Re: 8x2.5" or 6x3.5" disks

You don't mention the capacity of the disks you are looking at. Here is
something you might want to consider.

I've seen a few performance posts on using different hardware
technologies to gain improvements. Most of those comments are on raid,
interface and rotation speed. One area that doesn't seem to have been
mentioned is to run your disks empty.

One of the key roadblocks in disk performance is the time for the disk
heads to seek, settle and find the start of the data. Another is the
time to transfer from disk to interface. Everyone may instinctively
know this but its often ignored.

Hard disks are CRV ( constant rotational velocity) = they spin at the
same speed all the time

Hard disk drives use a technology called ZBR = Zone Bit Recording = a
lot more data on the outside tracks than the inner ones.

Hard disk fill up from outside track to inside track generally unless
you've done some weird partitioning.

On the outside of the disk you get a lot more data per seek than on the
inside. Double whammy you get it faster.

Performance can vary more than 100% between the outer and inner tracks
of the disk. So running a slower disk twice as big may give you more
benefit than running a small capacity 15K disk full. The slower disks
are also generally more reliable and mostly much cheaper.

The other issue for full disks especially with lots of random small
transactions is the heads are seeking and settling across the whole
disk but typically with most of those seeks being on the latest
transactions which are placed nicely towards the middle of the disk.

I know of a major bank that has a rule of thumb 25% of the disk
partioned as a target maximum for high performance disk systems in a
key application. They also only pay for used capacity from their disk
vendor.

This is not very green as you need to buy more disks for the same amount
of data and its liable to upset your purchasing department who won't
understand why you don't want to fill your disks up.

Mike