[rescue] Re: DiskSuite configuration suggestions

[Robert Novak]

On Thu, 14 Feb 2002, Brian Hechinger wrote:

> On Thu, Feb 14, 2002 at 02:48:13AM -0600, Bill Bradford wrote:
> > > RAID 10 or 0+1.  Each disk is mirrored by a second disk(level 1), then
> > > striped (level 0) for speed.  Downside is that 4x18GB drives = 36GB of
> > 
> > Okay, how is this different from: stripe, containing two disks
> > mirror the stripe
> 
> 1+0 vs 0+1  do we stripe first or mirror first.  i vote for stripe first and
> then mirror, like you have it.

Actually, two fun notes on this.

But first, software RAID5 is a waste. It might be usable on a
never-written-to filesystem, but there's a big performance hit on writes
even if you have, say, 8 disks on four different SCSI buses spread across
two PCI buses (I tried it on a fresh E450 once). Now if you could mirror
that setup to an identical match somehow, with lots of cache, maybe... but
then you'd be able to get some sort of hardware RAID solution. Too bad the
Sun PCI RAID card didn't pan out as well as expected. Too bad there aren't
SPARC Solaris drivers for Mylex/IBM or even AMI/DellPERC RAID cards. 

The two fun notes:

1) With four disks, you don't get as much as advantage as if you had a few
more. The benefit to mirror first, then stripe (1+0) is that you can lose
half of each mirror and keep going. So with:

	d0  			d100==	d120/d130
	==			d120==	d11+d12+d13+d14
	d1==d11/d21		d130==	d21+d22+d23+d24
	d2==d12/d22
	d3==d13/d23
	d4==d14/d24

This fictional example would have d1 as a mirror of submirrors d11 and
d21. d0 is a stripe of d1+d2+d3+d4. If d11, d12, d13, and d14 all die,
you're still up and running.

The second side has d100 as a mirror of submirrors d120 and d130. d120 is
a stripe of d11 through d14 and so forth. On this one, if you lose one
disk (say d14), its submirror goes offline. Then if you lose a disk in the
remaining submirror, your whole mirror goes offline and you're looking for
backups you probably forgot to make.

--

Now in Disksuite you cannot create a stripe of mirrors explicitly. You can
only add stripes/concats to a mirror, and stripes/concats can only contain
slices. HOWEVER... and I've been cursing Brian Wong over this since the
first Sun server I built but finally found the explanation recently...
Disksuite treats 0+1 as 1+0 if the elements of the stripes are identical
in size and interlacing.

http://docs.sun.com/ab2/coll.260.2/DISKSUITEREF/@Ab2PageView/idmatch(CH2METADEVICES-24868)?Ab2Lang=C&Ab2Enc=iso-8859-1#CH2METADEVICES-24868
 (referred from http://www.ofb.net/~jheiss/raid10/disksuite.txt )

You can apparently do explicit 1+0 in Veritas but you get the same basic
functionality in Disksuite even if it doesn't look like it. Disksuite is
almost always cheaper than Veritas Volume Manager too. :)

--Rob (who was up at 3am a few years back trying to make Disksuite do 1+0
whilst rebuilding a fileserver)


Robert Novak, Indyramp Consulting * rnovak at indyramp.com * indyramp.com/~rnovak
	"I don't want to doubt you, Know everything about you
      I don't want to sit Across the table from you Wishing I could run."