Subject: Re: slow disk access with large cpg values
To: David Laight <david@l8s.co.uk>
From: Lubos Vrbka <shnek@tiscali.cz>
List: netbsd-users
Date: 04/22/2004 16:12:46
David Laight wrote:

> The 'cpg' field of the disklabel no longer contains the number of cylinders
> in a cylinder group.  The filesystems 'cylinder groups' are no longer
> aligned to physical cylinders, and the filesystem data areas are defined
> as if there were a single cylinder in each group.
> Because the allocation bitmaps for a cg have to fit inside a fs block,
> with 8k blocks (the default) each cg has a maximum size of (about) 55MB.
> This means that large (ie any modern) disk has a lot of cylinder groups.
whoa, i'm confused now... :) if i got it correctly - the value of the 
corresponding column no longer means "cylinders per group" but rather 
something else. it is probably not number of 'cylinder groups' - it 
would mean that my 18gb disk (the fast one) has only 86 'cylinder 
groups' of max. 55mb each (ie max 4730mb). with the same logic, cylinder 
groups on the second drive would have to be much less than 55 mb 
(0.7mb), since i have 28546 of them (should be approx. 350 of them for 
55mb 'cylinder group')
so what does the number mean if it is not number of cylinders per group? 
is it 'sgs' value - segment shift (i don't know what's that)...

frederick mentioned that there was some change of the method how newfs 
works with these numbers. cannot be system confused how to handle disks 
with so different 'cpg/sgs' values?

i'm probably totally out... :)

thanks in advance for clarification,

-- 
Lubos
_@_"