> I cannot imagine why this basic textbook stuff would not already be
> implemented...

Actually it is [1], and is partially responsible for the delays!
The problem is that a given request can be delayed for a very
long time because of a sequential read/write to a different part
of the disk.

	David

[1] I suspect the 'escalator' algorithm is actually used.

-- 
David Laight: david@l8s.co.uk