lot more likely that the microprocessor will sit around waiting for data than it
is that the hard drive will pump information faster than the microprocessor
can work on it.
A number of factors go into the calculation of speed:
The revolutions per minute of the platters. The faster the platter spins,
the quicker the read/write heads can move into position to a particular
cluster and the faster the stream of data that comes off the disk.
Designers talk about latencyas a factor here, defining it as the average
amount of time for the spinning drive to bring a particular cluster
beneath or above the read/write head. Modern laptops generally use
hard drives that spin at 4,200 rpm, although faster motors are beginning
to be installed.
The seek time for the read/write heads. This is the average amount of
time required for the heads to move in or out to locate a track. Obviously,
it is much quicker to move from track 49 to track 50 than it is to go all
the way from 0 to 79. The hard drive mechanism can benefit from soft-
ware utilities that attempt to reorganize data in a way that attempts to
anticipate the system needs.The density of the data on the disk. The closer together the bits of infor-
mation, the more data there is in a particular cluster and the faster the
stream of data.
The data transfer speed— the thickness and pumping power of the pipe
between the disk and the processor. Although a number of data inter-
faces are in use on desktop machines (including ATA/IDE and a number
of flavors of SCSI standards — more on these in the next section), nearly
all laptops use a parallel ATA/IDE connection. In coming years, most
hard drives are expected to migrate to the developing serial version of
ATA (called SATA), which gives a slight speed boost while it reduces
some of the complexity of the internal cabling.When hard drives were first developed, each track had the same number of
sectors, which was a simpler scheme but very wasteful because the outer
tracks have much more space than the inner ones. Modern drives vary the
number of sectors in each track, subdividing the outer circles into many
more sectors than are found in the smaller inner ones. Therefore the stream
of information from the outer tracks of a hard drive is much greater than
from inside tracks: The outer edge is traveling much faster than the core and
there is more information stored there.
Advanced hard drives also include data buffers,which are small blocks of
RAM intended to help keep the transfer pipe filled at all times; they compen-
sating for any differences in transfer rates amongst various devices.
Chapter 7: Easing In to Hard Disks 121