Degraded objects while OSD is being added/filled

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Hi cephers,

I noticed something I don't understand about ceph's behavior when adding an OSD.  When I start with a clean cluster (all PG's active+clean) and add an OSD (via ceph-deploy for example), the crush map gets updated and PGs get reassigned to different OSDs, and the new OSD starts getting filled with data.  As the new OSD gets filled, I start seeing PGs in degraded states.  Here is an example:

      pgmap v52068792: 42496 pgs, 6 pools, 1305 TB data, 390 Mobjects
            3164 TB used, 781 TB / 3946 TB avail
            8017/994261437 objects degraded (0.001%)
            2220581/994261437 objects misplaced (0.223%)
               42393 active+clean
                  91 active+remapped+wait_backfill
                   9 active+clean+scrubbing+deep
                   1 active+recovery_wait+degraded
                   1 active+clean+scrubbing
                   1 active+remapped+backfilling

Any ideas why there would be any persistent degradation in the cluster while the newly added drive is being filled?  It takes perhaps a day or two to fill the drive - and during all this time the cluster seems to be running degraded.  As data is written to the cluster, the number of degraded objects increases over time.  Once the newly added OSD is filled, the cluster comes back to clean again.

Here is the PG that is degraded in this picture:

7.87c    1    0    2    0    0    4194304    7    7    active+recovery_wait+degraded    2017-06-20 14:12:44.119921    344610'7    583572:2797    [402,521]    402    [402,521]    402    344610'7    2017-06-16 06:04:55.822503    344610'7    2017-06-16 06:04:55.822503

The newly added osd here is 521.  Before it got added, this PG had two replicas clean, but one got forgotten somehow?

Other remapped PGs have 521 in their "up" set but still have the two existing copies in their "acting" set - and no degradation is shown.  Examples:

2.f24    14282    0    16    28564    0    51014850801    3102    3102    active+remapped+wait_backfill    2017-06-20 14:12:42.650308    583553'2033479    583573:2033266    [467,521]    467    [467,499]    467    582430'2033337    2017-06-16 09:08:51.055131    582036'2030837    2017-05-31 20:37:54.831178
6.2b7d    10499    0    140    20998    0    37242874687    3673    3673    active+remapped+wait_backfill    2017-06-20 14:12:42.070019    583569'165163    583572:342128    [541,37,521]    541    [541,37,532]    541    582430'161890    2017-06-18 09:42:49.148402    582430'161890    2017-06-18 09:42:49.148402

We are running the latest Jewel patch level everywhere (10.2.7).  Any insights would be appreciated.

Andras

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