This document is for a development version of Ceph.
Log Based PG¶
Currently, consistency for all ceph pool types is ensured by primary log-based replication. This goes for both erasure-coded (EC) and replicated pools.
Primary log-based replication¶
Reads must return data written by any write which completed (where the
client could possibly have received a commit message). There are lots
of ways to handle this, but Ceph’s architecture makes it easy for
everyone at any map epoch to know who the primary is. Thus, the easy
answer is to route all writes for a particular PG through a single
ordering primary and then out to the replicas. Though we only
actually need to serialize writes on a single RADOS object (and even then,
the partial ordering only really needs to provide an ordering between
writes on overlapping regions), we might as well serialize writes on
the whole PG since it lets us represent the current state of the PG
using two numbers: the epoch of the map on the primary in which the
most recent write started (this is a bit stranger than it might seem
since map distribution itself is asynchronous – see Peering and the
concept of interval changes) and an increasing per-PG version number
– this is referred to in the code with type
eversion_t and stored as
pg_info_t::last_update. Furthermore, we maintain a log of “recent”
operations extending back at least far enough to include any
unstable writes (writes which have been started but not committed)
and objects which aren’t uptodate locally (see recovery and
backfill). In practice, the log will extend much further
osd_min_pg_log_entries when clean and
osd_max_pg_log_entries when not
clean) because it’s handy for quickly performing recovery.
Using this log, as long as we talk to a non-empty subset of the OSDs
which must have accepted any completed writes from the most recent
interval in which we accepted writes, we can determine a conservative
log which must contain any write which has been reported to a client
as committed. There is some freedom here, we can choose any log entry
between the oldest head remembered by an element of that set (any
newer cannot have completed without that log containing it) and the
newest head remembered (clearly, all writes in the log were started,
so it’s fine for us to remember them) as the new head. This is the
main point of divergence between replicated pools and EC pools in
PG/PrimaryLogPG: replicated pools try to choose the newest valid
option to avoid the client needing to replay those operations and
instead recover the other copies. EC pools instead try to choose
the oldest option available to them.
The reason for this gets to the heart of the rest of the differences
in implementation: one copy will not generally be enough to
reconstruct an EC object. Indeed, there are encodings where some log
combinations would leave unrecoverable objects (as with a
where 3 of the replicas remember a write, but the other 3 do not – we
don’t have 3 copies of either version). For this reason, log entries
representing unstable writes (writes not yet committed to the
client) must be rollbackable using only local information on EC pools.
Log entries in general may therefore be rollbackable (and in that case,
via a delayed application or via a set of instructions for rolling
back an inplace update) or not. Replicated pool log entries are
never able to be rolled back.
For more details, see
osd_types.h:pg_log_entry_t, and peering in general.
ReplicatedBackend/ECBackend unification strategy¶
The fundamental difference between replication and erasure coding
is that replication can do destructive updates while erasure coding
cannot. It would be really annoying if we needed to have two entire
PrimaryLogPG since there
are really only a few fundamental differences:
How reads work – async only, requires remote reads for EC
How writes work – either restricted to append, or must write aside and do a tpc
Whether we choose the oldest or newest possible head entry during peering
A bit of extra information in the log entry to enable rollback
and so many similarities
All of the stats and metadata for objects
The high level locking rules for mixing client IO with recovery and scrub
The high level locking rules for mixing reads and writes without exposing uncommitted state (which might be rolled back or forgotten later)
The process, metadata, and protocol needed to determine the set of osds which participated in the most recent interval in which we accepted writes
Instead, we choose a few abstractions (and a few kludges) to paper over the differences:
Various bits of the write pipeline disallow some operations based on pool type – like omap operations, class operation reads, and writes which are not aligned appends (officially, so far) for EC
Misc other kludges here and there
PGTransaction enable abstraction of differences 1 and 2 above
and the addition of 4 as needed to the log entries.
The replicated implementation is in
ReplicatedBackend.h/cc and doesn’t
require much additional explanation. More detail on the
ECBackend can be
PGBackend Interface Explanation¶
Note: this is from a design document that predated the Firefly release and is probably out of date w.r.t. some of the method names.
Readable vs Degraded¶
For a replicated pool, an object is readable IFF it is present on
the primary (at the right version). For an EC pool, we need at least
m shards present to perform a read, and we need it on the primary. For
PGBackend needs to include some interfaces for determining
when recovery is required to serve a read vs a write. This also
changes the rules for when peering has enough logs to prove that it
PGBackendneeds to be able to return
IsPG(Recoverable|Readable)Predicateobjects to allow the user to make these determinations.
Reads from a replicated pool can always be satisfied
synchronously by the primary OSD. Within an erasure coded pool,
the primary will need to request data from some number of replicas in
order to satisfy a read.
PGBackend will therefore need to provide
objects_read_async interfaces where
the former won’t be implemented by the
We currently have two scrub modes with different default frequencies:
[shallow] scrub: compares the set of objects and metadata, but not the contents
deep scrub: compares the set of objects, metadata, and a CRC32 of the object contents (including omap)
The primary requests a scrubmap from each replica for a particular range of objects. The replica fills out this scrubmap for the range of objects including, if the scrub is deep, a CRC32 of the contents of each object. The primary gathers these scrubmaps from each replica and performs a comparison identifying inconsistent objects.
Most of this can work essentially unchanged with erasure coded PG with
the caveat that the
PGBackend implementation must be in charge of
actually doing the scan.
The logic for recovering an object depends on the backend. With the current replicated strategy, we first pull the object replica to the primary and then concurrently push it out to the replicas. With the erasure coded strategy, we probably want to read the minimum number of replica chunks required to reconstruct the object and push out the replacement chunks concurrently.
Another difference is that objects in erasure coded PG may be
unrecoverable without being unfound. The
should probably be renamed to
unrecoverable. Also, the
PGBackend implementation will have to be able to direct the search
for PG replicas with unrecoverable object chunks and to be able
to determine whether a particular object is recoverable.