Information Technology Reference
In-Depth Information
Beware of “average seek time”
Although the name average seek time makes it tempting to use this metric when
estimating the time it will take a disk to complete a particular workload, it is often the
wrong metric to use. Average seek time—the average across seeks between each
possible pair of tracks on disk—was defined this way to make it a well-defined, standard
metric, not because it is representative of common workloads.
The definition of average seek time essentially assumes no locality in a workload,
so it is very nearly a worst-case scenario. Many workloads access sectors that are
likely to be near one another; for example, most operating systems attempt to place
files sequentially on disk and to place different files in a directory on the same track
or on tracks near one another. For these (common) workloads, the seek times ob-
served may be closer to the disk's minimum seek time than its “average” seek time.
The demise of the cylinder
A cylinder on a disk is a set of tracks on different surfaces with the same track index.
For example, on a 2-platter drive, the 8th tracks on surfaces 0, 1, 2, and 3 would form
the 8th cylinder of the drive.
Some early file systems put related data on different surfaces but in the same cylinder.
The idea was that data from the different tracks in the cylinder could be read without a
requiring a seek. Once a cylinder was full, the file system would start placing data in
one of the adjacent cylinders.
As disk densities have increased, the importance of the cylinder has declined. Today,
a disk's tracks can be less than a micron wide. To follow a track at these densities, a
controller monitors the signals from a disk's head to control the disk arm assembly's
motor to keep the head centered on a track. Furthermore, at these densities, the tracks
of a cylinder may not be perfectly aligned. As a result, when a disk switches disk heads,
the new head must center itself over the desired track. So, switching heads within a
cylinder ends up being similar to a short 1-track seek: the controller chooses the new
cylinder/track and the disk head settles over the target track. Today, accessing different
tracks within the same cylinder costs about the same as accessing adjoining tracks on
the same platter.
