Comment 3 for bug 1492742

This warning appears to be due to a change that went into the Hammer release of Ceph in commit https://github.com/ceph/ceph/commit/7f3dcdb089b4342e8624d37abfe5437db8a15c39. The default threshold for the mon_pg_warn_max_per_osd has since been tuned down to 300, however in this commit the warning is raised if there are more than 300 Placement Groups which are spread across the number of OSDs which are UP & IN the cluster.

For >= Hammer releases of Ceph, I think what Ed lays out is appropriate - increasing the number of OSDs available (which is more likely in a production cluster), increasing the number of replicas required, or decreasing the number of services which use Ceph.

Another option is to simply change the default threshold at which the warnings appear. Not necessarily ideal, but it is a tunable parameter. It's a monitor option and is settable via the following key:

mon pg warn max per osd = 500

For the record, if you undersize things and put fewer than 30 pgs per OSD in the cluster, you'll also get a warning.