format vs diag and Wren VII (sun-spots digest v9n60)

[Daniel Strick]

The following comments are vaguely in response to the the query:

	" ... and those artifacts are carefully preserved in the
	disk label and format.dat for no good reason i know of.
	anyone know to the contrary?"

In theory, using the correct geometry will improve performance with a
Berkelix fast file system.  Unfortunately, all the SCSI systems I have
played with (including the ones I cook up) are either too slow to begin
with or simply don't try very hard to optimize for the exact disk
geometry.  They tend to assume the geometry is a lie and do some sort of
elevator algorithm in the block address space instead of the cylinder
address space.  I don't think anybody tries to do rotational optimization
with SCSI.

The amount of disk space that can be saved by carefully selecting cylinder
geometry is trivial.  When I label a SCSI disk, I usually use the correct
number of heads and the actual track length for the first cylinder.  At
least this way I get the first few cylinders correct.  Then I lie about
the number of cylinders.  The arithmetic is easy and I only lose an
average of half a cylinder (no big deal when you have ~1500 cylinders).

Dan Strick, aka dan at bellcore.com or bellcore!dan, (201)829-4624