In mdadm's /proc/mdstat, what does [U_U] mean?

Question

I just set up a raid 5 array for my media server. I set got 2 new disks, and set up an array with one missing drive, to copy the data from the old single drive to the new array, and later I will add the old drive into the array (after the data has been copied).

This is all going fine. I'm just curious about the contents of /proc/mdstat

Personalities : [raid6] [raid5] [raid4]
md127 : active raid5 sdc1[2] sda1[0]
      3906762752 blocks super 1.2 level 5, 1024k chunk, algorithm 2 [3/2] [U_U]

unused devices: <none>

What does [U_U] mean on the third line? when I see it, I think of an emoticon unhappy face, which kind of matches the state of the array, but I don't think that is it :).

Answer 1

It means 1 device is missing.

• U: device is up
• _: device is down/missing

For more information, read the md status line wiki page.

Answer 2

It's showing the drives, and that one drive is missing.

The 3/2 also represents this, showing that there are supposed to be 3 drives in the array, but only 2 are up.