Error message displayed when mon_osd_max_split_count would be exceeded is not as user-friendly as it could be
Under certain circumstances, an attempt to increase the PG count of a pool can fail like this:
$ ceph osd pool set foopool pg_num 512 Error E2BIG: specified pg_num 512 is too large (creating 504 new PGs on ~8 OSDs exceeds per-OSD max of 32)
This error can be addressed in two ways: either by increasing mon_osd_max_split_count or by simply increasing the pg_num in smaller increments. However, the error message does not mention either possibility.
This issue affects only mimic and luminous, because the error message (and indeed the mon_osd_max_split_count option itself) was removed in Nautilus.
The issue was partially addressed by https://github.com/ceph/ceph/commit/d2c0fe9b531 in Mimic.