Kevin Brown <kevin(at)sysexperts(dot)com> writes:
> In the case of pure random reads, you'll end up having to wait an
> average of half of a rotation before beginning the read.
You're assuming the conclusion. The above is true if the disk is handed
one request at a time by a kernel that doesn't have any low-level timing
information. If there are multiple random requests on the same track,
the drive has an opportunity to do better than that --- if it's got all
the requests in hand.
regards, tom lane