Hi Andras. Assuming that you've already tightened the mgr/balancer/upmap_max_deviation to 1, I suspect that this cluster already has too many upmaps.
Last time I checked, the balancer implementation is not able to improve a pg-upmap-items entry if one already exists for a PG. (It can add an OSD mapping pair to an PG, but not change an existing pair from one osd to another). So I think that what happens in this case is the balancer gets stuck in a sort of local minimum in the overall optimization. It can therefore help to simply remove some upmaps, and then wait for the balancer to do a better job when it re-creates new entries for those PGs. And there's usually some low hanging fruit -- you can start by removing pg-upmap-items which are mapping PGs away from the least full OSDs. (Those upmap entries are making the least full OSDs even *less* full.) We have a script for that: https://github.com/cernceph/ceph-scripts/blob/master/tools/upmap/rm-upmaps-underfull.py It's a pretty hacky and I don't use it often, so please use it with caution -- you can run it and review which upmaps it would remove. Hope this helps, Dan On Fri, Apr 2, 2021 at 10:18 AM Andras Pataki <apat...@flatironinstitute.org> wrote: > > Dear ceph users, > > On one of our clusters I have some difficulties with the upmap > balancer. We started with a reasonably well balanced cluster (using the > balancer in upmap mode). After a node failure, we crush reweighted all > the OSDs of the node to take it out of the cluster - and waited for the > cluster to rebalance. Obviously, this significantly changes the crush > map - hence the nice balance created by the balancer was gone. The > recovery mostly completed - but some of the OSDs became too full - so we > neded up with a few PGs that were backfill_toofull. The cluster has > plenty of space (overall perhaps 65% full), only a few OSDs are >90% (we > have backfillfull_ratio at 92%). The balancer refuses to change > anything since the cluster is not clean. Yet - the cluster can't become > clean without a few upmaps to help the top 3 or 4 most full OSDs. > > I would think this is a fairly common situation - trying to recover > after some failure. Are there any recommendations on how to proceed? > Obviously I can manually find and insert upmaps - but for a large > cluster with tens of thousands of PGs, that isn't too practical. Is > there a way to tell the balancer to still do something even though some > PGs are undersized (with a quick look at the python module - I didn't > see any)? > > The cluster is on Nautilus 14.2.15. > > Thanks, > > Andras > _______________________________________________ > ceph-users mailing list -- ceph-users@ceph.io > To unsubscribe send an email to ceph-users-le...@ceph.io _______________________________________________ ceph-users mailing list -- ceph-users@ceph.io To unsubscribe send an email to ceph-users-le...@ceph.io
