The hard disk spindle is the shaft upon which the platters
are mounted; it is driven by the spindle motor, one of
the most important components in the hard disk. Obviously, the faster the motor turns, the
faster the platters spin. The spindle speed of the hard disk is always given in RPM
(revolutions per minute). Typical speeds of drives today range from 4,200 RPM to 15,000
RPM, with 5,400 to 10,000 RPM being most common on desktop machines. See this operational discussion of spindle speed for
a table of the most common speeds employed today and in the past, and a list of different
applications that use them.
Spindle speed has gone from being one of the least-discussed to one of the
most-discussed hard disk specifications in only a few short years. The reason is the
creation of increasingly fast spindles. For the first several years that hard disks were
used in PCs, they all had the same spindle speed--3,600 RPM--so there was literally
nothing to talk about in this regard! Over time, faster drives began to appear on the
market, but slowly, and starting with high-end SCSI
drives not used in most systems. Once the trend started, however, and the obvious
advantages of higher spin speeds became apparent, the trend accelerated. Still, it is only
since about 1998 that mainstream IDE/ATA drives
have been readily available for the desktop in spindle speeds higher than 5,400 RPM. The
most common speeds today are 5,400 and 7,200 RPM, and 10,000 RPM IDE/ATA drives are likely
just around the corner (since they are now standard on SCSI with the SCSI high-end moving
to 15,000 RPM!)
Today, spindle speed is the first thing you really should look for when assessing a
drive; the speed of the spindle is the primary method by which drives are categorized into
"classes". Almost any 7,200 RPM drive will be faster, and more expensive, than a
5,400 RPM drive of the same size and generation. The spindle speed directly correlates to
the drive's rotational latency, affecting positioning
performance, and also influences the drive's internal transfer
rate. However, there is more to this: the difference in speed between different
classes of drives is due not only to the speed of the spindle, but the fact that
manufacturers tend to design these drives to be faster in other ways as well, knowing they
are targeting a market more concerned with all facets of performance.
The spindle speed is of course influenced primarily by the spindle motor's speed and power. However, there are
other issues involved in designing higher-RPM drives: you can't just slap a faster motor
into an existing model! The size and number of platters
is also an important design consideration, and the areal
density of the drive also has an impact--faster drives sometimes require reductions in
density compared to slower drives.
In fact, the overall quality of the entire hard disk becomes much more critical the
faster you spin the platters. Issues with higher-speed drives include increased noise and vibration, and cooling concerns, though these have improved
greatly with second, third and subsequent generation high-speed drives.
Next: Areal Density
