PG's become undersize+degraded if OSD's restart during backfill

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Hi,

I had recently re-added some old OSD's by zapping them and reintroducing them into cluster as new OSD's. I'm using Ansible to add
the OSD's and because there was an outstanding config change, it restarted all OSD's on the host where I was adding the OSD's at the
end of the play.

I noticed something a bit strange when this happened. First here is the ceph.log just before the restart happened

2016-12-05 15:53:49.234039 mon.0 10.1.2.71:6789/0 5394558 : cluster [INF] pgmap v11064938: 4352 pgs: 1 active+remapped+backfilling,
9 active+clean+scrubbing, 437 active+remapped+wait_backfill, 3 active+clean+scrubbing+deep, 3902 active+clean; 70290 GB data, 207 TB
used, 114 TB / 321 TB avail; 1796 kB/s rd, 449 B/s wr, 0 op/s; 112/58482219 objects degraded (0.000%); 5751145/58482219 objects
misplaced (9.834%); 150 MB/s, 37 objects/s recovering
2016-12-05 15:53:49.436154 mon.0 10.1.2.71:6789/0 5394559 : cluster [INF] osd.50 marked itself down
2016-12-05 15:53:49.436334 mon.0 10.1.2.71:6789/0 5394560 : cluster [INF] osd.59 marked itself down
2016-12-05 15:53:49.436555 mon.0 10.1.2.71:6789/0 5394561 : cluster [INF] osd.56 marked itself down
2016-12-05 15:53:49.437382 mon.0 10.1.2.71:6789/0 5394562 : cluster [INF] osd.54 marked itself down
2016-12-05 15:53:49.437560 mon.0 10.1.2.71:6789/0 5394563 : cluster [INF] osd.57 marked itself down
2016-12-05 15:53:49.437650 mon.0 10.1.2.71:6789/0 5394564 : cluster [INF] osd.52 marked itself down
2016-12-05 15:53:49.438224 mon.0 10.1.2.71:6789/0 5394565 : cluster [INF] osd.58 marked itself down
2016-12-05 15:53:49.438599 mon.0 10.1.2.71:6789/0 5394566 : cluster [INF] osd.49 marked itself down
2016-12-05 15:53:49.438717 mon.0 10.1.2.71:6789/0 5394567 : cluster [INF] osd.55 marked itself down
2016-12-05 15:53:49.439303 mon.0 10.1.2.71:6789/0 5394568 : cluster [INF] osd.48 marked itself down
2016-12-05 15:53:49.439399 mon.0 10.1.2.71:6789/0 5394569 : cluster [INF] osd.51 marked itself down
2016-12-05 15:53:49.439710 mon.0 10.1.2.71:6789/0 5394570 : cluster [INF] osd.53 marked itself down
2016-12-05 15:53:49.966611 mon.0 10.1.2.71:6789/0 5394571 : cluster [INF] osdmap e2089614: 60 osds: 47 up, 59 in

You can see there was only a handful of degraded objects and no undersized ones.

During the period between the down and up of OSD's, lots of messages like these:

2016-12-05 15:53:57.924148 osd.15 10.1.111.2:6800/4024 2734 : cluster [INF] 1.c58 continuing backfill to osd.49 from
(2060554'272739,2089547'275741] MIN to 208954
7'275741
2016-12-05 15:53:58.569751 osd.5 10.1.111.1:6866/1787721 4143 : cluster [INF] 1.bc6 continuing backfill to osd.10 from
(2049483'242150,2089562'245156] MIN to 2089
562'245156
2016-12-05 15:53:58.569760 osd.5 10.1.111.1:6866/1787721 4144 : cluster [INF] 1.bc6 continuing backfill to osd.28 from
(2049483'242150,2089562'245156] MIN to 2089
562'245156
2016-12-05 15:53:58.569827 osd.5 10.1.111.1:6866/1787721 4145 : cluster [INF] 1.bc6 continuing backfill to osd.50 from
(2049490'242155,2089562'245156] MIN to 2089
562'245156
2016-12-05 15:53:58.569883 osd.5 10.1.111.1:6866/1787721 4146 : cluster [INF] 1.add continuing backfill to osd.19 from
(2064881'215318,2089235'218318] MIN to 2089
235'218318
2016-12-05 15:53:58.569933 osd.5 10.1.111.1:6866/1787721 4147 : cluster [INF] 1.add continuing backfill to osd.40 from
(2064881'215318,2089235'218318] MIN to 2089
235'218318
2016-12-05 15:53:58.570026 osd.5 10.1.111.1:6866/1787721 4148 : cluster [INF] 1.add continuing backfill to osd.50 from
(2064881'215318,2089235'218318] MIN to 2089
235'218318
2016-12-05 15:53:58.570816 osd.5 10.1.111.1:6866/1787721 4149 : cluster [INF] 1.cc9 continuing backfill to osd.50 from
(2067348'326147,2089547'329149] MIN to 2089
547'329149


After the restart:

2016-12-05 15:54:30.983780 mon.0 10.1.2.71:6789/0 5394697 : cluster [INF] pgmap v11064986: 4352 pgs: 1
active+undersized+degraded+remapped+backfilling, 5 active+degraded, 142 active+recovery_wait+degraded, 1 remapped+peering, 191
active+undersized+degraded+remapped+wait_backfill, 11 active+clean+scrubbing, 246 active+remapped+wait_backfill, 3755 active+clean;
70290 GB data, 207 TB used, 114 TB / 321 TB avail; 74496 kB/s rd, 179 kB/s wr, 57 op/s; 819165/57663296 objects degraded (1.421%);
4930596/57663296 objects misplaced (8.551%); 5950 kB/s, 6 objects/s recovering

I have lots of undersized PG's and quite a lot of degraded objects. However, I do notice that the degraded objects number will not
move and then suddenly jump down by several thousand. Almost like as the PG is completed backfilling, all the objects are marked as
healthy again.

And an example PG dump
1.e8c   4554    0       4554    4554    0       18995052544     3040    3040    active+undersized+degraded+remapped+wait_backfill
2016-12-05 15:53:58.0540892090202'516479  2090202:308795  [17,49,40]      17      [17,40] 17      2074023'511913  2016-12-04
22:51:46.035122      2074023'511913  2016-12-04 22:51:46.035122

49 is one of the OSD's I reintroduced.

Questions
1. Are these PG's really undersized?
2. Why did restarting the OSD's cause the reported degraded objects to change?

Nick

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