[lug] software RAID

[Sean Reifschneider]

On Fri, Feb 10, 2006 at 08:19:41PM -0700, D. Stimits wrote:
>Worse than that: IDE does not allow two drives simultaneous access. 

Yeah, it's just a performance issue there.  The other issue is not only
that you could fry two discs with a controller spike, but that one of the
drives can fail in such a way as to take down the communications channel.
This means that, while your other drive is alive and could be giving you
data, it can't.  This isn't just an IDE issue, we started doing that on
SCSI, because you can have so many more (potentially misbehaving) devices.

Really it just all goes back to eliminating Single Points of Failure
(SPOF).  That's what HA stuff is all about.

>least for regular IDE, somehow I doubt they did any better with 
>SATA...if they had it would be called SCSI).

They do, it's called "Serial Attached SCSI" and it can use regular SATA
devices.  I'd imagine that SATA is better than IDE because you only get one
device per bus, as far as I can tell.  It's unclear whether two port
controllers that show up as "master" and "slave" suffer the same
performance issues as regular IDE, but that's a controller issue.
Certainly on things like the 3ware SATA RAID cards, they are different
busses.  On lower-cost gear, that might not be the case.

Thanks,
Sean
-- 
 [...] who asked "Why do we do it, this science?"  No one had an answer until
 I stood up and said "Isn't there money in a Nobel?"  -- Steve Martin
Sean Reifschneider, Member of Technical Staff <jafo at tummy.com>
tummy.com, ltd. - Linux Consulting since 1995: Ask me about High Availability