Btrfs devel - Aug 2013 - check for reflink capability and for shared data

Hi,

how can I do the following in a shell script:

- check whether my file system supports cp --reflink?
- check whether two files share the same data on disk, i.e. one has been 
created by cp --reflink of the other?

Thank you!

Thomas Koch
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On Sat, Aug 24, 2013 at 06:09:58PM +0200, Thomas Koch
wrote:
> Hi,
> 
> how can I do the following in a shell script:
> 
> - check whether my file system supports cp --reflink?
touch foo; if cp --reflink=always foo bar; then ...; fi; rm -f foo bar
> - check whether two files share the same data on disk, i.e. one has been 
> created by cp --reflink of the other?
   You can''t, using simple userspace tools. I think the only way would
be to use the tree search ioctl to inspect the extents for each file,
and see whether any of them overlap. Why do you need to know this?

   Hugo.

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On Sat, August 24, 2013 at 18:20 (+0200), Hugo Mills
wrote:
> On Sat, Aug 24, 2013 at 06:09:58PM +0200, Thomas Koch wrote:
>> Hi,
>>
>> how can I do the following in a shell script:
>>
>> - check whether my file system supports cp --reflink?
> 
> touch foo; if cp --reflink=always foo bar; then ...; fi; rm -f foo bar
> 
>> - check whether two files share the same data on disk, i.e. one has
been
>> created by cp --reflink of the other?
> 
>    You can''t, using simple userspace tools. I think the only way
would
> be to use the tree search ioctl to inspect the extents for each file,
> and see whether any of them overlap. Why do you need to know this?
It should work with fiemap to a file in question and btrfs inspect logical to
what ever extents fiemap reported as "physical".

-Jan
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On Sat, Aug 24, 2013 at 06:09:58PM +0200, Thomas Koch
wrote:
> Hi,
> 
> how can I do the following in a shell script:
> 
> - check whether my file system supports cp --reflink?
> - check whether two files share the same data on disk, i.e. one has been 
> created by cp --reflink of the other?
Someone has proposed to teach ''du(1)'' to show this, ie.
''du --shared''[1].

It''s written for ocfs2 originally, besides btrfs, ocfs2 also can share
data between two files.

[1]: https://oss.oracle.com/pipermail/ocfs2-devel/2010-September/007287.html

-liubo
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On Sat, Aug 24, 2013 at 12:09 PM, Thomas Koch <thomas@koch.ro>
wrote:
> - check whether two files share the same data on disk, i.e. one has been
> created by cp --reflink of the other?
How about inspecting the output of filefrag -v $filename?
For example, you could filter out with grep all lines except the ones
that contain information about the extents.
Then you could hash the lines left and use that single hash to compare
the two files.
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