[geeks] recommended dual channel LVD controller, PC

[velociraptor]

On Fri, 23 Jul 2004 10:49:27 -0500, mrl <vlack-lists at vlack.com> wrote:
> On Fri, 2004-07-23 at 02:21, Bjvrn Ramqvist wrote:
> 
> > People here would probably go for the
> > "striped-mirror" approach rather than the "mirrored-stripe" approach,
> > since it's more tolerant to failures.
> 
> How could that be?
> 
> 1 2    (assumes 2 mirrors or stripes, total of 4 disks in array)
> 
> - -    This is a mirrored stripe.
> - -    Any one disk in a column can fail. Beyond that, you're toast.
> 
> : :    This is a striped mirror.
>        Again, any one disk in a column can fail.
> 
> (sorry if my ASCII "art" doesn't do it for you... it was half for my
> benefit, to make sure I was asking an intelligent question. :P)
> 
> So how's it more tolerant to failures?

It's more fault tolerant in the case of a double-disk failure.

If you stripe first, any failure takes out a whole stripe.  A double-disk
failure occuring in opposite sides of the mirror will cause complete
failure of the RAID.

If you mirror first then stripe, a double-disk failure, even if occurring
on opposite sides of the mirror will allow the RAID to be recovered.

It's best understood with a graphic--it took me a bit to understand
this at my Veritas class...

=Nadine=