Messages in this thread | | | Date | Fri, 19 Mar 1999 10:49:48 -0500 | From | Arvind Sankar <> | Subject | Re: disk head scheduling |
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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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