Placement Groups stuck in activating
When migrating from FileStore with BlueStore with Ceph Luminuous you might run into the problem that certain Placement Groups stay stuck in the activating state.
This is a side-effect of the new PG overdose protection in Ceph Luminous.
Too many PGs on your OSDs can cause serious performance or availability problems.
You can see the amount of Placement Groups per OSD using this command:
$ ceph osd df
Increase Max PG per OSD
The default value is a maximum of 200 PGs per OSD and you should stay below that! However, if you are hit by PGs in the activating state you can set this configuration value:
[global] mon_max_pg_per_osd = 500
Then restart the OSDs and MONs which are serving the affected by this.
Usually you shouldn’t run into this, but if this hits you in the middle of a migration or upgrade this might save you.