(^190) PC Hardware: A Beginner’s Guide
Performance Indicators
The most common of the performance specifications are Seek Time This is one of the more important performance indicators of the
speed of a hard disk. Manufacturers like to dazzle you with their rotation speed,
which is an important element of the disk’s access time, butseek time, which is
measured in milliseconds (ms), is the time it takes the head actuator to move the
head arm and read/write heads from one track to the next. Seek time does not
include the time required to move to a specific data location.Average seek timeis a
commonly used benchmark for comparing drive performance. Average seek time
is calculated from the drive’s performance over a number of randomly located
requests. Most current disk drives have average seek times of 8 to 14 ms.
Access Time This time includes seek time and measures the time required
to position the read/write heads on a particular track and to find the sector
containing a particular disk location. Access time involves latency, or rotational
delay, which is the time required for the disk to rotate the sector being accessed
under the read/write head. Latency is also measured in milliseconds and is
generally around one-half the time required for the disk to make a single
revolution. At 7,200 rpm (revolutions per minute), a very common disk
rotational speed today, the latency is around 4 ms. As the rotational speed
of the drive increases, the latency time decreases proportionately.
Data Transfer Rate This is the amount of data that can be moved between
the disk and RAM in one second. The data transfer rate is normally given as
a number of megabytes (MB) per second. The higher the data transfer rate, the
less time a user will wait for software to load or data to be retrieved. Data
transfer rates of 5 to 40 MBps are common on today’s hard disk drives.
Data Access Time (QBench) This measurement combines the data access
time and the data transfer rate to provide a rating of a disk drive’s overall
performance. This specification was developed by a hard disk manufacturer,
Quantam (www.quantum.com), who also developed a benchmarking tool
called Qbench, which has become a widely used standard for drive
performance measurement and comparison.
Disk Capacity A very important criteria for disk performance is how much data
it can actually store. Disk drives typically have two capacity ratings: unformatted
and formatted. The formatted capacity is usually the most important metric for
most people, since it is the one that states the usable disk space on the drive. Nearly
all drives being sold today are in the range of 500MB to 30GB.
Areal Density While not technically a performance measurement, the areal
density of a disk is an indication of a disk drive’s storage capacity. Theareal
densityof a disk is calculated by multiplying the number ofbits per inch (bpi)
(the number of bits in the total length of a track) by the number or tracks on the