Performance Measures of Disks

The main measures of the qualities of a disk are capacity, access time, data transfer rate, and reliability,

1. access time: the time from when a read or write request is issued to when data transfer begins. To access data on a given sector of a disk, the arm first must move so that it is positioned over the correct track, and then must wait for the sector to appear under it as the disk rotates. The time for repositioning the arm is called seek time, and it increases with the distance the arm must move. Typical seek time range from 2 to 30 milliseconds.

   Average seek time is the average of the seek time, measured over a sequence of (uniformly distributed) random requests, and it is about one third of the worst-case seek time.

   Once the seek has occurred, the time spent waiting for the sector to be accesses to appear under the head is called rotational latency time. Average rotational latency time is about half of the time for a full rotation of the disk. (Typical rotational speeds of disks ranges from 60 to 120 rotations per second).

   The access time is then the sum of the seek time and the latency and ranges from 10 to 40 milli-sec.

2. data transfer rate, the rate at which data can be retrieved from or stored to the disk. Current disk systems support transfer rate from 1 to 5 megabytes per second.
3. reliability, measured by the mean time to failure. The typical mean time to failure of disks today ranges from 30,000 to 800,000 hours (about 3.4 to 91 years).