Mapped Drive

[Paul Slootman]

On Mon 24 Sep 2007, Stephen Zemlicka wrote:

> I am having trouble running rsync over a mapped drive.  Basically it only
> copies whole files.  I use the -rvcS switches.  Any suggestions?

>From the manpage:

    -W, --whole-file
            With this option the incremental rsync algorithm is not used and
            the  whole  file  is  sent  as-is  instead.  The transfer may be
            faster if this option is used when  the  bandwidth  between  the
            source  and destination machines is higher than the bandwidth to
            disk  (especially  when  the  "disk"  is  actually  a  networked
            filesystem).   This is the default when both the source and des-
            tination are specified as local paths.

This is because there's no advantage to updating only parts instead of
the whole file. If rsync finds it needs to update a file, it now only
has to read the source, write the destination (via a temp file so that
the destination filename either points to the old version or the new
version; not an incomplete new version. If you want that, use
--inplace).

If it did it incrementally, then rsync would need to read the source,
read the old destination file, compare, and while writing the new temp
file read from either the source or the old destination. This is more
IO, and hence less efficient.

Rsync's incremental algorithm is for optimizing network traffic at the
expense of more local disk IO, in the assumption that the network is
slower than local disk IO.  If you're doing a local transfer, it
optimizes the disk IO... Using a mapped drive appears to be a local
disk. If you can transfer directly between the server from which the
drive is mapped instead of going through the mapping, then that's
preferable (and will decrease network traffic...).


Paul Slootman