I have an automated cron that sends me mdadm status to ensure everything is running fine. Sometimes it looks like this:

Personalities : [raid1]
md2 : active raid1 sda2[0] sdb2[1]
225278912 blocks [2/2] [UU]
[>....................] check = 3.1% (7080320/225278912) finish=23.4min speed=155272K/sec
bitmap: 0/2 pages [0KB], 65536KB chunk

md3 : active raid1 sda3[0] sdb3[1]
242781120 blocks [2/2] [UU]
resync=DELAYED
bitmap: 2/2 pages [8KB], 65536KB chunk

md1 : active raid1 sdb1[1] sda1[0]
261056 blocks [2/2] [UU]
resync=DELAYED

unused devices: <none>

Does that mean that the raid is not synced at that moment? Does the delay relate to the checking process?

Thanks.

Yes. It happens when you have many md devices sharing a single disk, and you are rebuilding them.

What you are seing here is resyncing between sda and sdb. It is a lot faster to resync the md devices after one another, than doing them in parallel. After md2 is finished, you will se it continues with md3 and so on. If it did all at once, the disk heads would have to move a lot - kind of like the difference between random and sequential IO.

Why is it resyncing however? These are not new drives. What happens if there is a HD crash in the middle of this?