ceph/doc/dev/osd_internals/log_based_pg.rst

============
Log Based PG
============

Background
==========

Why ReplicatedPG?
-----------------

Currently, consistency for all ceph pool types is ensured by primary
log-based replication.  This goes for both erasure-coded and
replicated pools.  If you ever find yourself asking "why is it that
both replicated and erasure coded pools are implemented by
ReplicatedPG.h/cc", that's why (though it definitely should be called
LogBasedPG, should include peering, and PG should be an abstract
interface defining only those things the OSD needs to know to route
messages etc -- but we live in an imperfect world where git deals
imperfectly with cherry-picking between branches where the file has
different names).

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 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 asyncronous -- 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 and objects which aren't uptodate locally (see recovery and
backfill).  In practice, the log will extend much further
(osd_pg_min_log_entries when clean, osd_pg_max_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/ReplicatedPG: 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 4+2 encoding
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 PGLog.h/cc, osd_types.h:pg_log_t,
osd_types.h:pg_log_entry_t, and peering in general.

ReplicatedBackend/ECBackend unification strategy
================================================

PGBackend
---------

So, 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
implementations of ReplicatedPG, one for each of the two, if there are
really only a few fundamental differences:

  1. How reads work -- async only, requires remote reads for ec
  2. How writes work -- either restricted to append, or must
     write aside and do a tpc
  3. Whether we choose the oldest or newest possible head entry
     during peering
  4. A bit of extra information in the log entry to enable rollback

and so many similarities

  1. All of the stats and metadata for objects
  2. The high level locking rules for mixing client IO with recovery
     and scrub
  3. The high level locking rules for mixing reads and writes without
     exposing uncommitted state (which might be rolled back or
     forgotten later)
  4. The process, metadata, and protocol needed to determine the set
     of osds which partcipated in the most recent interval in which we
     accepted writes
  5. etc.

Instead, we choose a few abstractions (and a few kludges) to paper
over the difference:

  1. PGBackend
  2. PGTransaction
  2. PG::choose_acting chooses between calc_replicated_acting and
     calc_ec_acting
  3. 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
  4. Misc other kludges here and there

PGBackend and PGTransaction enables abstraction of differences 1, 2,
and the addition of 4 as needed to the log entries.

The replicated implementation is in ReplicatedBackend.h/cc and doesn't
require much explanation, I think.  More detail on the ECBackend can be
found in doc/dev/osd_internals/erasure_coding/ecbackend.rst.

PGBackend Interface Explanation
===============================

Note: this is from a design document from before the original firefly
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 do a read, and we need it on the primary.  For
this reason, PGBackend needs to include some interfaces for determing
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

Core Changes:

- PGBackend needs to be able to return
	IsPG(Recoverable|Readable)Predicate objects to allow the user
	to make these determinations.

Client Reads
------------

Reads with the replicated strategy can always be satisfied
synchronously out of the primary OSD.  With an erasure coded strategy,
the primary will need to request data from some number of replicas in
order to satisfy a read.  PGBackend will therefore need to provide
seperate objects_read_sync and objects_read_async interfaces where
the former won't be implemented by the ECBackend.

PGBackend interfaces:

- objects_read_sync
- objects_read_async

Scrub
-----

We currently have two scrub modes with different default frequencies:

1. [shallow] scrub: compares the set of objects and metadata, but not
   the contents
2. 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.


PGBackend interfaces:

- be_*

Recovery
--------

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 "unfound" concept
should probably then 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.


Core changes:

- s/unfound/unrecoverable

PGBackend interfaces:

- `on_local_recover_start <https://github.com/ceph/ceph/blob/firefly/src/osd/PGBackend.h#L60>`_
- `on_local_recover <https://github.com/ceph/ceph/blob/firefly/src/osd/PGBackend.h#L66>`_
- `on_global_recover <https://github.com/ceph/ceph/blob/firefly/src/osd/PGBackend.h#L78>`_
- `on_peer_recover <https://github.com/ceph/ceph/blob/firefly/src/osd/PGBackend.h#L83>`_
- `begin_peer_recover <https://github.com/ceph/ceph/blob/firefly/src/osd/PGBackend.h#L90>`_