[Gllug] re: software vs hardware RAID

[Huw Lynes]

On Wed,  3 Nov 2004 17:00:14 +0 00
"t.clarke" <tim at seacon.co.uk> wrote:

> Can't I have more than a superficial understanding of this  - so while
> the 'thread' is current maybe someone can explain better.
> 
> My impression is that hardware controller cards are configured prior to O-S
> installation using their 'bios' functions (much the sme as a SCSI controller
> is configured), so that a number of physical discs are simply presented to
> the operating system(s) as one or more logical discs. Thus, the o-s need
> have no knowledge of the underlying hardware and simply writes/reads to the
> logical disc(s) it sees; relying on the hardware controller to do the actual
> work or mirroring/striping or whatever.   This means no extra work for the
> o-s.

Essentially yes.
 
> On the other hand, I presume software raid requires the o-s to do all the
> hard work, including scheduling and performing two writes for the
> price-of-one when mirroring is done, striping etc etc when other levels of
> raid are done; all of which would seem to impose extra load on the system.

Yes more or less but it's not as computationally expensive as it sounds.

> So, seems to me, as previously stated by Martin, that if you really NEED
> raid, go for the hardware variety !  

It depends on why you are using RAID. There are really two reasons: redundancy
and performance. If you are using RAID for redundancy such as mirroring the
root partition then software RAID is just fine. It means your machine will
keep on trucking if one of the root mirrors goes to never-never land. This is
a very good thing. Also you can take your software RAID mirror and plug in
into pretty much any other chasis and it will just work. I was very glad of
this when our web server's power-supply went up in smoke (lietrally).

In terms of performance hardware RAID wins every time. With the proviso that
you need to keep a spare RAID card around or else you have just moved your
reliability problems to a different point of failure.

> Personally, I reckon except for systems that
> absolutely HAVE to be up all the time, modern discs are sufficiently
> reliable

I submit that 70Gb Seagate Cheetahs(SCSI) and the IBM DeathStars prove you
wrong.

> to make raid a luxury which can be done without.  

Is 50 quid for a second root disk really a luxury? That's the beauty of
software RAID.

-- 
| Huw Lynes               | The Moving Picture Company  |
| System Administrator    | 127 Wardour Street          |
|.........................| London, W1F 0NL             | 


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