mdadm: Resume initial sync

Solution 1:

You need to use the repair action - ie: echo repair > /sys/block/md2/md/sync_action

In theory you can use the array during the repair, but I would let this first-time repair/resync to finish before putting valuable data on the disks.

From md man page:

md/sync_action

This can be used to monitor and control the resync/recovery process of MD. In particular, writing "check" here will cause the array to read all data block and check that they are consistent (e.g. parity is correct, or all mirror replicas are the same). Any discrepancies found are NOT corrected. A count of problems found will be stored in md/mismatch_count. Alternately, "repair" can be written which will cause the same check to be performed, but any errors will be corrected. Finally, "idle" can be written to stop the check/repair process.

In other words, resync is ignored because it is not an accepted action - while check, repair and idle are.