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

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Date:	Fri, 29 Jan 2010 10:30:08 -0500
From:	Nick Dokos <nicholas.dokos@...com>
To:	Giel de Nijs <giel@...torwise.com>
Cc:	linux-ext4@...r.kernel.org, nicholas.dokos@...com
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.

Thanks,
Nick

Transcript:

root@...fter:~/src/ext4/giel-de-nijs# ./a.out
opening file ext4_bug.testfile
submitting write of 524288 bytes at offset 6442450944
waiting for write to be finished
got 1 events
written 524288 bytes
submitting write of 524288 bytes at offset 5368709120
waiting for write to be finished
got 1 events
written 524288 bytes
root@...fter:~/src/ext4/giel-de-nijs#  dd if=ext4_bug.testfile bs=512k count=1 skip=10K|hexdump
0000000 ffff ffff ffff ffff ffff ffff ffff ffff
*
1+0 records in
1+0 records out
524288 bytes (524 kB) copied, 0.0045471 s, 115 MB/s
0080000
root@...fter:~/src/ext4/giel-de-nijs#  dd if=ext4_bug.testfile bs=512k count=1 skip=12K|hexdump
0000000 ffff ffff ffff ffff ffff ffff ffff ffff
*
1+0 records in
1+0 records out
524288 bytes (524 kB) copied, 0.00474075 s, 111 MB/s
0080000
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