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SubjectRe: elevator algorithm bug in ll_rw_blk.c
On Fri, 13 Nov 1998, Chip Salzenberg wrote:

> But that's a *one-way* elevator. Ideal elevators are two-way, aren't they?

Normally you use a one-way scan. A two-way scan is unfair, it will service
different parts of the disks diffrently. Specifically the edges will on
average have a longer wait than the central parts.

Imagine a request being placed just after the head passed the track. If it
is in the central region it will at most wait for the head to seek
1/2+1/2=1 disk (seek to the edge and back). On the periphery it will have
to wait at most 1+1=2.

A long seek does not take that much longer than a medium one using a
unidirectional scan lowers the max-norm of the wait.

Peter
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