Knowing how to setup your storage is all about measuring and budgeting the IOPS and bandwidth. (I'm being simplistic here because the size of the percentage mix of read/write, average IOs, RAID stripe size, and cache hit percentages matter greatly. If you can get those numbers you can make your calculations even more accurate.)
There's a really nice little IO calculator here that I frequently use when planning out storage. wmarow's storage directory is also nice for getting some fairly contemporary disk performance numbers.
If I dedicate a pair spindles to a
particular task, such as transaction
logs, and they're not even breaking a
sweat with the workload...why not just
put that workload onto a larger RAID
10?
Remember that putting sequential IO onto a spindle with random IO makes that sequential IO random. Your transaction log disks may look like they're not breaking a sweat because you're seeing sequential IO operations. Sequential reads and writes to a RAID-1 volume will be quite fast, so if you're basing "not breaking a sweat" on disk queue length, for example, you're not getting the whole story.
Measure or calculate the maximum possible random IOPS for the intended destination volume, take a baseline of the current workload on that volume, and then decide if you have enough headroom to put those transaction log IOPS into the remaining random IO in the destination volume. Also, be sure to budget the space necessary for the workload (obviously). If you're so inclined, build in a percentage of additional "headroom" in your IO workload / space allocation.
Continue with this methodology for all of the other workloads that you want to put into the destination RAID-10 volume. If you run out of random IOPS then you're piling too much into the volume-- add more disks or put some of the workload on dedicated volumes. If you run out of space, add more disks.
Best Answer
You can't simply subtract the number of parity disks from number of total spindles and end up with useful results if you're doing IOP transaction calculations. There's more going on behind the scenes:
RAID 10:
RAID 5:
RAID 6: (not 100% sure on these numbers - someone please correct me if they are off)
So for example in a RAID set with 8 disks:
Note that for the RAID10 calculations the method of "subtract number of parity spindles from total number of spindles" gives you the right answer. However, this falls over in RAID5/RAID6 calculations.