Great info to learn on determining IOPS required for Horde

Determining disk drives if you know the IOPS required

If you can make an estimate of how many I/O's Per Second (IOPS) you need you can determine how many disks you'll need as RAID10 and how many disks you'll need as RAID5.

Generally, a single disk drive can do this many IOPS per disk:

15k rpm: 180-210 IOPS
10k rpm: 130-150 IOPS
7200 rpm: 80-100 IOPS
5400 rpm: 50-80 IOPS

In a mirrored configuration:

Disk IOPS = Read IOPS + (2 * Write IOPS)

In a parity (RAID5) configuration:

Disk IOPS = Read IOPS + (4 * Write IOPS)

Example calculations

Now let's look at an example. If you estimate that you need to support 40 Read IOPS (40 reads/sec) and 80 Write IOPS (80 writes/sec).

If you want to use a mirrored configuration of drives:

Disk IOPS = Read IOPS + (2 * Write IOPS) = 40r/s + (2 * 80w/s) = 200 Disk IOPSUsing 7200 rpm drives, you need: 200 / 50 = 4 disk drives
Using 10k rpm drives, you need: 200 / 130 = 2 disk drives (always round up)

If you want to use a parity (RAID5) configuration of drives:

Disk IOPS = Read IOPS + (4 * Write IOPS) = 40r/s + (4 * 80w/s) = 360 Disk IOPSUsing 7200 rpm drives, you need: 360 / 50 = 8 disk drives (always round up)
Using 10k rpm drives, you need: 360 / 130 = 3 disk drives

You can also refer to this page, http://www.pcguide.com/ref/hdd/op/spin_Speed.htm where you can benchmark your IOPS base on their expected latency.

ref. http://wiki.horde.org/HardwareRequirements