Hi again, Oops, I'd missed the part about some PGs being degraded, which prevents the balancer from continuing. So I assume that you have PGs which are simultaneously undersized+backfill_toofull? That case does indeed sound tricky. To solve that you would either need to move PGs out of the toofull OSD, to make room for the undersized PGs; or, upmap those undersized PGs to some other less-full OSDs. For the former, you could either use the rm-upmaps-underfull script and hope that it incidentally moves data out of those toofull OSDs. Or a similar script with some variables reversed could be used to remove any upmaps which are directing PGs *to* those toofull OSDs. Or maybe it will be enough to just reweight those OSDs to 0.9. -- Dan On Fri, Apr 2, 2021 at 10:47 AM Dan van der Ster <dan@vanderster.com> wrote:
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-u... 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 <apataki@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-leave@ceph.io