[rescue] Re: RAID (was Re: Insane drives)

[Robert Novak]

On Tue, 10 Jul 2001, Jonathan Katz wrote:

> Rob once wrote:
> > > Note that software RAID 5 sucks.
> David wrote:
> > Hardware RAID5 also sucks.
> 
> But, hardware RAID5>Software RAID5, and nothing
> beats $ for disk (esp. when compared to 0+1/1+0.)

RAID5 with good sparing can be pretty good. Note that some of these
comments on RAID5 also apply to RAID3 or RAID4, since the base concepts
are somewhat similar.

In general though, * sucks. Or .* for you regexp fans.

> > IIRC disksuite actually does 1+0.
> 
> Depending on how you set DiskSuite up you can do either.
> 
> DS works best when you setup a stripe first then mirror it, although
> you can setup multiple mirrors and then stripe them.

I've only been able to get it to do 0+1 (make two stripes, add them to a
mirror metadevice); last time I tried 1+0 (make four or so mirrors, add
them to a concat/stripe device) I got an error saying I can't add a
metadevice to a stripe. Brian Wong's book says SDS only does one of the
two, and I think he said SDS only does 1+0 but it looks like 0+1. Has this
changed in a later version? However, you can add stripes to a mirror (just
did that this morning in fact).

Joshua wrote: 
>
> So, you are saying that you always only use one disk for parity?  I
> thought that was what raid 3 did.

Nope. I'm saying one disk's worth of capacity is "lost" to parity. This is
common to RAID 3/4/5. RAID-3 uses a distinct disk for all parity
calculations. RAID-4 uses a distinct disk for parity but lets the disks
operate independently, from what I can find in a quick google search.
RAID-5 spreads the parity chunks across the disks. 

Nifty pictures:
	http://www.sagitta-ps.com/Technology/raid/raid3.htm
	http://www.sagitta-ps.com/Technology/raid/raid4.htm
	http://www.sagitta-ps.com/Technology/raid/raid5.htm

RAID-5 with a hardware controller and hefty battery-backed cache can be
quite usable. I actually had 3 18gb disks on a Mylex controller with 8MB
of cache, RAID-5, and it was comfortable. Darned slow to build the set,
but once it was there, FreeBSD ran happily on it.

--Rob

-- 
Robert Novak, Indyramp Consulting * rnovak at indyramp.com * indyramp.com/~rnovak
        "And it's been a long December and there's reason to believe
           Maybe this year will be better than the last...." -- counting crows