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Date:	Thu, 14 Apr 2011 16:52:11 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	Eric Sandeen <sandeen@...deen.net>
CC:	xfs-oss <xfs@....sgi.com>, linux-ext4@...r.kernel.org,
	coreutils@....org, Markus Trippelsdorf <markus@...ppelsdorf.de>
Subject: Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

On 14/04/11 16:50, Eric Sandeen wrote:
> On 4/14/11 9:59 AM, Pádraig Brady wrote:
>> On 14/04/11 15:02, Markus Trippelsdorf wrote:
>>>>> Hi Pádraig,
>>>>>
>>>>> here you go:
>>>>> + filefrag -v unwritten.withdata                                                                                                                     
>>>>> Filesystem type is: ef53                                                                                                                             
>>>>> File size of unwritten.withdata is 5120 (2 blocks, blocksize 4096)                                                                                   
>>>>>  ext logical physical expected length flags                                                                                                          
>>>>>    0       0   274432            2560 unwritten,eof                                                                                                  
>>>>> unwritten.withdata: 1 extent found
>>>>>
>>>>> Please notice that this also happens with ext4 on the same kernel. 
>>>>> Btrfs is fine.
>>>>
>>> `filefrag -vs` fixes the issue on both xfs and ext4.
>>
>> So in summary, currently on (2.6.39-rc3), the following
>> will (usually?) report a single unwritten extent,
>> on both ext4 and xfs
>>
>>   fallocate -l 10MiB -n k
>>   dd count=10 if=/dev/urandom conv=notrunc iflag=fullblock of=k
>>   filefrag -v k # grep for an extent without unwritten || fail
> 
> right, that's what I see too in testing.
> 
> But would the coreutils install have done a preallocation of the destination file?
> 
> Otherwise this looks like a different bug...
> 
>> This particular issue has been discussed so far at:
>> http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8411
>> Note there it was stated there that ext4 had this
>> fixed as of 2.6.39-rc1, so maybe there is something lurking?
> 
> ext4 got a fix, but not xfs, I guess.  My poor brain can't remember, I think I started looking into it, but it's clearly still broken.
> 
> Still, I don't know for sure what happened to Markus - did something preallocate, in his case?

Well that preallocate test is failing for him
when the source file is either on ext4 or xfs.
He noticed the issue initially on XFS when copying
none preallocated files, so XFS probably just has
the general issue of needing a sync before fiemap,
where as EXT4 just has this preallocate one
(though I've not seen it myself).

cheers,
Pádraig.
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