Re: Bug in crush algorithm? 1 PG with the same OSD twice.

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Hi Frank,

CRUSH can only find 5 OSDs, given your current tree, rule, and
reweights. This is why there is a NONE in the UP set for shard 6.
But in ACTING we see that it is refusing to remove shard 6 from osd.1
-- that is the only copy of that shard, so in this case it's helping
you rather than deleting the shard altogether.
ACTING == what the OSDs are serving now.
UP == where CRUSH wants to place the shards.

I suspect that this is a case of CRUSH tunables + your reweights
putting CRUSH in a corner case of not finding 6 OSDs for that
particular PG.
If you set the reweights all back to 1, it probably finds 6 OSDs?

Cheers, Dan


On Mon, Aug 29, 2022 at 4:44 PM Frank Schilder <frans@xxxxxx> wrote:
>
> Hi all,
>
> I'm investigating a problem with a degenerated PG on an octopus 15.2.16 test cluster. It has 3Hosts x 3OSDs and a 4+2 EC pool with failure domain OSD. After simulating a disk fail by removing an OSD and letting the cluster recover (all under load), I end up with a PG with the same OSD allocated twice:
>
> PG 4.1c, UP: [6,1,4,5,3,NONE] ACTING: [6,1,4,5,3,1]
>
> OSD 1 is allocated twice. How is this even possible?
>
> Here the OSD tree:
>
> ID  CLASS  WEIGHT   TYPE NAME          STATUS     REWEIGHT  PRI-AFF
> -1         2.44798  root default
> -7         0.81599      host tceph-01
>  0    hdd  0.27199          osd.0             up   0.87999  1.00000
>  3    hdd  0.27199          osd.3             up   0.98000  1.00000
>  6    hdd  0.27199          osd.6             up   0.92999  1.00000
> -3         0.81599      host tceph-02
>  2    hdd  0.27199          osd.2             up   0.95999  1.00000
>  4    hdd  0.27199          osd.4             up   0.89999  1.00000
>  8    hdd  0.27199          osd.8             up   0.89999  1.00000
> -5         0.81599      host tceph-03
>  1    hdd  0.27199          osd.1             up   0.89999  1.00000
>  5    hdd  0.27199          osd.5             up   1.00000  1.00000
>  7    hdd  0.27199          osd.7      destroyed         0  1.00000
>
> I tried already to change some tunables thinking about https://docs.ceph.com/en/octopus/rados/troubleshooting/troubleshooting-pg/#crush-gives-up-too-soon, but giving up too soon is obviously not the problem. It is accepting a wrong mapping.
>
> Is there a way out of this? Clearly this is calling for trouble if not data loss and should not happen at all.
>
> Best regards,
> =================
> Frank Schilder
> AIT Risø Campus
> Bygning 109, rum S14
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