[geeks] recommended dual channel LVD controller, PC

[Jonathan C. Patschke]

On Fri, 23 Jul 2004, mrl 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?

If a component of a stripe goes down, the whole stripe goes down, so if
your stripes are mirrored, the loss of any one disk means the entire
metadevice is running unmirrored.

If the individual mirrors are striped, the loss of any one disk only
breaks the mirror between that disk and its partner--the rest of the
stripe is still mirrored.  While you still will lose data if the failed
disk's partner also fails, the chances for data loss is much less.

Let's put it this way, if you have N disks, and two disks fail over time
without replacement, in the first case, your chances of data loss are[0]
0.5*((n-1)/n)--or a little less likely than even odds.  In the second
case, your chances of data loss are 1/(n-1)--MUCH better odds.

You're right, though, with four disks, it comes out the same either way.


[0] That's assuming that the failed drive and its parter arne't both
      subjected to exceptional stress.  If the two drives are sitting next
      to each other[1], and the fan tray servicing both disks dies, you're
      probably going to lose both disks.
[1] Dumb!
-- 
Jonathan Patschke )"We're Germans and we use Unix.  That's a combination
Elgin, TX        (  of two demographic groups known to have no sense of
USA               ) humour whatsoever." - Hanno Mueller in de.c.o.u.p