures
  22 -------------------------
  23 * SnapRealm: A `SnapRealm` is created whenever you create a snapshot at a new
  24   point in the hierarchy (or, when a snapshotted inode is move outside of its
  25   parent snapshot). SnapRealms contain an `sr_t srnode`, and `inodes_with_caps`
  26   that are part of the snapshot. Clients also have a SnapRealm concept that
  27   maintains less data but is used to associate a `SnapContext` with each open
  28   file for writing.
  29 * sr_t: An `sr_t` is the on-disk snapshot metadata. It is part of the containing
  30   directory and contains sequence counters, timestamps, the list of associated
  31   snapshot IDs, and `past_parent_snaps`.
  32 * SnapServer: SnapServer manages snapshot ID allocation, snapshot deletion and
  33   tracks list of effective snapshots in the file system. A file system only has
  34   one instance of snapserver.
  35 * SnapClient: SnapClient is used to communicate with snapserver, each MDS rank
  36   has its own snapclient instance. SnapClient also caches effective snapshots
  37   locally.
  38
  39 Creating a snapshot
  40 -------------------
  41 CephFS snapshot feature is enabled by default on new file system. To enable it
  42 on existing file systems, use command below.
  43
  44 .. code::
  45
  46        $ ceph fs set <fs_name> allow_new_snaps true
  47
  48 When snapshots are enabled, all directories in CephFS will have a special
  49 ``.snap`` directory. (You may configure a different name with the ``client
  50 snapdir`` setting if you wish.)
  51
  52 To create a CephFS snapshot, create a subdirectory under
  53 ``.snap`` with a name of your choice. For example, to create a snapshot on
  54 directory "/1/2/3/", invoke ``mkdir /1/2/3/.snap/my-snapshot-name`` .
  55
  56 This is transmitted to the MDS Server as a
  57 CEPH_MDS_OP_MKSNAP-tagged `MClientRequest`, and initially handled in
  58 Server::handle_client_mksnap(). It allocates a `snapid` from the `SnapServer`,
  59 projects a new inode with the new SnapRealm, and commits it to the MDLog as
  60 usual. When committed, it invokes
  61 `MDCache::do_realm_invalidate_and_update_notify()`, which notifies all clients
  62 with caps on files under "/1/2/3/", about the new SnapRealm. When clients get
  63 the notifications, they update client-side SnapRealm hierarchy, link files
  64 under "/1/2/3/" to the new SnapRealm and generate a `SnapContext` for the
  65 new SnapRealm.
  66
  67 Note that this *is not* a synchronous part of the snapshot creation!
  68
  69 Updating a snapshot
  70 -------------------
  71 If you delete a snapshot, a similar process is followed. If you remove an inode
  72 out of its parent SnapRealm, the rename code creates a new SnapRealm for the
  73 renamed inode (if SnapRealm does not already exist), saves IDs of snapshots that
  74 are effective on the original parent SnapRealm into `past_parent_snaps` of the
  75 new SnapRealm, then follows a process similar to creating snapshot.
  76
  77 Generating a SnapContext
  78 ------------------------
  79 A RADOS `SnapContext` consists of a snapshot sequence ID (`snapid`) and all
  80 the snapshot IDs that an object is already part of. To generate that list, we
  81 combine `snapids` associated with the SnapRealm and all valid `snapids` in
  82 `past_parent_snaps`. Stale `snapids` are filtered out by SnapClient's cached
  83 effective snapshots.
  84
  85 Storing snapshot data
  86 ---------------------
  87 File data is stored in RADOS "self-managed" snapshots. Clients are careful to
  88 use the correct `SnapContext` when writing file data to the OSDs.
  89
  90 Storing snapshot metadata
  91 -------------------------
  92 Snapshotted dentries (and their inodes) are stored in-line as part of the
  93 directory they were in at the time of the snapshot. *All dentries* include a
  94 `first` and `last` snapid for which they are valid. (Non-snapshotted dentries
  95 will have their `last` set to CEPH_NOSNAP).
  96
  97 Snapshot writeback
  98 ------------------
  99 There is a great deal of code to handle writeback efficiently. When a Client
 100 receives an `MClientSnap` message, it updates the local `SnapRealm`
 101 representation and its links to specific `Inodes`, and generates a `CapSnap`
 102 for the `Inode`. The `CapSnap` is flushed out as part of capability writeback,
 103 and if there is dirty data the `CapSnap` is used to block fresh data writes
 104 until the snapshot is completely flushed to the OSDs.
 105
 106 In the MDS, we generate snapshot-representing dentries as part of the regular
 107 process for flushing them. Dentries with outstanding `CapSnap` data is kept
 108 pinned and in the journal.
 109
 110 Deleting snapshots
 111 ------------------
 112 Snapshots are deleted by invoking "rmdir" on the ".snap" directory they are
 113 rooted in. (Attempts to delete a directory which roots snapshots *will fail*;
 114 you must delete the snapshots first.) Once deleted, they are entered into the
 115 `OSDMap` list of deleted snapshots and the file data is removed by the OSDs.
 116 Metadata is cleaned up as the directory objects are read in and written back
 117 out again.
 118
 119 Hard links
 120 ----------
 121 Inode with multiple hard links is moved to a dummy global SnapRealm. The
 122 dummy SnapRealm covers all snapshots in the file system. The inode's data
 123 will be preserved for any new snapshot. These preserved data will cover
 124 snapshots on any linkage of the inode.
 125
 126 Multi-FS
 127 ---------
 128 Snapshots and multiple file systems don't interact well. Specifically, each
 129 MDS cluster allocates `snapids` independently; if you have multiple file systems
 130 sharing a single pool (via namespaces), their snapshots *will* collide and
 131 deleting one will result in missing file data for others. (This may even be
 132 invisible, not throwing errors to the user.) If each FS gets its own
 133 pool things probably work, but this isn't tested and may not be true.