Hardware Reference
In-Depth Information
As you can see, the
true
transfer rates for these drives are much lower than the 600MBps SATA
interface transfer rate. Although all these support the 6Gbps (600MBps) SATA transfer rate
specification, they have media transfer rates of up to 210MBps maximum, with individual drives
averaging between about 104MBps and 156MBps. Note that the fastest performing drive is not the
fastest rotating drive because physical size and density has as much or more to do with actual
performance as rotational speed does.
Two primary factors contribute to transfer rate performance between drives with equal sized platters:
rotational speed and the linear recording density or sector-per-track figures. When you're comparing
two drives with the same number of sectors per track, the drive that spins more quickly transfers data
more quickly. Likewise, when you're comparing two drives with identical rotational speeds, the
drive with the higher recording density (more sectors per track) is faster. A higher-density drive can
be faster than one that spins faster—both factors have to be taken into account.
As you can see from these examples, the interface transfer speed is almost meaningless. So, if you
were thinking about getting a new motherboard or maybe a separate host adapter card offering a faster
interface rate for the sole purpose of increasing drive performance, save your money. To be fair,
there will be a slight benefit to higher interface transfer speeds in that data from the buffer on the
drive controller can be transferred to the motherboard at interface speed, rather than media speed.
These buffers are usually 32MB or less and help only with repetitive transfers of small amounts of
data. However, if you perform repetitive transfers frequently, drives with larger buffers can improve
performance with applications that perform repetitive transfers. More recently, drives with large
Flash memory buffers, called
hybrid
drives, are being manufactured to improve performance for
startup and commonly read files. These drives offer near SSD read performance while costing only a
little bit more than standard drives.
All other things being equal, a drive that spins faster transfers data faster, regardless of the interface
transfer rate. Unfortunately, it is rare that all other things are exactly equal, so you should consult the
drive specifications listed in the data sheet or manual for the drive to be sure.
To find the transfer specifications for a given drive, look in the data sheet or preferably the
documentation or manual for the drive. These can usually be downloaded from the drive
manufacturer's website.
Average Seek Time
Average seek time
, usually measured in milliseconds (ms), is the average amount of time it takes to
move the heads from one cylinder to another a random distance away. One way to measure this
specification is to run many random track-seek operations and then divide the timed results by the
number of seeks performed. This method provides an average time for a single seek.
