linux-kernel - Re: fiemap is broken for sparse file in ext4?

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Message-ID: <4C11C41C.9070000@oracle.com>
Date:	Fri, 11 Jun 2010 13:05:32 +0800
From:	Tao Ma <tao.ma@...cle.com>
To:	Eric Sandeen <sandeen@...hat.com>
CC:	linux-ext4@...r.kernel.org, Theodore Tso <tytso@....edu>,
	linux-kernel@...r.kernel.org
Subject: Re: fiemap is broken for sparse file in ext4?

Hi Eric,
	Thanks for the quick response.

On 06/11/2010 12:08 PM, Eric Sandeen wrote:
> Tao Ma wrote:
>> Hi Ted and other ext4 gurus,
>>      I found fiemap may be broken for sparse files in ext4. Here is a
>> simple example.
>>
>> dd if=/dev/zero of=testfile1 bs=1M count=1
>> using fiemap shows that it has a delalloc extent.
>> Logical: 0 Ext length: 1048576    Physical: 0    flags: 7
>>
>> flags 7 means FIEMAP_EXTENT_LAST, FIEMAP_EXTENT_UNKNOWN and
>> FIEMAP_EXTENT_DELALLOC,
>>
>> while if we create a sparse file, fiemap will not show the delalloc extent.
>> dd if=/dev/zero of=testfile1 bs=1M count=1 seek=1
>> using fiemap shows that it has no extent for the file. while we should
>> have some output like:
>> Logical: 1048576 Ext length: 1048576    Physical: 0    flags: 7
>>
>> So we have different output with sparse and non-sparse file. Is it a bug
>> for ext4?
>
> What are you using to call fiemap?  Here it seems to be working:
I just wrote a simple test program by calling ioctl. It is attached. 
btw, you need to call it immediately after dd so that we have a chance 
that ext4 don't have time to allocate extents. ;)
>
> # dd if=/dev/zero of=testfile1 bs=1M count=1 seek=1;
>
> # filefrag -v testfile1
> Filesystem type is: ef53
> Filesystem cylinder groups is approximately 119
> File size of testfile1 is 2097152 (512 blocks, blocksize 4096)
>   ext logical physical expected length flags
>     0     256   151946               1 merged
>     1     257   151951   151946      2 merged
>     2     259   152434   151952    253 merged,eof
> testfile1: 4 extents found
I guess maybe filefrag use the diffrent ioctl flag, maybe 
FIEMAP_FLAG_SYNC to let ext4 sync first.

Regards,
Tao

View attachment "fiemap.c" of type "text/x-csrc" (2752 bytes)