Re: [PATCH 2/2] Handle group offsets in UFS1

[Vladimir 'phcoder' Serbinenko]

On Thu, Aug 13, 2009 at 10:05 PM, Robert Millan<address@hidden> wrote:
> On Thu, Aug 13, 2009 at 06:04:35PM +0200, Vladimir 'phcoder' Serbinenko wrote:
>
> This looks a bit odd (a mask applied to an integer?), but if it's really
> this way, please go ahead with it.
>
It's the so-called skewness.
Let's say you place inodes on addresses (C/H/S)
(0/0/1) and (1/0/1) you first read the metadata at (0/0/1) then you
try to fetch the metadata from (0/1/1). Responding to your request
harddrive moves the head to cylinder number 1 but it takes some time.
Meanwhile the plates have spinned (they are spinning constantly) and
perhaps head is above sector (1/0/10) and you need to wait for
complete rotation to fetch your sector.
If you write inodes at (0/0/1) and (1/0/15) you will need to wait only
for 4 sectors. This is called skewness and was an optimisation
technique in the past. But now OS doesn't know about physical geometry
and so can't do such kind of optimisation. I suppose it's why it's not
used anymore for UFS2. I don't know if FreeBSD variant of UFS1 still
uses this feature.
> Btw, your mailer marked this attachment as application/octet-stream (I can't
> context-reply :-( )
>
I forgot the extension. Sorry.
> --
> Robert Millan
>
>  The DRM opt-in fallacy: "Your data belongs to us. We will decide when (and
>  how) you may access your data; but nobody's threatening your freedom: we
>  still allow you to remove your data and not access it at all."
>
>
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-- 
Regards
Vladimir 'phcoder' Serbinenko

Personal git repository: http://repo.or.cz/w/grub2/phcoder.git