Ceph » RADOS

When an osd that is part of current up set gets chosen as an
async_recovery_target, it gets removed from the acting set.
Since we don't allow any want that is larger than the pool size,
a pg must transit into UNDERSIZED when asynchronous recovery
eventually happens.
However, if that pg has one or more backfill targets, it might
spin UNDERSIZED for a long time during which mon will keep issuing
"PG_AVAILABILITY" warns until all backfill targets finally completes.

Below is an example:

 [root@host-192-168-9-13 ~]# ceph pg 1.1fc query
        {
            "state": "active+undersized+remapped+backfilling",
            "snap_trimq": "[]",
            "snap_trimq_len": 0,
            "epoch": 16777,
            "up": [
                29,
                15,
                0
            ],
            "acting": [
                29,
                54
            ],
            "backfill_targets": [
                "15" 
            ],
            "async_recovery_targets": [
                "0" 
            ],
            "acting_recovery_backfill": [
                "0",
                "15",
                "29",
                "54" 
            ],