Comments:	Authenticated sender is <alaric+abwillms AT sdps DOT demon DOT co DOT uk>
From:	"Alaric B. Williams" <alaric AT abwillms DOT demon DOT co DOT uk>
To:	Lorier <lorier AT ihug DOT co DOT nz>
Date:	Fri, 18 Apr 1997 21:24:25 +0000
MIME-Version:	1.0
Subject:	Re: Usage of directory entries
Reply-to:	alaric AT abwillms DOT demon DOT co DOT uk
CC:	Matthias DOT Paul AT post DOT rwth-aachen DOT de, opendos-developer AT delorie DOT com
In-reply-to:	<m0wIAP0-000Fm9C@hn.planet.gen.nz>
Message-ID:	<861394882.116759.0@abwillms.demon.co.uk>

On 18 Apr 97 at 21:56, Lorier wrote:

> >An extent based filer like VSTa uses or, I deduce, NTFS is (it uses
> >512 byte allocation units) can be more efficient, I think; disk
> >space is managed like malloc allocates blocks of RAM, in runs of 
> >sectors. Unless it gets really fragmented, this is smaller than having 
> >free-space bitmaps and indirection blocks and all that. And smaller
> >means faster, no? :-)
 
> Er, not always :) It's usually a trade off between Speed and Size, having to
> do it that way means you have to do mass calculations, and also trying to
> handle adding to to the end of a file is a problem if the next file is right
> at the end, thus leading to high fragmentation :)

To extend a file, just increment that extent's length counter until you reach
the next extent, then start with a fresh extent. Easy peasy?!?!?

Fragmentation can be solved by moving fragmented files (found when the filer
notes that accessing a certain file has entailed a lot of extent seeks)
into contiguous areas from time to time, a sort of background defrag that
works on individual files when it feels the need.
 
ABW
--
Alaric B. Williams (alaric AT abwillms DOT demon DOT co DOT uk)

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