[Sussex] Debian and Software RAID and hello

[Stephen Williams]

On Mon, 2005-09-05 at 21:42 +0100, Al Bennett wrote:
> Hi Guys
> 
> Firstly, apologies for not being around on the list or at Moots for what 
> must be a year!  The old ME has thrown me around a bit this year, and on the 
> ups I always seem to be away or busy to coincide with the Moots.  Still, I 
> promise to chip in on the list more often!
> 
> Anyway, I have a project to be keeping me busy with in the next weekish 
> (after building a Windows XP box (the money's ok)).  I need to build a cheap 
> and Linuxful small business type server.  I've ordered the bits (which I'm 
> hoping are all vaguely compatible), and stuff should arrive tomorrow (it is 
> Ebuyer though...).  I've ordered two 120Gb SATA disks which I'm hoping to 
> make into a RAID mirror.  I thought there might be some more useful 
> documentation on Raid available considering it's popularity but I can't 
> really get straight answers to anything related to it!
> 
> I hoping to stick Sarge on this and have it all just work, but I'm pretty 
> sure it's not that simple...
> 
> The mobo is a GA-8IG1000MK with an Intel 82801EB southbridge, which may or 
> may not include some form of RAID (I'm a bit baffled), either way I'm happy 
> to use software raid on JBOD (I'm picking up the lingo!).

If you're thinking of using these 2 disks as a RAID array for a server
to hold data, then don't use JBOD, use RAID 1 instead. This will allow
you to recover from a failure of one of the 2 disks.

> 
> Now, in the reading up I've been doing there seems to be this kernel stuff 
> called 'md' and a user space control thing mdadm, however that all seems to 
> be a bit old hat.  I read something about it not taking care of bad blocks 
> or something.

I've not had any trouble from kernel based md devices. If you want to
boot from a RAID array you'll have to compile md and RAID 1 support into
the kernel. If you use modules to provide this, or use evms I suspect
you'll need to compile an initrd image - good luck. On the other hand,
the Debian Sarge installer might take care of this for you, but I'm a
Gentoo user myself, so I can't tell you how it works. With Gentoo you
configure the RAID arrays before installation.

You'll need to set the partition types as fd - Linux RAID autodetect -
again the Sarge installer may do this for you. With md and RAID support
in the kernel and using udev, the kernel autodetects, assembles and
starts the RAID arrays during boot prior to starting the initscript. The
only issue is whether you can persuade grub to read the /boot partition
if it's on a RAID array. I've not found an infalliable way to do this,
so you may have to consider having an ext2 /boot partition on the RAID
boot disk.

> 
> However, I did find this great sounding thing called EVMS (evms.sf.net) 
> which seems to do everything including toast making.  However I'd need to do 
> some kernel compiling, which I've never really done.  How easy is adding 
> EVMS to a standard Sarge install?  Can I set up an array at installation 
> time or do I raidify a standard installation (I have another 120Gb IDE disk 
> to play with, if needed)?

Not sure about Sarge, but with Gentoo you can do either.

If you have 3 120Gb disks, have you considered RAID 5?

> 
> Also, whichever route I go, how do I handle booting from an array, and 
> handling a failed disk (for booting)?

See above for booting, for failures use mdadm (assuming hot-swap
capability - not very common for IDE/SATA disks):


Unmount your array:

umount /dev/md/2

Stop your array:

mdadm -S /dev/md/2

Add the spare:

mdadm /dev/md/2 -a /dev/hd??

Check sync progress:

cat /proc/mdstat

When done, remount your array.

You can't do this for your / partition, you have to carry out any
troubleshooting from a CD-ROM boot - Knoppix is great for this.
Alternatively you could install a vanilla hard disk and install a backup
Sarge install on that for the times when your / RAID partition is up the
creek. I've recently done this on my server and can recommend it.

There's plenty of help available if required - please call.

Steve W.


> 
> Help!
> 
> Al 
> 
> 
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