Hi Frank, Just a guess, but I wonder if for small values rounding/precision start to impact the placement like you observed. Do you see the same issue if you reweight to 2x the original? -- Dan On Tue, Nov 15, 2022 at 10:09 AM Frank Schilder <frans@xxxxxx> wrote: > > Hi all, > > I re-weighted all OSDs in a pool down from 1.0 to the same value 0.052 (see reason below). After this, all hell broke loose. OSDs were marked down, slow OPS all over the place and the MDSes started complaining about slow ops/requests. Basically all PGs were remapped. After setting all re-weights back to 1.0 the situation went back to normal. > > Expected behaviour: No (!!!) PGs are remapped and everything continues to work. Why did things go down? > > More details: We have 24 OSDs with weight=1.74699 in a pool. I wanted to add OSDs with weight=0.09099 in such a way that the small OSDs receive approximately the same number of PGs as the large ones. Setting a re-weight factor of 0.052 for the large ones should achieve just that: 1.74699*0.05=0.09084. So, procedure was: > > - ceph osd crush reweight osd.N 0.052 for all OSDs in that pool > - add the small disks and re-balance > > I would expect that the crush mapping is invariant under a uniform change of weight. That is, if I apply the same relative weight-change to all OSDs (new_weight=old_weight*common_factor) in a pool, the mappings should be preserved. However, this is not what I observed. How is it possible that PG mappings change if the relative weight of all OSDs to each other stays the same (the probabilities of picking an OSD are unchanged over all OSDs)? > > Thanks for any hints. > > Best regards, > ================= > Frank Schilder > AIT Risø Campus > Bygning 109, rum S14 > _______________________________________________ > ceph-users mailing list -- ceph-users@xxxxxxx > To unsubscribe send an email to ceph-users-leave@xxxxxxx _______________________________________________ ceph-users mailing list -- ceph-users@xxxxxxx To unsubscribe send an email to ceph-users-leave@xxxxxxx
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