linux-ext4 - Re: Files full of zeros with coreutils-8.11 and xfs (FIEMAP related?)

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Date:	Thu, 14 Apr 2011 10:56:16 -0500
From:	Eric Sandeen <sandeen@...deen.net>
To:	Pádraig Brady <P@...igBrady.com>
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 4/14/11 10:52 AM, Pádraig Brady wrote:
> 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.
> 

well, if I simply take the preallocation step out of the testcase, it works fine on xfs without a sync.

So I still don't know what Markus hit...

-Eric
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