Joe,
Are you actually mixing drive models (and spindle speeds?!) in the same array, or did you build multiple arrays on the same controller from like discs?
I would also like to give an example of what Joe was indicating: four or more drives on a single U160 chain is fine, under certain conditions.
Small mail server, <10,000 users: Four 15K disks in RAID5 for swap and /var (mail spool) plus a pair of 10K OS disks in RAID1. In this example, I'm expecting the RAID5 to get hammered all day long (it's a mail queue, so writes are about as common a reads, possibly more so), while the RAID1 disks basically sit idle, since most everything on it that is useful hangs around in memory. Heck, throw on a CDROM and a tape drive (which rarely get used, and are slow even when they do...) and you're still probably below the ceiling for U160.
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