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

Yongqiang Yang
2011-04-14T16:00:47Z

2011/4/14 Eric Sandeen <sandeen@sandeen.net>:
> 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...
Sorry for that my patch ignored fallocate. The situation is like this:
An user allocated space for a file with fallocate, and write a small
part of it. and the written is not flushed.
The extent stays one unwritten extent in disk and memory with delayed
allocation.

In ext4 ext4_ext_walk_space() thinks an extent does not exist only if
there is no any extents on disk. So ext4_ext_walk_space()
thinks there is a extent and ext4_ext_fiemap_cb() thus ignores pagecache.

I think ext4_ext_walk_space() should take unwritten extent not exist.

Yongqiang.
>
> -Eric
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--
Best Wishes
Yongqiang Yang