[Bug 8918] Use fiemap to quickly detect zero ranges of source file

[samba-bugs at samba.org]

https://bugzilla.samba.org/show_bug.cgi?id=8918

--- Comment #2 from Theodore Ts'o <tytso at mit.edu> 2012-05-08 10:01:13 UTC ---
Using FIEMAP has some real potential problems if the file was just recently
written (and has blocks which were recently written, and where their final
location on disk has not been determined yet).   You can work around this using
the explicitly or implicitly forcing an fsync if this case is found, but
supporting SEEK_HOLE/SEEK_DATA avoids this problem.  The tradeoff is that only
the very latest kernels support SEEK_HOLE/SEEK_DATA.

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