# Disk utilization

Because transfer rates vary among disks, most operating systems do not report disk utilization directly. Instead, they report the number of data transfers per second (in operating-system memory-page-size units.)

Because each disk acts as a single resource, you can use the following basic formula to estimate the service time, which is described in detail in Resource utilization:
``S= P/(1-U)``

To compare the load on disks with similar access times, simply compare the average number of transfers per second.

If you know the access time for a given disk, you can use the number of transfers per second that the operating system reports to calculate utilization for the disk. To do so, multiply the average number of transfers per second by the access time for the disk as listed by the disk manufacturer. Depending on how your data is laid out on the disk, your access times can vary from the rating of the manufacturer. To account for this variability, you should add 20 percent to the access-time specification of the manufacturer.

The following example shows how to calculate the utilization for a disk with a 30-millisecond access time and an average of 10 transfer requests per second:
``````U =     (A * 1.2) * X
=     (.03 * 1.2) * 10
=     .36``````
U
is the resource utilization (this time of a disk).
A
is the access time (in seconds) that the manufacturer lists.
X
is the number of transfers per second that your operating system reports.
You can use the utilization to estimate the processing time at the disk for a transaction that requires a given number of disk transfers. To calculate the processing time at the disk, multiply the number of disk transfers by the average access time. Include an extra 20 percent to account for access-time variability:
``P = D (A * 1.2) ``
P
is the processing time at the disk.
D
is the number of disk transfers.
A
is the access time (in seconds) that the manufacturer lists.
For example, you can calculate the processing time for a transaction that requires 20 disk transfers from a 30-millisecond disk as follows:
``````P =    20 (.03 * 1.2)
=    20 * .036
=    .72``````
Use the processing time and utilization values that you calculated to estimate the expected service time for I/O at the particular disk, as the following example shows:
``````S =    P/(1-U)
=    .72 / (1 - .36)
=    .72 / .64
=    1.13``````