...

2. If a down host comes up again and it's osd are started. Is data

still being copied, or does ceph see that checksums(?)

PG or RADOS object epoch I think. So if data hasn’t changed, the recovery completes without having anything to do.

...

are the same and just sets a pointer(?)

back to the old location? Yes I mean the osdmap having the old osd on the node that was down.

Hmmm, I currently have pg's in 'active+remapped+backfill_wait' of a pool rbd.backup. Of which I know nothing in it has changed. And those osd's listed[1] are already up. Especially when you are getting at the 'end' of recovery where osd_max_backfills=X has less effect and recovery takes longer. It would be nice to have the "no work needed pg's" be processed quickly. [1] [18,25,41]p18 [18,41,17] [5,0,25]p5 [5,0,4