1 processor vs. 2

[Danny Pansters]

(enough CCing, back to list only)

On Wednesday 03 March 2004 22:36, Matthew Seaman wrote:
> On Wed, Mar 03, 2004 at 10:26:43PM +0100, Stefan Cars wrote:
> > Okey, but if you would compare RAID-1 on two disks compared to RAID-5 on
> > three disks then ? What would be the faster ?
>
> RAID1 is going to be faster, both reading and writing, but it will
> take a lot more raw disk space to provide the required usable space.

No, it would require 2 identical disks, whereas RAID5 with 3 disks would 
require 3 of those disks ;-) 

Physical disks are your unit of failure or of resilliance if you like. That's 
why you need 5+ drives for RAID5 to be any real fun. You want your data to be 
present at least twice on different physical drives. You want the same for 
your parity info. The mere fact that you stripe everything out with RAID5 
doesn't change your physical unit which is one disk. Resilliance means: what 
happens if a random drive fails. RAID5 on 3 disks defeats the purpose of 
RAID5 IMHO. Theoretically the more drives, the better RAID5 gets, so that 
might say something about Veritas if they warned against using more than 7 
drives. Perhaps grog can be the final referee here, not my turf ;-)

Having said that I don't have a RAID5, but I would recommend OP to use RAID1 
and use the 3rd drive as a (semi) hot spare for extra sleep security and less 
spending. It's much more interesting if you can (un)plug a spare on the fly 
BTW.

I just kinda fell back into the developed thread, hope you don't mind me 
adding a general remark: One doesn't do RAID to increase performance. Period. 
If budget is no problem, buy spare boxen and use them "secondary", always 
nominated to become "primary" at any time. That's better insurance against 
(any) hardware failure than mere RAID can ever be IMHO.


Greets,

Dan