Hi, Although I am yet to fully understand the logic of the placement group recovery ( I'm eager to read Sam's doc/dev/osd_internals/pg_recovery.rst :-), I wrote down my understanding of backfilling : http://dachary.org/?p=2009 . Cheers On 05/25/2013 09:15 PM, Loic Dachary wrote: > Hi ! > > On 05/25/2013 08:06 PM, Samuel Just wrote: >> Hi, thanks for taking the time to try to get all this documented! >> >> Placement groups are assigned to a set of OSDs by crush. >> >> (4.1, osdmap(e 1)) --CRUSH--> [3,1,2] >> >> where the primary is 3. When 3 dies, the osdmap is updated to reflect this >> and we get a new mapping for pg 4.1: >> >> (4,1, osdmap(e 2)) --CRUSH--> [1,2,4] >> >> Here, 1 and 2 already have up-to-date copies of 4.1. osd 4, however, needs >> to be brought up to date. During peering, osd 1 will learn that osd 4 >> falls into >> 1 of 2 cases. >> >> Case 1 is that osd 4 already had an old copy of pg 4.1 AND its pg log for pg >> 4.1 happens to overlap osd 1's pg log for pg 4.1. In that case, by running >> through the log of operations, we can determine exactly which objects need >> to be copied over. We usually refer to this as just "recovery" (or log based >> recovery). >> >> In case 2, either osd 4's pg log does not overlap that of osd 1. In this case, >> we cannot determine from the log which objects need to be copied over. >> To bring osd 4 up to date, we therefore need to backfill. >> >> Backfill involves the primary and the backfill peer (there is only ever one in >> the acting set at a time, see PG::choose_acting) scanning over their pg stores >> and copying the objects which are different or missing from the primary to the >> backfill peer. Because this may take a long time, we track the a last_backfill >> attribute for each local pg copy indicating how far the local copy has been >> backfilled. In the case that the copy is complete, last_backfill is >> hobject_t::max(). > > Is it true that if two osd briefly disconnect while backfilling, they may be in the case 1 above (i.e. log based recovery ) and then backfilling again when done, starting from last_backfill and up ? > >> More exactly, a local pg copy is described by a few pieces of information: >> 1) the local pg log > > pg_log_t https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1371 > pg_log_entry_t https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1277 > >> 2) the local last_backfill > > pg_info_t::last_backfill https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1102 > >> 3) the local last_complete > > pg_info_t::last_complete https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1089 > >> 4) the local missing set > > pg_missing_t https://github.com/ceph/ceph/blob/master/src/osd/osd_types.h#L1468 > >> The local pg store reflects all updates up to version last_complete on all > > I assume you mean 'local pg log' instead of 'local pg log'. > >> hobject_ts hoid such that hoid < last_backfill AND hoid is not in the missing >> set. Comparing the pg logs is used to fill in the missing set for OSDs which >> were only down for a brief period thus avoiding a costly backfill in many cases. > > The pg logs are trimmed ( https://github.com/ceph/ceph/blob/master/src/osd/PG.cc#L216 ), this is why the pg logs of two OSDs that have been disconnected for too long are unlikely to overlap ? And therefore require a backfill because the two pg logs cannot be compared ? > >> This is a bit of a rough brain dump and may be somewhat misleading/wrong. > > It is very helpful as it is, thanks :-) > >> I'll get it cleaned up and put it into >> doc/dev/osd_internals/pg_recovery.rst next >> week. >> > > That would be great. > >> Also, rados objects currently have three pieces: >> 1) data - read, write, writefull, etc. >> 2) xattrs >> 3) omap >> The omap is much like the xattrs except that it can generally store a much >> larger number of keys and support efficient scans. It's used at the moment >> for a few things including rgw bucket indices. The omap entries are copied >> over along with the rest of the object in recovery. Behind the scenes, all >> omap entries for all objects stored on an OSD are stored prefixed in a single >> big leveldb instance. >> >> omap operations probably shouldn't be supported on objects in an >> ErasureCodedPG :) > > I thought omap / xattrs were mutually exclusive. I did not realize both could be used at the same time. > > Cheers > >> -Sam >> >> On Sat, May 25, 2013 at 10:37 AM, Loic Dachary <loic@xxxxxxxxxxx> wrote: >>> >>> >>> On 05/25/2013 04:48 PM, Leen Besselink wrote: >>>> On Sat, May 25, 2013 at 04:27:16PM +0200, Loic Dachary wrote: >>>>> >>>>> >>>>> On 05/25/2013 02:33 PM, Leen Besselink wrote: >>>>> Hi Leen, >>>>> >>>>>> - a Cehp object can store keys/values, not just data >>>>> >>>>> I did not know that. Could you explain or give me the URL ? >>>>> >>>> >>>> Well, I got that impression from some of the earlier talks and from this blog post: >>>> >>>> http://ceph.com/community/my-first-impressions-of-ceph-as-a-summer-intern/ >>>> >>>> But I haven't read it in while. >>>> >>>> But at this time I only see something like: >>>> >>>> http://ceph.com/docs/master/rados/api/librados/?highlight=rados_getxattr#rados_getxattr >>>> >>>> Which looks like it is storing it in filesystem attributes. >>>> >>>> So maybe an object can be a piece of data or a key/value store. >>> >>> Thanks for explaining: I did not know about the works of Eleanor Cawthon. I knew about the objects xattributes but I thought you meant that the data inside of the object could be structured as key/value pairs. My bad :-) >>> >>> Cheers >>> >>> -- >>> Loïc Dachary, Artisan Logiciel Libre >>> All that is necessary for the triumph of evil is that good people do nothing. >>> > -- Loïc Dachary, Artisan Logiciel Libre All that is necessary for the triumph of evil is that good people do nothing.
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