[Sussex] Why Linux doesn't need defrag

[Alan Pope]

On Thu, Aug 17, 2006 at 07:18:47AM -0600, linux at oneandoneis2.org wrote:
> Quoting Alan Pope <alan at popey.com>:
> 
> >I don't agree with:-
> >
> >"Linux pays attention to the hard drive's actual geometry, and would not
> >split this file so badly."
> >
> >I was under the impression that Linux has no clue about the real physical
> >geometry of the disk because that's abstracted by the circuitry on the
> >drive/controller. As far as Linux is concerned it's a large number of
> >sectors, it has no clue *really* how they're arranged.
> >
> >That's my understanding anyway.
> 
> You could well be right - I read somewhere that Linux *did* take the  
> geometry into account, but I'm damned if I can find the relevant  
> article again. .
> 
> Anybody know for sure which is right?
> 

http://www.linuxdocs.org/HOWTOs/Large-Disk-HOWTO-11.html is a good start.

"The first serious limit was the 4096 cylinder limit (that is, with 16 heads
and 63 sectors/track, 2.11 GB). For example, a Fujitsu MPB3032ATU 3.24 GB
disk has default geometry 6704/15/63, but can be jumpered to appear as
4092/16/63, and then reports LBAcapacity 4124736 sectors, so that the
operating system cannot guess that it is larger in reality. In such a case
(with a BIOS that crashes if it hears how big the disk is in reality, so
that the jumper is required) one needs boot parameters to tell Linux about
the size of the disk."

Ergo: the numbers of heads (the middle one of the 3 numbers separated by
slashes) - 15 or 16 - is clearly not correct. How can you change the
physical number of heads in a disk with one jumper setting?

Short answer is you can't, you're change the logical representation that the
drive presents itself as.

I pulled a disk apart the other day and noted it had just two platters.

Cheers,
Al.