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

[samba-bugs at samba.org]

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

           Summary: Use fiemap to quickly detect zero ranges of source
                    file
           Product: rsync
           Version: 3.1.0
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P5
         Component: core
        AssignedTo: wayned at samba.org
        ReportedBy: matt at mattmccutchen.net
         QAContact: rsync-qa at samba.org


Rsync could use the fiemap ioctl
(https://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=Documentation/filesystems/fiemap.txt;hb=HEAD)
on the source file to detect any ranges that do not have data on the filesystem
and thus are guaranteed to read as zero, without actually reading them.  This
would be most useful in combination with writing the destination file sparsely
(--sparse), but it would be safe to use in any case as a sender-side
optimization.

Originally proposed at https://bugzilla.redhat.com/show_bug.cgi?id=525545.

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