[ceph-users] Balancer module not balancing perfectly

Steve Taylor steve.taylor at storagecraft.com
Tue Oct 30 08:14:06 PDT 2018

I have a Luminous 12.2.7 cluster with 2 EC pools, both using k=8 and
m=2. Each pool lives on 20 dedicated OSD hosts with 18 OSDs each. Each
pool has 2048 PGs and is distributed across its 360 OSDs with host
failure domains. The OSDs are identical (4TB) and are weighted with
default weights (3.73).

Initially, and not surprisingly, the PG distribution was all over the
place with PG counts per OSD ranging from 40 to 83. I enabled the
balancer module in upmap mode and let it work its magic, which reduced
the range of the per-OSD PG counts to 56-61.

While 56-61 is obviously a whole lot better than 40-83, with upmap I
expected the range to be 56-57. If I run 'ceph balancer optimize
<plan>' again to attempt to create a new plan I get 'Error EALREADY:
Unable to find further optimization,or distribution is already
perfect.' I set the balancer's max_misplaced value to 1 in case that
was preventing further optimization, but I still get the same error.

I'm sure I'm missing some config option or something that will allow it
to do better, but thus far I haven't been able to find anything in the
docs, mailing list archives, or balancer source code that helps. Any

Steve Taylor | Senior Software Engineer | StorageCraft Technology Corporation
380 Data Drive Suite 300 | Draper | Utah | 84020
Office: 801.871.2799 | 
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