I recently created a raid1 out of 2 hard drives (8tb wd red). The content of /proc/mdstat is confusing to me.
This is it:
$ cat /proc/mdstat
Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md127 : active raid1 sde1[0] sdd1[2]
7813893952 blocks super 1.2 [2/2] [UU]
unused devices: <none>
I read an article somewhere about understanding the output. It stated, that the numbers in the brackets after the drive-names (0 and 2 in this case) are there to mark drives as active or spare (so in this case, sdd1 is considered a spare drive):
sde1[0] sdd1[2]
In other words: The sdd1[2] should in fact be sdd1[1] if it were considered an active disk in the raid.
On the other hand, it states on the end of the line below, that there are 2 of 2 disks present [2/2] and that they are ok, synced and ready to go [UU].
I tried to add sdd1 to the raid1 (as non-spare actual active disk) but I failed:
$ sudo mdadm --manage /dev/md127 --add /dev/sdd1
mdadm: Cannot open /dev/sdd1: Device or resource busy
$ sudo mdadm --grow /dev/md127 --raid-disks=2
mdadm: /dev/md127: no change requested
Note, that those messages stay the same, if the drive is mounted or not mounted .. so that is not the issue why the device is busy.
It seems to me, that they might be both active after all .. but why then is the drive showing as sdd1[2] instead of sdd1[1]?
My questions are: Is sdd1 a spare or is it active? If it is a spare: How can I activate it?
