============================
 PG (Placement Group) notes
============================

Miscellaneous copy-pastes from emails, when this gets cleaned up it
should move out of /dev.

Overview
========

PG = "placement group". When placing data in the cluster, objects are
mapped into PGs, and those PGs are mapped onto OSDs. We use the
indirection so that we can group objects, which reduces the amount of
per-object metadata we need to keep track of and processes we need to
run (it would be prohibitively expensive to track eg the placement
history on a per-object basis). Increasing the number of PGs can
reduce the variance in per-OSD load across your cluster, but each PG
requires a bit more CPU and memory on the OSDs that are storing it. We
try and ballpark it at 100 PGs/OSD, although it can vary widely
without ill effects depending on your cluster. You hit a bug in how we
calculate the initial PG number from a cluster description.

There are a couple of different categories of PGs; the 6 that exist
(in the original emailer's ``ceph -s`` output) are "local" PGs which
are tied to a specific OSD. However, those aren't actually used in a
standard Ceph configuration.


Mapping algorithm (simplified)
==============================

| > How does the Object->PG mapping look like, do you map more than one object on
| > one PG, or do you sometimes map an object to more than one PG? How about the
| > mapping of PGs to OSDs, does one PG belong to exactly one OSD?
| >
| > Does one PG represent a fixed amount of storage space?

Many objects map to one PG.

Each object maps to exactly one PG.

One PG maps to a single list of OSDs, where the first one in the list
is the primary and the rest are replicas.

Many PGs can map to one OSD.

A PG represents nothing but a grouping of objects; you configure the
number of PGs you want (see
http://ceph.newdream.net/wiki/Changing_the_number_of_PGs ), number of
OSDs * 100 is a good starting point, and all of your stored objects
are pseudo-randomly evenly distributed to the PGs. So a PG explicitly
does NOT represent a fixed amount of storage; it represents 1/pg_num
'th of the storage you happen to have on your OSDs.

Ignoring the finer points of CRUSH and custom placement, it goes
something like this in pseudocode::

	locator = object_name
	obj_hash = hash(locator)
	pg = obj_hash % num_pg
	osds_for_pg = crush(pg)  # returns a list of osds
	primary = osds_for_pg[0]
	replicas = osds_for_pg[1:]

If you want to understand the crush() part in the above, imagine a
perfectly spherical datacenter in a vacuum ;) that is, if all osds
have weight 1.0, and there is no topology to the data center (all OSDs
are on the top level), and you use defaults, etc, it simplifies to
consistent hashing; you can think of it as::

	def crush(pg):
	   all_osds = ['osd.0', 'osd.1', 'osd.2', ...]
	   result = []
	   # size is the number of copies; primary+replicas
	   while len(result) < size:
	       r = hash(pg)
	       chosen = all_osds[ r % len(all_osds) ]
	       if chosen in result:
	           # osd can be picked only once
	           continue
	       result.append(chosen)
	   return result

User-visible PG States
======================

.. todo:: diagram of states and how they can overlap

*creating*
  the PG is still being created

*active*
  requests to the PG will be processed

*clean*
  all objects in the PG are replicated the correct number of times

*down*
  a replica with necessary data is down, so the pg is offline

*replay*
  the PG is waiting for clients to replay operations after an OSD crashed

*splitting*
  the PG is being split into multiple PGs (not functional as of 2012-02)

*scrubbing*
  the PG is being checked for inconsistencies

*degraded*
  some objects in the PG are not replicated enough times yet

*inconsistent*
  replicas of the PG are not consistent (e.g. objects are
  the wrong size, objects are missing from one replica *after* recovery
  finished, etc.)

*peering*
  the PG is undergoing the :doc:`/dev/peering` process

*repair*
  the PG is being checked and any inconsistencies found will be repaired (if possible)

*recovering*
  objects are being migrated/synchronized with replicas

*backfill*
  a special case of recovery, in which the entire contents of
  the PG are scanned and synchronized, instead of inferring what
  needs to be transferred from the PG logs of recent operations

*incomplete*
  a pg is missing a necessary period of history from its
  log.  If you see this state, report a bug, and try to start any
  failed OSDs that may contain the needed information.

*stale*
  the PG is in an unknown state - the monitors have not received
  an update for it since the PG mapping changed.

*remapped*
  the PG is temporarily mapped to a different set of OSDs from what
  CRUSH specified

