RE: 4 Disk Raid5 Performance over 3 Disk Raid5 Performance

midrange-l-request@xxxxxxxxxxxx wrote:

  9. 4 Disk Raid5 Performance over 3 Disk Raid5 Performance
     (kirkg)

I know from real life that a system with a 3 drive raid5 set performs 
almost as bad as a 2 drive mirrored  set. Now I need some official doc 
that says a 4 drive raid5 set out performs a 3 drive raid5  set.  Anyone 
seen something like this?

Unfortunately, no, I haven't seen actual docs. But for the sake of completeness 
on the concept, I want to add a "lunatic fringe" comment... NOT to be confused 
with any form of "recommendation"!

While a 3-drive RAID5 set may perform far worse than 4 or more drives in a set, 
it _might_ not perform badly enough to ignore as a possibility. We moved a 
couple systems from some old hardware to a couple of the partitions on a new 
box, both partitions with 3-drive RAID5 sets, and both are so much blazingly 
faster than the old that we fail to see what the complaints are about.

MAJOR caveat -- our processing is ***NOT*** typical and definitely not database 
bound. But when we run a *FULL build of a significant product, we still get a 
big chunk of AuxIO.

The only point is that _sometimes_ for unusual circumstances, a 3-drive set can 
work well. The sum of performance boosts from various components after 
upgrading to new hardware can be impressive.

Tom Liotta