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diff --git a/doc/dev/osd_internals/snaps.rst b/doc/dev/osd_internals/snaps.rst new file mode 100644 index 000000000..5ebd0884a --- /dev/null +++ b/doc/dev/osd_internals/snaps.rst @@ -0,0 +1,128 @@ +====== +Snaps +====== + +Overview +-------- +Rados supports two related snapshotting mechanisms: + + 1. *pool snaps*: snapshots are implicitly 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 PrimaryLogPG::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). + If the object is a clone which no longer belongs to any live snapshots, + it is removed here. (See PrimaryLogPG::trim_object() when new_snaps + is empty.) + #. 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 file system +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 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 +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. |