How do I calculate the disk subsystem i/o capacity

I am trying to figure out the number of 8K page i/o capacity for a disk subsystem. The drives are SATA 7200 RPM - 4 drives in a RAID-5 configuration. I am not sure about the SCSI controller but the server is about 5 years old.

Another thing you'll need to know is the ratio of sequential vs. random I/O requests. That can affect I/O op speed by a good degree. For rotational media, 100% random requests is your lower bound for I/O ops, and 100% sequential is your upper bound.

Also, since you'll be working with RAID5, your read/write percentage will also affect I/O Ops. This can be greatly affected by your RAID card, so there aren't many hard and fast rules of thumb other than 'writes will be slower than reads', and even THAT can be spoiled by intelligent caching on the RAID card.

7.2K RPM drives will reach I/O saturation at some point, though it is possible your RAID card will hit CPU saturation well before then when handling lots of writes.

The only way to know for sure is to test. As Evan said, iometer has a long track-record. I've also used iozone to good effect. It's not as advanced as iometer, but it's a bit simpler to use.