On Mon, 2 Apr 2012, Wayne Johnson wrote:

> cat /proc/mdstat
> is your best friend when running mdm raid.  It tells you the current state of each "drive" as well as the state of the components.  
>
>
> Example from my machine:
> Personalities : [raid1]
> md0 : active raid1 sda3[1]
>       243175808 blocks [2/1] [_U]
>
> unused devices: <none>
>
> I have one raid one device, md0.  It has one dead device (the _). 


Here's what my messed-up machine is showing:


Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5] [raid4] [raid10]
md1 : active raid1 sda2[0]
       8200960 blocks [2/1] [U_]

md0 : active raid1 sda1[0]
       1945312384 blocks [2/1] [U_]

unused devices: <none>



Here's what my other (probably OK) machine is showing:

Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10] 
md1 : active raid1 sdb3[1] sda3[0]
       241641600 blocks [2/2] [UU]

md0 : active raid1 sdb1[1] sda1[0]
       497856 blocks [2/2] [UU]

unused devices: <none>


I think I'm starting to understand this a little bit.  So the md1 and md0 
are the two partitions that are being mirrored on the two drives.  When I 
see this...

[2/2] [UU]

That means things are working OK, but when I see this...

[2/1] [U_]

It means that one of the drives has failed.

Thanks!

Mike