Hardware Reference
In-Depth Information
hard disks
to differentiate them from the long-vanished
floppy disks
used on the
very first personal computers. In this business, it is difficult to pick a name for
anything and not have it be ridiculous 30 years later.
Most disks consist of multiple platters stacked vertically, as depicted in
Fig. 2-20. Each surface has its own arm and head. All the arms are ganged toget-
her so they move to different radial positions all at once. The set of tracks at a
given radial position is called a
cylinder
. Current PC and server disks typically
have 1 to 12 platters per drive, giving 2 to 24 recording surfaces. High-end disks
can store 1 TB on a single platter and that limit is sure to grow with time.
Read/write head (1 per surface)
Surface 7
Surface 6
Surface 5
Surface 4
Surface 3
Direction of arm motion
Surface 2
Surface 1
Surface 0
Figure 2-20.
A disk with four platters.
Disk performance depends on a variety of factors. To read or write a sector,
first the arm must be moved to the right radial position. This action is called a
seek
. Average seek times (between random tracks) range in the 5- to 10-msec
range, although seeks between consecutive tracks are now down below 1 msec.
Once the head is positioned radially, there is a delay, called the
rotational latency
,
until the desired sector rotates under the head. Most disks rotate at 5400 RPM,
7200 RPM, or 10,800 RPM, so the average delay (half a rotation) is 3 to 6 msec.
Transfer time depends on the linear density and rotation speed. With typical inter-
nal transfer rate of 150 MB/sec, a 512-byte sector takes about 3.5
μ
sec. Conse-
quently, the seek time and rotational latency dominate the transfer time. Reading
random sectors all over the disk is clearly an inefficient way to operate.
It is worth mentioning that on account of the preambles, the ECCs, the inter-
sector gaps, the seek times, and the rotational latencies, there is a big difference be-
tween a drive's maximum burst rate and its maximum sustained rate. The maxi-
mum burst rate is the data rate once the head is over the first data bit. The com-
puter must be able to handle data coming in this fast. However, the drive can keep
up that rate only for one sector. For some applications, such as multimedia, what
