RE: Software RAID

["Mark McCulligh" <mmcculli visualtech ca>]

Thanks Nigel for all the information. I know already if I unplug the primary
drive the system won't boot. I will try some of your other tips to see how
the system react.

thanks again,
Mark.

-----Original Message-----
From: redhat-install-list-admin redhat com
[mailto:redhat-install-list-admin redhat com]On Behalf Of Nigel Wade
Sent: Thursday, December 11, 2003 4:53 AM
To: redhat-install-list redhat com
Subject: Re: Software RAID


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


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