mirror of
https://github.com/ceph/ceph
synced 2024-12-25 04:43:17 +00:00
145 lines
4.7 KiB
ReStructuredText
145 lines
4.7 KiB
ReStructuredText
============================
|
|
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.com/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
|
|
|