Just so it's in the archives... If anyone is planning on using some sort of software RAID5 solution for TSM disk storagepool volumes, a few things to note. We're managing to get up to about 40 MiB/s to/from fibre attached disk using Veritas Volume Manager RAID5. Here's how.
TSM does disk storagepool I/Os in blocks of 256 KiB. Optimally, a write I/O should represent a full-stripe write. If it is not a full stripe, the RAID software must do a read-modify-write cycle which can reduce bandwidth by an order of magnitude, or more. This doesn't apply to hardware RAID that has either battery backed caches or NVRAM, since they can delay and combine many small I/Os into a few larger I/Os. I'd imagine it would apply to hardware RAID solutions without cache, or with write-through cache.
To force full stripe writes, use raw devices for volumes. This also bypasses the OS file-cache which can be a big win (especially with older versions of Solaris). Next is to make the size of a stripe 256 KiB. This requires that the number of disks in the RAID5 set be one more than a power of two. i.e. 3, 5, 9, 17. There are other combinations that work, I'll leave that to your imagination. So, hopefully a write will hit all disks and not involve any unnecessary reads. In our case, we have a A5x00 series array with 14 disks, which are broken up into 5 disk RAID5 and 9 disk RAID5, and I've seen the disk pool peak at about 40 MiB/s writes, especially on the 9 disk set.
Cheers, -- Paul Ripke Unix/OpenVMS/TSM/DBA 101 reasons why you can't find your Sysadmin: 68: It's 9AM. He/She is not working that late. -- Koos van den Hout