Subject: Re: start of the partition c
To: <>
From: David Laight <>
List: netbsd-users
Date: 07/16/2004 21:10:42
> I've read somewhere that all partitions should start and end on cylinder 
> boundaries.

If you have a disk that has several 14" platters (and might be 20MB)
then it is true - OTOH they went out of fashion in about 1980.

Modern disks don't have fixed geometries, and lie about the geometry
they do have.  So even if there were marginal benefits in aligning
on cylinder boundaries you can't tell where they are.

Worse still, for IDE disks on i386 there are two geometries!
One reported by the disk, and one by the BIOS.  You can't actually
align to both types of cylinder.

The only time you do need to align partitions to cylinders is when
using Sun's labels (eg on sparc) where the label itself contains
cylinder numbers - not block numbers.

> How come disklabel does not do that automatically?

Because it can't know what you really mean - and it doesn't matter.
(and it doesn't do anything automatically)

I suspect that sysinst will align the mbr (fdisk) partitions on
BIOS cylinder boundaries (typically 255*63 = 16065) sectors.
But then try to align (or maybe just size) the disklabel partitions
to disk cylinders - but inside that mbr partition.

In any case the performance effects are probably minimal.


David Laight: