On Sun, Sep 23, 2012 at 09:56:34AM +1200, Jp Wise wrote:

quoted

Afaik without playing with it myself fiemap can give you information
about the mappings of each file. If the mappings of 2 files match,
the data is shared.

OK, have just done some searching on fiemap and located a code example using
it to pull the file extent data.
http://smackerelofopinion.blogspot.com/2010/01/using-fiemap-ioctl-to-get-file-extents.html
Will have a play around to see if i might be able to hack it up to compare
two files, or just parse it's output between two files to identify matches.
Thank you for the pointer. :)

Likewise if anyone else knows of an existing utility to do a non-bytewise
compare between two files, and just check if they share the same datablocks
please let me know. :)

The FIEMAP is a way with stable and defined interface to show file
extents, there's the filefrag utility (from e2fsprogs). It does not
have a parser-friendly output, so you may want to call the ioctl
directly. The key information is in the (struct
fiemap_extent)->fe_physical field. If physical block ranges from two
files overlap, they're shared.

There's another way how to get the extent info, via btrfs' SEARCH_TREE
ioctl, but it's more low-level and needs basic knowledge about the
internal b-tree items and how they're linked together.

david