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How much time does it take for the head in a harddrive

S

soccerballtux

Lifer
Trying to fall asleep last night, I was considering what the bottleneck in harddrives is. I realized the ~8ms latency spec'd on harddrives is the amount of time it takes for the platter to make 1 revolution at 7200RPM. Which means...this is the limiting factor in harddrive speed.

So I'm wondering how much time it takes for the head to move. (IE, assuming the platter were spinning and infinite number of revolutions per minute, what would the latency be?)

Moved to Memory and Storage Forum - Moderator Rubycon
 
http://www.storagereview.com/g...perf/spec/posSeek.html

At one point many years ago seek times were difficult to use because manufacturers wouldn't agree on a standardized way of reporting them. Today, this has largely been corrected. While seek time is usually given as a single number, in fact there are three different seek time specifications you should examine for a drive, as they represent the drive's performance when doing different types of seeks:

* Average: As discussed, this is meant to represent an average seek time from one random track (cylinder) to any other. This is the most common seek time metric, and is usually 8 to 10 ms, though older drives had much higher numbers, and top-of-the-line SCSI drives are now down to as low as 4 ms!
* Track-to-Track: This is the amount of time that is required to seek between adjacent tracks. This is similar in concept (but not exactly the same as) the track switch time and is usually around 1 ms. (Incidentally, getting this figure without at least two significant digits is pretty meaningless; don't accept "1 ms" for an answer, get the number after the decimal point! Otherwise every drive will probably round off to "1 ms".)
* Full Stroke: This number is the amount of time to seek the entire width of the disk, from the innermost track to the outermost. This is of course the largest number, typically being in the 15 to 20 ms range. In some ways, combining this number with the average seek time represents the way the drive will behave when it is close to being full.
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