Difference between Seek Time and Latency

Seek Time

The seek time of a hard disk measures the amount of time required for the read/write heads to move between tracks over the surfaces of the platters. Seek time is one of the most commonly discussed metrics for hard disks, and it is one of the most important positioning performance specifications. However, using this number to compare drives can be somewhat fraught with danger. Alright, that's a bit melodramatic; nobody's going to get hurt or anything. :^) Still, to use seek time properly, we must figure out exactly what it means.

Switching between tracks requires the head actuator to move the head arms physically, which being a mechanical process, takes a specific amount of time. The amount of time required to switch between two tracks depends on the distance between the tracks. However, there is a certain amount of "overhead" involved in track switching, so the relationship is not linear. It does not take double the time to switch from track 1 to track 3 that it does to switch from track 1 to track 2, much as a trip to the drug store 2 miles away does not take double the time of a trip to the grocery store 1 mile away, when you include the overhead of getting into the car, starting it, etc.

Seek time is normally expressed in milliseconds (commonly abbreviated "msec" or "ms"), with average seek times for most modern drives today in a rather tight range of 8 to 10 ms. Of course, in the modern PC, a millisecond is an enormous amount of time: your system memory has speed measured in nanoseconds, for example (one million times smaller). A 1 GHz processor can (theoretically) execute over one million instructions in a millisecond! Obviously, even small reductions in seek times can result in improvements in overall system performance, because the rest of the system is often sitting and waiting for the hard disk during this time. It is for this reason that seek time is usually considered one of the most important hard disk performance specifications. Some consider it the most important.

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 on the hard disk. Therefore, at the time that the heads arrive at the correct cylinder, the actual sector that is needed may be anywhere. After the actuator assembly has completed its seek to the correct track, the drive must wait for the correct sector to come around to where the read/write heads are located. This time is called latency. Latency is directly related to the spindle speed of the drive and such is influenced solely by the drive's spindle characteristics. This operation page discussing spindle speeds also contains information relevant to latency.

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. Sometimes the sector will be at just the right spot when the seek is completed, and the latency for that access will be close to zero. Sometimes the needed sector will have just passed the head and in this "worst case", a full rotation will be needed before the sector can be read. On average, latency will be half the time it takes for a full rotation of the disk.