How fast are your disks?

[pri=-10 Stuart Lynne]

In article <3842 at watcgl.waterloo.edu> tbray at watsol.waterloo.edu (Tim Bray) writes:
>that Unix can do a maximum of 30 disk I/O's a second".  Somebody else remarked
>that that figure had been remarkably constant for quite some time.  Somebody
>else proposed that it was a fundamental law of Computer Science.  (Of course,
>we are poor peons restricted to the use of Vaxes and Suns).

Probably related to your average seek time plus rotational delay plus data
transfer. 

On most popular, extant Unix systems 20 - 30 ms is a reasonable figure
for average seek. Average rotational latency is 8.5 ms. Transfer time for a
one sector, say about 1 ms. 

Given a fast 20 ms drive, you probably should approach 30 disk I/O's per
second. 

Given a slow 30 ms drive, probably closer to 25, 40 ms about 20.

Other factors which will help are controllers which will overlap seeks;
multiple disk's to localize file accesses (allowing average seeks times to
decline); larger block sizes (actually getting the information in is only a
small part of the battle, getting there is the largest component for small
random reads).


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