Still didn't find out what will happen when the pool is full - but tried a 
little bit in our testing environment and i were not able to get the pool full 
before an OSD got full. So in first place one OSD reached the full ratio (pool 
not quite full, about 98%) and IO stopped (like expected when an OSD reaches 
full ratio).

I were able to re-balance the OSDs by manually doing reweights. Now, the 
cluster is much more balanced and even the pool shows more free space (about 
75% used).

Also the pg-autoscaler does not really play well with the stretch crush rule 
... had to increase / adjust the PGs manually to get a better distribution.

Regards,
Kilian
________________________________
Von: Kilian Ries <m...@kilian-ries.de>
Gesendet: Mittwoch, 19. April 2023 12:18:06
An: ceph-users
Betreff: [ceph-users] Ceph stretch mode / POOL_BACKFILLFULL

Hi,


we run a ceph cluster in stretch mode with one pool. We know about this bug:


https://tracker.ceph.com/issues/56650

https://github.com/ceph/ceph/pull/47189


Can anyone tell me what happens when a pool gets to 100% full? At the moment 
raw OSD usage is about 54% but ceph throws me a "POOL_BACKFILLFULL" error:


$ ceph df

--- RAW STORAGE ---

CLASS    SIZE   AVAIL    USED  RAW USED  %RAW USED

ssd    63 TiB  29 TiB  34 TiB    34 TiB      54.19

TOTAL  63 TiB  29 TiB  34 TiB    34 TiB      54.19



--- POOLS ---

POOL             ID  PGS   STORED  OBJECTS     USED  %USED  MAX AVAIL

.mgr              1    1  415 MiB      105  1.2 GiB   0.04    1.1 TiB

vm_stretch_live   2   64   15 TiB    4.02M   34 TiB  95.53    406 GiB



So the pool warning / calculation is just a bug, because it thinks its 50% of 
the total size. I know ceph will stop IO / set OSDs to read only if the hit a 
"backfillfull_ratio" ... but what will happen if the pool gets to 100% full ?


Will IO still be possible?


No limits / quotas are set on the pool ...


Thanks

Regards,

Kilian
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