mirror of https://github.com/ceph/ceph
126 lines
5.3 KiB
ReStructuredText
126 lines
5.3 KiB
ReStructuredText
======
|
|
Snaps
|
|
======
|
|
|
|
Overview
|
|
--------
|
|
Rados supports two related snapshotting mechanisms:
|
|
|
|
1. *pool snaps*: snapshots are implicitely applied to all objects
|
|
in a pool
|
|
2. *self managed snaps*: the user must provide the current *SnapContext*
|
|
on each write.
|
|
|
|
These two are mutually exclusive, only one or the other can be used on
|
|
a particular pool.
|
|
|
|
The *SnapContext* is the set of snapshots currently defined for an object
|
|
as well as the most recent snapshot (the *seq*) requested from the mon for
|
|
sequencing purposes (a *SnapContext* with a newer *seq* is considered to
|
|
be more recent).
|
|
|
|
The difference between *pool snaps* and *self managed snaps* from the
|
|
OSD's point of view lies in whether the *SnapContext* comes to the OSD
|
|
via the client's MOSDOp or via the most recent OSDMap.
|
|
|
|
See OSD::make_writeable
|
|
|
|
Ondisk Structures
|
|
-----------------
|
|
Each object has in the pg collection a *head* object (or *snapdir*, which we
|
|
will come to shortly) and possibly a set of *clone* objects.
|
|
Each hobject_t has a snap field. For the *head* (the only writeable version
|
|
of an object), the snap field is set to CEPH_NOSNAP. For the *clones*, the
|
|
snap field is set to the *seq* of the *SnapContext* at their creation.
|
|
When the OSD services a write, it first checks whether the most recent
|
|
*clone* is tagged with a snapid prior to the most recent snap represented
|
|
in the *SnapContext*. If so, at least one snapshot has occurred between
|
|
the time of the write and the time of the last clone. Therefore, prior
|
|
to performing the mutation, the OSD creates a new clone for servicing
|
|
reads on snaps between the snapid of the last clone and the most recent
|
|
snapid.
|
|
|
|
The *head* object contains a *SnapSet* encoded in an attribute, which tracks
|
|
|
|
1. The full set of snaps defined for the object
|
|
2. The full set of clones which currently exist
|
|
3. Overlapping intervals between clones for tracking space usage
|
|
4. Clone size
|
|
|
|
If the *head* is deleted while there are still clones, a *snapdir* object
|
|
is created instead to house the *SnapSet*.
|
|
|
|
Additionally, the *object_info_t* on each clone includes a vector of snaps
|
|
for which clone is defined.
|
|
|
|
Snap Removal
|
|
------------
|
|
To remove a snapshot, a request is made to the *Monitor* cluster to
|
|
add the snapshot id to the list of purged snaps (or to remove it from
|
|
the set of pool snaps in the case of *pool snaps*). In either case,
|
|
the *PG* adds the snap to its *snap_trimq* for trimming.
|
|
|
|
A clone can be removed when all of its snaps have been removed. In
|
|
order to determine which clones might need to be removed upon snap
|
|
removal, we maintain a mapping from snap to *hobject_t* using the
|
|
*SnapMapper*.
|
|
|
|
See ReplicatedPG::SnapTrimmer, SnapMapper
|
|
|
|
This trimming is performed asynchronously by the snap_trim_wq while the
|
|
pg is clean and not scrubbing.
|
|
|
|
#. The next snap in PG::snap_trimq is selected for trimming
|
|
#. We determine the next object for trimming out of PG::snap_mapper.
|
|
For each object, we create a log entry and repop updating the
|
|
object info and the snap set (including adjusting the overlaps).
|
|
#. We also locally update our *SnapMapper* instance with the object's
|
|
new snaps.
|
|
#. The log entry containing the modification of the object also
|
|
contains the new set of snaps, which the replica uses to update
|
|
its own *SnapMapper* instance.
|
|
#. The primary shares the info with the replica, which persists
|
|
the new set of purged_snaps along with the rest of the info.
|
|
|
|
|
|
Recovery
|
|
--------
|
|
Because the trim operations are implemented using repops and log entries,
|
|
normal pg peering and recovery maintain the snap trimmer operations with
|
|
the caveat that push and removal operations need to update the local
|
|
*SnapMapper* instance. If the purged_snaps update is lost, we merely
|
|
retrim a now empty snap.
|
|
|
|
SnapMapper
|
|
----------
|
|
*SnapMapper* is implemented on top of map_cacher<string, bufferlist>,
|
|
which provides an interface over a backing store such as the filesystem
|
|
with async transactions. While transactions are incomplete, the map_cacher
|
|
instance buffers unstable keys allowing consistent access without having
|
|
to flush the filestore. *SnapMapper* provides two mappings:
|
|
|
|
1. hobject_t -> set<snapid_t>: stores the set of snaps for each clone
|
|
object
|
|
2. snapid_t -> hobject_t: stores the set of hobjects with a the snapshot
|
|
as one of its snaps
|
|
|
|
Assumption: there are lots of hobjects and relatively few snaps. The
|
|
first encoding has a stringification of the object as the key and an
|
|
encoding of the set of snaps as a value. The second mapping, because there
|
|
might be many hobjects for a single snap, is stored as a collection of keys
|
|
of the form stringify(snap)_stringify(object) such that stringify(snap)
|
|
is constant length. These keys have a bufferlist encoding
|
|
pair<snapid, hobject_t> as a value. Thus, creating or trimming a single
|
|
object does not involve reading all objects for any snap. Additionally,
|
|
upon construction, the *SnapMapper* is provided with a mask for filtering
|
|
identifying the objects in the single SnapMapper keyspace belonging to that
|
|
pg.
|
|
|
|
Split
|
|
-----
|
|
The snapid_t -> hobject_t key entries are arranged such that for any pg,
|
|
up to 8 prefixes need to be checked to determine all hobjects in a particular
|
|
snap for a particular pg. Upon split, the prefixes to check on the parent
|
|
are adjusted such that only the objects remaining in the pg will be visible.
|
|
The children will immediately have the correct mapping.
|