linux-ext4 - Re: Possible ext4 data corruption with large files and async I/O

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Message-id: <9721.1264780167@gamaville.dokosmarshall.org>
Date:	Fri, 29 Jan 2010 10:49:27 -0500
From:	Nick Dokos <nicholas.dokos@...com>
To:	nicholas.dokos@...com
Cc:	Giel de Nijs <giel@...torwise.com>, linux-ext4@...r.kernel.org
Subject: Re: Possible ext4 data corruption with large files and async I/O

> > 
> > Dear ext4 devs,
> > 
> > Today I hit a situation where seemingly blocks did not get written to 
> > disk. I've narrowed it down to the following test case.
> > 
> > Running Fedora Core 12 with kernel 2.6.31.9-174.fc12.x86_64, both on an 
> > i7 920 and a Core2 Q6600, I executed the following steps:
> > 
> > - create a file
> > - with kernel async i/o, write a 512kb (haven't tried other sizes) block 
> > to an offset >4GB, effectively creating a large sparse file
> > - again with async i/o, write a 512kb block to an offset smaller than 
> > the previous write, but >4GB
> > - wait for the kernel async i/o to tell you the writes have succeeded
> > 
> > Now, looking at the file, the second write never seems to have happened. 
> > When doing this on the same machines on ext3, the behavior is as expected.
> > 
> > As far as I can tell (from the bigger program that triggered this), all 
> > writes >4GB but < EOF to a sparse file with async i/o aren't executed. 
> > When creating a large file first (i.e., with dd), everything does work 
> > as expected.
> > 
> > Attached is some C code that triggers this bug for me.
> > 
> > If you need more information or want me to test some more things, please 
> > do ask.
> > 
> 
> I ran your program on FC-11 with a 2.6.33-rc4 upstream kernel: it worked fine.
> Both dd's gave the expected output.
> 

Scratch that: I goofed. I can reproduce it too.

Sorry for the confusion.

Nick
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