Oracle Internals Notes

I/O Service Times

Disk drives contain multiple disk platters. The information on each platter is arranged in tracks. In frame A of the figure below, the dashed circular lines represent tracks. Each track is divided into sectors. Sectors normally store 512 bytes of data, but larger sector sizes are becoming fairly common. Although a sector is the smallest possible unit of disk I/O, most I/O operations actually address a number of adjacent sectors. Frame A shows a group of sectors about to be addressed by a typical I/O operation.

Seek, rotational latency and transfer times

In general, the time taken to perform such an I/O operation is comprised of the following component times.

seek time The time required to move the read/write heads over the disk surface to the required track. Of course, if the heads happen to be over the correct track already, then there is no seek time. Otherwise, the seek time is roughly proportional to the distance the heads must move.
rotational latency The time taken, after the completion of the seek, for the disk platter to spin until the first sector addressed passes under the read/write heads. On average, the rotational latency is half of a full rotation.
transfer time The time taken for the disk platter to spin until all the addressed sectors have passed under the heads. Of course, the transfer time is directly proportional to the number of sectors addressed.

These times are normally measured in milliseconds. Although disk characteristics vary greatly, the average seek time is commonly twice as long as the average rotational latency, which is in turn significantly longer than most transfer times.

There is one exception to this general case. Most modern disks have track buffers that can make the rotational latency and transfer times trivial on reads for which no seek is required. If the entire track has already been buffered, then any read from that track can be satisfied immediately from the track buffer, without waiting for the disk to rotate.


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12-Oct-2007 22:22
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