linux-ext4 - Re: [RFC] Defragmentation interface

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Message-ID: <20061103193800.GD17306@atrey.karlin.mff.cuni.cz>
Date:	Fri, 3 Nov 2006 20:38:00 +0100
From:	Jan Kara <jack@....cz>
To:	Andreas Dilger <adilger@...sterfs.com>
Cc:	David Chinner <dgc@....com>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [RFC] Defragmentation interface

> On Nov 03, 2006  15:30 +0100, Jan Kara wrote:
> > > - stripe unit and stripe width need to be exposed so defrag too
> > >   can make correct placement decisions.
> >   fs-specific thing...
> 
> I think this is not just XFS-specific.  It is very desirable to align
> large IO to the RAID stripe so that if you write stripe_width bytes
> you don't do 2 expensive read-modify-write steps, but rather a single
> write operation.  Also RAID controllers have (large) internal cache
> "lines" and having aligned reads can help noticably, and also if array
> is doing RAID parity checking to avoid overhead of doing this for two
> stripes instead of one.
  I see, thanks for information. On the other hand this is rather an
underlying block device feature than a fs thing, isn't it? So does it
make sence to pass this information via fs interface?

								Honza
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