2018-02-08 23:38 GMT+01:00 Simon Ironside <sironside@xxxxxxxxxxxxx>:
-- Hi Everyone,
I recently added an OSD to an active+clean Jewel (10.2.3) cluster and was surprised to see a peak of 23% objects degraded. Surely this should be at or near zero and the objects should show as misplaced?
I've searched and found Chad William Seys' thread from 2015 but didn't see any conclusion that explains this:
http://lists.ceph.com/pipermail/ceph-users-ceph.com/2015- July/003355.html
I agree, I always viewed it as if you had three copies of your PG, add a new OSD and that PG decides one of the copies should be on that OSD instead of one of the 3 older ones, it would just stop caring about the old PG, create a new empty PG on the new OSD and then as the synch is going towards the new PG it is "behind" in the data it contains until sync is done, but it (and its 2 previous copies) are correctly placed for the new crush map. Misplaced would probably be a more natural way of seeing it, at least if the now-abandoned PG was still being updated while the sync is done, but I don't think it is. It gets orphaned rather quickly as the new OSD kicks in.
I guess this design choice boils down to "being able to handle someone adding more OSDs to a cluster that is close to getting full", at the expense of "discarding one or more of the old copies and scaring the admin as if there was a huge issue when just adding one or many new shiny OSDs".
May the most significant bit of your life be positive.
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