linux-kernel - Re: write 'O_DIRECT' file w/odd amount of data: desirable result?

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Message-ID: <20110224092625.GA3087@dastard>
Date:	Thu, 24 Feb 2011 20:26:25 +1100
From:	Dave Chinner <david@...morbit.com>
To:	"Linda A. Walsh" <lkml@...nx.org>
Cc:	PXXdraig Brady <P@...igBrady.com>,
	LKML <linux-kernel@...r.kernel.org>, xfs-oss <xfs@....sgi.com>
Subject: Re: write 'O_DIRECT' file w/odd amount of data: desirable result?

On Wed, Feb 23, 2011 at 10:04:30AM -0800, Linda A. Walsh wrote:
> 
> 
> FWIW -- xfs-oss, included as 'last line' was of minor interest; known bug on
> this kernel?:
> Linux Ishtar 2.6.35.7-T610-Vanilla-1 #2 SMP PREEMPT Mon Oct 11
> 17:19:41 PDT 2010 x86_64 x86_64 x86_64 GNU/Linux
....
> Of *SIGNIFICANT* note.  In trying to create an empty file of the size
> used, from scratch, using 'xfs_mkfile', I got an error:
> 
> > xfs_mkfile 5776419696 testfile
> pwrite64: Invalid argument

xfs_mkfile does not create an "empty" file. It creates a file that
is full of zeros.

iAnd you're getting that erro because:

5776419696 / 512 = 11,282,069.7188

the last write is not a multiple of the sector size and xfs_mkfile
uses direct IO. It has always failed when you try to do this. If you
want to create allocated, zeroed files of abitrary size, then use:

xfs_io -f -c "truncate $size" -c "resvsp 0 $size" $filename

to preallocate it. it'll be much, much faster than xfs_mkfile.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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