Re: Software RAID

[Nigel Wade <nmw ion le ac uk>]

Mark McCulligh wrote:
> Hi Group,
>
> I have software raid running on a server. Two harddrives on one ribbon.
> Primary and Secondary. I setup software raid to mirror between the two
> drives.
>
> My question is if the Primary drive fails, then the secondary drive will
> kick in. 

Possibly, but there's a possibilty that a dead drive will make that IDE 
channel inoperative and render the slave useless. It's much better to have 
the 2 drives on separate channels, not just for the above reason but also 
for speed as having both disks in a RAID on the same channel will be slower.

> But what happens if I reboot the machine without a working Primary
> drive. Will it boot or will I have to first open the case remove the primary
> drive and make the secondary drive primary before it will boot. Then add a
> new Secondary drive and rebuild the raid array.  OR can I just replace the
> primary drive with a new drive, the server booting from the secondary and
> then rebuilds the raid array on the primary.

That depends on how you've configured your drives and which boot manager you 
use. Is your boot partition mirrored, and do you use lilo or grub? If the 
secondary doesn't have a boot partition then it won't be able to boot. If it 
has a boot partition which is a mirror of the primary, and you use grub it 
won't be able to boot as the secondary, you'd have to swap it to primary, 
but if you use lilo then it should boot as the secondary. lilo is RAID aware 
and if your boot partition is mirrored it writes a different boot sector to 
each device in the RAID so it is bootable in it's current location.

> I am trying to figure out the recovery method from a failed primary drive.
> No good to have raid if you don't know to use it.

With grub you would have to swap drives, insert a new secondary and rebuild 
the RAID. With lilo you just insert a new primary, boot off the secondary, 
partition the new disk and rebuild the raid. At least, that's how it worked 
when I was testing my firewall...

There's no substitute for testing. Try it out. Remove the primary and see if 
 the system boots. Remove the partition table from the primary and see what 
steps are necessary in rebuilding the RAID. It's all good experience.
When I was building my firewall this is exactly what I did to see how the 
system handled a failed disk, and how I should go about adding a 
replacement. I ended up writing a Python script which would partition a new, 
blank, replacement disk and add it into the raid. I'm currently building a 
new mail server with mirrored disks and will do exactly the same again with 
this system.

> Thanks,
> Mark.


--
Nigel Wade, System Administrator, Space Plasma Physics Group,
            University of Leicester, Leicester, LE1 7RH, UK
E-mail :    nmw ion le ac uk
Phone :     +44 (0)116 2523548, Fax : +44 (0)116 2523555