Direct I/O support (patches included)

[Dag Wieers]

Hi,

Since a --direct-io feature was requested a few times the past decade with 
little response and the actual patch is quite trivial, I patched both 
v3.0.9 and master branch and included the patches here.

If this functionality is acceptable I don't mind spending the additional 
effort to update the documentation, etc.

Beware that the underlying filesystem needs Direct I/O support, therefore 
for this to work on NFS, one needs the kernel CONFIG_NFS_DIRECTIO enabled 
(probably default these days anyway). This is the reason why --direct-io 
is not enabled by default and should be used with care. I don't know what 
the behavior is if the local or remote side do not support O_DIRECT.

In my tests it provides a much more stable copy process (on NFS) with 
improved performance, but I didn't do any prolonged repeated tests yet (I 
only used it during a large copy that was ongoing so I prefer not to 
juggle with numbers here). Please test yourself and report back.

Kind regards,
-- 
-- dag wieers, dag at wieers.com, http://dag.wieers.com/
-- dagit linux solutions, info at dagit.net, http://dagit.net/

[Any errors in spelling, tact or fact are transmission errors]
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