Disk Drive Capacity; Disk Drive Performance - Compaq BL10e - HP ProLiant - 512 MB RAM Technology Overview

and the reliability of these drives, combined with their lower power and smaller size, have hastened
this transition. By the end of 2010, all new enterprise class drives and all new 10k and 15k RPM
drives will be SFF. HP expects to continue to develop 3.5-inch midline and entry class drives.

Disk drive capacity

The capacity of a drive, measured in gigabytes, is set at manufacturing. Today's drives are capable
of storing hundreds of gigabytes and some are capable of storing a terabyte or greater. The drive's
capacity is determined by the number of platters it contains, the surface area of each platter, and the
number of bits that can be stored per unit area (called areal density). Areal density is determined by
the number of tracks-per-inch of disk radius multiplied by the number of bits--per-inch of track.
A common source of confusion regarding disk drive capacity is the definition of a gigabyte. In a disk
drive, a gigabyte is exactly 1,000,000,000 bytes, but operating systems often use the binary-based
approximation of 2 30 , or 1,073,741,824 bytes, per gigabyte. Thus, the operating system may report
that a disk drive with 100 actual gigabytes of storage has only 93 gigabytes.

Disk drive performance

Several factors determine the performance of a disk drive. These include the rotational speed of the
platters, seek performance, mechanical latency, read/write bandwidth, queuing strategies, and
interface technologies.
When preparing to read data from the disk, the drive head must move to the position above the
correct track and then wait for the target segment to pass under the head. This mechanical delay—the
time to move the head to the correct track and then wait for the target segment—is called the latency
or seek time.
Latency, which is fundamental to disk system performance, is measured in milliseconds (ms). Typical
values are 4 to 10 ms. A number of strategies have been developed to directly or indirectly avoid or
reduce this mechanical latency (Table 2). For example, doubling the rotation rate of the disk platter
can reduce the time spent waiting for the target segment to pass under the head.
Disk drive performance is usually characterized under one of two data transfer scenarios—continuous
data transfer rate of the media and random Input/Output operations per second (IOPs).
Continuous data transfer occurs when reading or writing relatively large blocks of data to sequential
disk sectors. It sets the upper boundary of performance for the drive. It should be noted, however, that
the maximum continuous data rate is valid only for the outermost tracks on the drive, and that this rate
can be up to 50% lower on the inner tracks.
Random access occurs when reading or writing relatively small blocks of data to sectors that may be
scattered across the disk. The speed of the actuator and the spindle determine performance in this
scenario and set the lower boundary of performance for the drive.
The performance of disk drives deployed in actual computing environments is heavily dependent on
the nature of the application, for example, whether it is dealing with large blocks of sequential data
(video files) or small blocks of unrelated data (customer records in an e-commerce database). As a
disk drive fills up, large blocks of data may have to be written to non-sequential segments or non-
adjacent tracks. This scattering of data across the disk, called fragmentation, can significantly
degrade performance.
4