osd/OSD: Add pg_num_history comment

Signed-off-by: Matan Breizman <[email protected]>

Author: Matan-B

src/osd/OSD.h

@@ -1666,6 +1666,12 @@ class OSD : public Dispatcher,
     return osdmap ? osdmap->get_epoch() : 0;
   }
 
+  /* When handling OSDMaps pg_num_history is used to
+   * track any changes to number of PGs of each pool
+   * to be used later in order to identify PG splits and merges.
+   * See: OSD::track_pools_and_pg_num_changes
+   *      and OSDService::identify_splits_and_merges.
+   */
   pool_pg_num_history_t pg_num_history;
 
   ceph::shared_mutex map_lock = ceph::make_shared_mutex("OSD::map_lock");