Write-intent bitmaps speed-up RAID resyncs significantly, at the expense of a little write performance.
Examine array component
Displays superblock information, including last event, RAID UUID, other components, etc.
mdadm --examine /dev/sda1
If an array comes out as inactive, e.g.:
md8 : inactive sdp1(S) sds1(S) sdx1(S) sdj1(S) 7814054094 blocks super 1.2
Stop the array before we continue to work on it.
mdadm --stop /dev/md8
First, use examine the array components and figure out which component is out of sync (it's best to do this manually). Then, with the remaining components, reassemble the array. E.g. if /dev/sds1 was the bad device:
mdadm --assemble /dev/md8 /dev/sd[pxj]1 --force
to restart the array. If supposedly bad device is fine, go ahead and re-add it:
mdadm --manage /dev/md8 --add /dev/sds1