----- Message from michael.harris@rmit.edu.au ---------
     Date: Wed, 23 Jan 2008 16:05:40 +1100
     From: Michael Harris [off-list ref]
Reply-To: Michael Harris [off-list ref]
  Subject: identifying failed disk/s in an array.
       To: linux-raid@vger.kernel.org

Hi,

I have just built a Raid 5 array  using mdadm and while it is   
running fine I have a question, about identifying the order of disks  
 in the array.

In the pre sata days you would connect your drives as follows:

Primary Master - HDA
Primary Slave - HDB
Secondary - Master - HDC
Secondary - Slave -HDD

So if disk HDC failed i would know it was the primary disk on the   
secondary controller and would replace that drive.

My current setup is as follows

MB Primary Master (PATA) Primary Master - Operating System

The array disks are attached to:

MB Sata port 1
MB Sata port 2
PCI card Sata port 1

When i setup the array the OS drive was SDA and the other SDB,SDC,SDD.

Now the problem is everytime i reboot, the drives are sometimes   
detected in a different order, now because i mount root via the UUID  
 of the OS disk and the kernel looks at the superblocks of the  
raided  drives everything comes up fine, but I'm worried that if i  
move the  array to another machine and need to do a mdadm --assemble  
that i  won't know the correct order of the disks and what is more  
worrying  if i have a disk fail say HDC for example, i wont know  
which disk  HDC is as it could be any of the 5 disks in the PC. Is  
there anyway  to make it easier to identify which disk is which?.

----- End message from michael.harris@rmit.edu.au -----

Try this:
mdadm -Q --detail /dev/md0
to see which disk is which disk in the raid.
To identify a disk you can examine it using:
mdadm -E /dev/sd[b-d]
and read your dmesg.
And finally you can use "blkid" to associate UUIDs with devices.
I hope this helps.
Kind regards,
Alex.

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