Disk sector to be read should be under the head of disk. To read a sector, head waits for sector to be under it. The time that a disk takes for data to be under head is called rotational latency. Rotational latency is added to response time for access of disk. As spin of disk is involved in this case, so spin speed of disk is most important. Spin speed of disk is measured in revolutions per minute (RPM). More the revolutions per minute (RPM) of a disk, less the rotational latency will be. So revolutions per minute (RPM) of a disk directly affect the I/O performance of your disk and thus database. Normally high performance disks operate at 10,000 RPM, but this speed is subject to improvement as time passes.
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