CephFS supports snapshots, generally created by invoking mkdir within the
.snap directory. Note this is a hidden, special directory, not visible
during a directory listing.
Generally, snapshots do what they sound like: they create an immutable view of the file system at the point in time they’re taken. There are some headline features that make CephFS snapshots different from what you might expect:
Arbitrary subtrees. Snapshots are created within any directory you choose, and cover all data in the file system under that directory.
Asynchronous. If you create a snapshot, buffered data is flushed out lazily, including from other clients. As a result, “creating” the snapshot is very fast.
Important Data Structures
SnapRealm: A SnapRealm is created whenever you create a snapshot at a new point in the hierarchy (or, when a snapshotted inode is move outside of its parent snapshot). SnapRealms contain an sr_t srnode, and inodes_with_caps that are part of the snapshot. Clients also have a SnapRealm concept that maintains less data but is used to associate a SnapContext with each open file for writing.
sr_t: An sr_t is the on-disk snapshot metadata. It is part of the containing directory and contains sequence counters, timestamps, the list of associated snapshot IDs, and past_parent_snaps.
SnapServer: SnapServer manages snapshot ID allocation, snapshot deletion and tracks list of effective snapshots in the file system. A file system only has one instance of snapserver.
SnapClient: SnapClient is used to communicate with snapserver, each MDS rank has its own snapclient instance. SnapClient also caches effective snapshots locally.
Creating a snapshot
CephFS snapshot feature is enabled by default on new file system. To enable it on existing file systems, use command below.
$ ceph fs set <fs_name> allow_new_snaps true
When snapshots are enabled, all directories in CephFS will have a special
.snap directory. (You may configure a different name with the
snapdir setting if you wish.)
To create a CephFS snapshot, create a subdirectory under
.snap with a name of your choice. For example, to create a snapshot on
directory “/1/2/3/”, invoke
mkdir /1/2/3/.snap/my-snapshot-name .
This is transmitted to the MDS Server as a CEPH_MDS_OP_MKSNAP-tagged MClientRequest, and initially handled in Server::handle_client_mksnap(). It allocates a snapid from the SnapServer, projects a new inode with the new SnapRealm, and commits it to the MDLog as usual. When committed, it invokes MDCache::do_realm_invalidate_and_update_notify(), which notifies all clients with caps on files under “/1/2/3/”, about the new SnapRealm. When clients get the notifications, they update client-side SnapRealm hierarchy, link files under “/1/2/3/” to the new SnapRealm and generate a SnapContext for the new SnapRealm.
Note that this is not a synchronous part of the snapshot creation!
Updating a snapshot
If you delete a snapshot, a similar process is followed. If you remove an inode out of its parent SnapRealm, the rename code creates a new SnapRealm for the renamed inode (if SnapRealm does not already exist), saves IDs of snapshots that are effective on the original parent SnapRealm into past_parent_snaps of the new SnapRealm, then follows a process similar to creating snapshot.
Generating a SnapContext
A RADOS SnapContext consists of a snapshot sequence ID (snapid) and all the snapshot IDs that an object is already part of. To generate that list, we combine snapids associated with the SnapRealm and all valid snapids in past_parent_snaps. Stale snapids are filtered out by SnapClient’s cached effective snapshots.
Storing snapshot data
File data is stored in RADOS “self-managed” snapshots. Clients are careful to use the correct SnapContext when writing file data to the OSDs.
Storing snapshot metadata
Snapshotted dentries (and their inodes) are stored in-line as part of the directory they were in at the time of the snapshot. All dentries include a first and last snapid for which they are valid. (Non-snapshotted dentries will have their last set to CEPH_NOSNAP).
There is a great deal of code to handle writeback efficiently. When a Client receives an MClientSnap message, it updates the local SnapRealm representation and its links to specific Inodes, and generates a CapSnap for the Inode. The CapSnap is flushed out as part of capability writeback, and if there is dirty data the CapSnap is used to block fresh data writes until the snapshot is completely flushed to the OSDs.
In the MDS, we generate snapshot-representing dentries as part of the regular process for flushing them. Dentries with outstanding CapSnap data is kept pinned and in the journal.
Snapshots are deleted by invoking “rmdir” on the “.snap” directory they are rooted in. (Attempts to delete a directory which roots snapshots will fail; you must delete the snapshots first.) Once deleted, they are entered into the OSDMap list of deleted snapshots and the file data is removed by the OSDs. Metadata is cleaned up as the directory objects are read in and written back out again.
Snapshots and multiple file systems don’t interact well. Specifically, each MDS cluster allocates snapids independently; if you have multiple file systems sharing a single pool (via namespaces), their snapshots will collide and deleting one will result in missing file data for others. (This may even be invisible, not throwing errors to the user.) If each FS gets its own pool things probably work, but this isn’t tested and may not be true.