Next: Access Time[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance | Hard Disk Internal Performance Factors ]
Latency
The hard disk platters are spinning around at high speed, and the spin speed is not synchronized to the process that moves the read/write heads to the correct cylinder on a random access of any sector of the hard disk. Therefore, at the time that the heads arrive at the correct cylinder, the actual sector that is needed may be anywhere. Latency is the time that the drive must wait for the correct sector to come around to where the read/write heads are waiting for it.
Conceptually, latency is rather simple to understand; it is also easy to calculate. The faster the disk is spinning, the quicker the correct sector will rotate under the heads, and the lower latency will be. On average, latency will be half the time it takes for a full rotation of the disk. This table shows the latency for a range of typical hard disk spindle speeds:
Spindle Speed (RPM) |
Average Latency (Half Rotation) (ms) |
Worst-Case Latency (Full Rotation) (ms) |
3,600 |
8.3 |
16.7 |
4,500 |
6.7 |
13.3 |
5,200 |
5.8 |
11.5 |
5,400 |
5.6 |
11.1 |
6,300 |
4.8 |
9.5 |
7,200 |
4.2 |
8.3 |
10,000 |
3.0 |
6.0 |
As with seek time, latency figures are in milliseconds, which are big numbers when dealing with computer system performance. Reducing latency figures greatly improves the performance of the system, particularly when dealing with access to random files on the disk. This is why newer and more expensive hard disks continue to push spindle speeds higher and higher. Even most mainstream disks now rotate at 5,200 or 5,400 RPM. You can see that a 5,400 RPM drive gets two-thirds of the latency improvement of a 7,200 RPM drive, compared to the old standard of 3,600 (which all PC hard disk drives used for many years.)
Again, as with seek times, latency is most relevant only to certain types of accesses. For multiple, frequent reads of random sectors on the disk, it is an important performance-limiting factor. For reading large continuous blocks of data, latency is a relatively minor factor because it will only happen while waiting to read the first sector of a file. The use of cylinder and head skewing on modern drives is intentionally designed to reduce latency considerations when switching between consecutive heads or cylinders.
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