[ceph-users] reducing min_size on erasure coded pool may allow recovery ?

Chad W Seys cwseys at physics.wisc.edu
Tue Oct 30 13:13:08 PDT 2018


Thanks for the clarification!  Glad to see this feature is being pursued.

Chad.


On 10/30/2018 12:24 PM, Gregory Farnum wrote:
> On Mon, Oct 29, 2018 at 7:43 PM David Turner <drakonstein at gmail.com 
> <mailto:drakonstein at gmail.com>> wrote:
> 
>     min_size should be at least k+1 for EC. There are times to use k for
>     emergencies like you had. I would suggest seeing it back to 3 once
>     your back to healthy.
> 
>     As far as why you needed to reduce min_size, my guess would be that
>     recovery would have happened as long as k copies were up. Were the
>     PG's refusing to backfill or just hang backfilled yet?
> 
> 
> Recovery on EC pools requires min_size rather than k shards at this 
> time. There were reasons; they weren't great. We're trying to get a fix 
> tested and merged at https://github.com/ceph/ceph/pull/17619
> -Greg
> 
> 
> 
>     On Mon, Oct 29, 2018, 9:24 PM Chad W Seys <cwseys at physics.wisc.edu
>     <mailto:cwseys at physics.wisc.edu>> wrote:
> 
>         Hi all,
>             Recently our cluster lost a drive and a node (3 drives) at
>         the same
>         time.  Our erasure coded pools are all k2m2, so if all is working
>         correctly no data is lost.
>             However, there were 4 PGs that stayed "incomplete" until I
>         finally
>         took the suggestion in 'ceph health detail' to reduce min_size .
>         (Thanks
>         for the hint!)  I'm not sure what it was (likely 3), but setting
>         it to 2
>         caused all PGs to become active (though degraded) and the
>         cluster is on
>         path to recovering fully.
> 
>             In replicated pools, would not ceph create replicas without
>         the need
>         to reduce min_size?  It seems odd to not recover automatically if
>         possible.  Could someone explain what was going on there?
> 
>             Also, how to decide what min_size should be?
> 
>         Thanks!
>         Chad.
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