>Now, I'm curious what sort of HDD subsystem you're using. We're running
>(don't hit me for that) a RAID5 UW SCSI system with 5 HDDs. How much
>performance could I gain setting up a RAID1-0 (striping over mirrors)
>array with 4 HDDs, dedicated for the mysql tree?
In general, you'll see a small (~25%) increase in read performance,
but a larger (~200%) increase in write performance, for sequential
reads & writes. For random reads and writes, the increase is less
substantial: ~5% for reads and ~70% for writes.
Big thing to consider, though, is that you'll lose disk space in a
RAID0+1 setup. Let's assume you have 4 9GB drives. With RAID5,
you get about 27GB usable, while with RAID0+1, you get only 18GB usable.
Plus, to be safer, you should be running them under some sort of
volume management (more $$) and an extra hot spare (even more $$).
But that cost may be worth it to you.