[PLUG] IDE RAID and Linux (revisions)

[Steve Beattie]

Hey Launi,

I'm going to preface my response by saying that I've only used software
raid (on ide) under linux, not hardware ide raid (though I've seen
others I work with beat their heads against the wall with hardware raid
driver issues).

On Sat, Mar 15, 2003 at 02:25:36PM -0800, Launi wrote:
> I'm going to build a replacement Linux server for one of my aging servers
> and want to do RAID 5.  I have done hardware SCSI RAID on Window$ so I know
> how it works in principle.  I have never done software IDE RAID before.  I
> have a few questions before I get started that may save me aggravation later
> on. I understand that the Linux 2.4.x kernel supports software RAID and I
> also know that in order to get the best performance you need your IDE drives
> on one channel each.  I want RAID 5 with one hot spare using 4 identical 72
> GB drives.

To be pedantic, the 2.2 linux kernels also support software RAID, which
is what I'm using. My understanding is that the software RAID format is
the same between 2.2 and 2.4, but don't know about 2.5 (I know there's
been drastic changes in the LVM stuff, not sure how it's affected
software RAID).

> I'm open to using any Linux distribution that will make this work.  I have
> used RedHat, Mandrake and Debian in the past.

AFAIK, they all will make it work. On the other hand, I have no idea
how well the installers handle it -- I set mine up manually long after
install time, as I was adding a mirror drive for a system without backing
up and reinstalling -- I had no backup device that could store the
amount of data reasonably.

>  1. Can I just have my 4 IDE drives connected to two separate two
> channel PCI IDE controllers and Linux will recognize them during
> startup?

I believe so, my raid 1 setup is two drives, one on each channel of the
ide controller. 

> 2. If I do buy a 4 channel controller (Rocket Raid 404) which does
> not support RAID 5 via hardware, will Linux support it using software?

Likely so (modulo my inexperience with hardware raid). Essentially you'd
be using it as a straight 4 channel ide controller, no raid as far as
the hardware device is concerned. 

> 3. Should I only use the controllers listed here
> (http://www.tldp.org/HOWTO/Hardware-HOWTO/ideraid.html)?

Dunno. You probably should also look at what the distribution you plan
on using supports for hardware raid, if that's the way you decide to go.

>  4. If I'm not concerned about performance can I use one 2 channel PCI IDE
> controller and connect the four drives (two on each channel)?

In software raid, I'm almost certain it will. All it looks at are the
block devices you give it, I don't think it has any meta knowledge about
them. You likely could be able to mirror across a mix of scsi, ide, cf
or other random devices, assuming they present themselves as block
devices (not having tried it, I don't this for sure, but it seems like
it). 

> Basic RAID configuration questions
>  
>  1. If I use Linux software RAID without a hardware driver should I boot off
> a single disk and then make RAID for just my data?  

You can, but you don't have to, you can have the root partition on raid
as well (I do this).  You will have to use an initrd (ram disk) with the
software raid modules (md) in it or have a kernel with software raid
compiled in so that the kernel has access to the software raid driver
before any of the file systems have been mounted.

> 2. If I have hardware
> RAID with Linux driver support can I install directly on the RAID?

Yes, I believe so.

> I'm on a budget and this is a home business system so that I can store my
> clients graphic work.  If I can use one PCI card with 4 ATA controllers on
> it at a cost of $100 then this would be ideal.  If I have to go with a
> higher end SX6000 or Escalade 7500-4 ATA 133 RAID Controller Card then I
> would have to spend about $300.  I'll spend the money if that's what it
> takes to work, but I wanted to use Linux software RAID and did not want to
> have to deal with third party hardware drivers.

I've been very happy with linux's software raid, though I've not had a
disk failure -- I did have to run in a failed configuration for a while
while I was moving my data from non-raid to raid. Personally, I'd lean
towards a raid 0+1 (or is it 1+0 that's the better of the two? I never
remember) if I had four disks and was willing to give up some space,
but that's just me.

Hope this helps.

-- 
Steve Beattie                               Don't trust programmers?
<steve at wirex.net>                         Complete StackGuard distro at
http://NxNW.org/~steve/                            immunix.org
http://www.personaltelco.net -- overthrowing QWest, one block at a time.
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