Re: disk head scheduling
Arvind Sankar (arvinds@mit.edu)
Fri, 19 Mar 1999 10:49:48 -0500
On Fri, Mar 19, 1999 at 02:38:53PM +0100, Rogier Wolff wrote:
> -- The parameters for modern disks make things less profitable. My first
> harddisk had a stepper motor for head-position that did around 1200
> steps per second seeking 10 tracks takes 10 times longer than one track.
> And a 100 tracks takes ten times longer than that. Modern disks seek
> track-to-track in a few milliseconds, and do the whole disk in under
> twenty. So fetching 20 random blocks in-order is going to be just
> a few milliseconds faster than just fetching in the random order.
No, the trouble with random order is that half the seeks are backward. I ran
a simple test on my hdd (IBM Deskstar), which just seeks forward 16Mbytes at
a time and (reads one byte from the drive to make sure something happens).
The difference between running forward doing this and running backward is a
factor of 5-6.
-- arvind
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