EXPERT RESPONSE
I'm glad you read the article, and thanks for the follow-up
question. You're right - RAID file placement isn't covered all that much.
I think it's one of those topics that's more art than science in some cases.
I hope this helps:
Start by categorizing the types of disk activity you have. For example,
fixed-size and dynamically-expanding VHDs could have a lot of write activity
(for example, if you're virtualizing a transaction-processing database), a
lot of read activity (for example, in the case of a file server), or neither
(in the case of Web server that caches most of its content). Next, look at
the types of reads or writes: Are they primarily sequential or random? You
can measure all of this using tools like the Windows Performance Monitor
(see my article "Designing Virtual Hard Disk Storage" for more details.)
Also, consider the activity patterns for undo disks and differencing disks.
Now, on to RAID: Your goal is to choose RAID configurations that allow you
to minimize contention for disk access. If you have two large VHDs that
generate a lot of random reads, for example, you might want to place them on
separate spindles. RAID 0+1 or RAID-5 would be good candidates here (since
you won't have to worry much about parity calculation overhead). Some
general tips are to separate base VHDs from their associated undo disks and
differencing disks (since these are often accessed concurrently).
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