RE: elevator algorithm bug in ll_rw_blk.c
Lyle Seaman (lyle@stormsystems.com)
Tue, 17 Nov 1998 20:25:12 -0500
On Tuesday, November 17, 1998 8:22 AM, Tomasz Przygoda
[SMTP:tprzyg@securities.com] wrote:
> "Stephen C. Tweedie" wrote:
> > Two-way elevators _do_ have this problem. A block in the middle
> > cylinder of the disk gets serviced twice per full two-way pass,
once
> > in
> > each direction. Maximum wait time is half a pass; average is a
> > quarter. For a block at the edge of the disk, it will get serviced
> > only
> > when the two-way pass approaches close to the edge of the disk, and
> > will
> > not get another chance for a full pass, average wait half a pass.
This is incorrect. Maximum wait time for a block in the middle
cylinder is also one full pass, unless you let the heads back up.
Guessing at averages requires assumptions about access patterns.
writing sectors backwards is the bigger problem, unless your disk
supports tagged queueing.
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