rsync - Dec 2010 - DO NOT REPLY [Bug 7876] New: please implement o_direct

https://bugzilla.samba.org/show_bug.cgi?id=7876

           Summary: please implement o_direct
           Product: rsync
           Version: 3.1.0
          Platform: Other
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P3
         Component: core
        AssignedTo: wayned at samba.org
        ReportedBy: costinel at gmail.com
         QAContact: rsync-qa at samba.org


running nightly rsync leaves a lot of unneeded data in the filesystem cache.
 an "--o_direct" argument would solve this.
thank you.


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------- Comment #1 from samba-bugs at thequod.de  2011-01-25 05:33 CST -------

https://bugzilla.samba.org/show_bug.cgi?id=7876





------- Comment #2 from samba-bugs at thequod.de  2011-01-25 13:35 CST -------
Created an attachment (id=6227)
 --> (https://bugzilla.samba.org/attachment.cgi?id=6227&action=view)
Proposed patch

This is a noobish attempt at getting this fixed.

I have forgotten to diff it using "-p", but the context is
"struct map_struct
*map_file", where every file appears to get mapped through.

Please note that this is a very amateurish attempt, but I would really like to
get feedback to improve on this. Of course the best would be to have an
official patch for this.. :)


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------- Comment #3 from wayned at samba.org  2011-01-25 14:30 CST -------
(In reply to comment #2)
> Please note that this is a very amateurish attempt, but I would really like
to
> get feedback to improve on this.
See the patch drop-cache.diff in older releases.  There is also a newer version
of the patch that someone else wrote.

This won't be in the official release until OS support is better.  At
present
the POSIX_FADV_DONTNEED option in linux does all kinds of weird things (or did
at the time the patch was written by Tobi Oetiker), such as uncaching files
that someone else put into the cache (so, if you copy a file that someone else
is using heavily, it totally vanishes from the disk cache).

I still say that a better solution than modifying various copy programs would
be to have a helper app (ala nice, etc.) that would use a pre-loaded library to
do whatever the current OS needs to do to make the file reading not flood the
cache.  Then it could be used with cp, tar, etc.


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------- Comment #4 from samba-bugs at thequod.de  2011-01-25 15:10 CST -------
Thanks for your feedback!

I had just found the patch by Tobi myself, and it is using mincore (now) to
check if a file has been cached before rsync touched it (and then skip removing
it from the cache).

Post: http://insights.oetiker.ch/linux/fadvise.html
Patch for 3.0.7: http://tobi.oetiker.ch/patches/rsync-3.0.7-fadvise.patch

I agree that something like "nice" and "ionice" would be the
best approach, and
I find it rather unusual to not have something like that available already.
I have found the following wrapper, which is using O_DIRECT however:
http://arighi.blogspot.com/2007/04/how-to-bypass-buffer-cache-in-linux.html

For what it's worth, here's a link to the POSIX_FADV_DONTNEED
implementation in
the current kernel:
http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.37.y.git;a=blob;f=mm/fadvise.c;hb=refs/heads/master#l118


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------- Comment #5 from samba-bugs at thequod.de  2011-01-25 17:15 CST -------
Just for information: rsync builds using the patch are available from my PPA
("Lucid", works on Squeeze, too):
https://launchpad.net/~blueyed/+archive/ppa


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--- Comment #6 from costin gusa <costinel at gmail.com> 2011-04-30
12:55:16 UTC ---
(In reply to comment #4)
> Thanks for your feedback!
> 
> I had just found the patch by Tobi myself, and it is using mincore (now) to
> check if a file has been cached before rsync touched it (and then skip
removing
> it from the cache).
> 
> Post: http://insights.oetiker.ch/linux/fadvise.html
> Patch for 3.0.7: http://tobi.oetiker.ch/patches/rsync-3.0.7-fadvise.patch
> 
> I agree that something like "nice" and "ionice" would
be the best approach, and
> I find it rather unusual to not have something like that available already.
> I have found the following wrapper, which is using O_DIRECT however:
> http://arighi.blogspot.com/2007/04/how-to-bypass-buffer-cache-in-linux.html
> 
> For what it's worth, here's a link to the POSIX_FADV_DONTNEED
implementation in
> the current kernel:
>
http://git.kernel.org/?p=linux/kernel/git/stable/linux-2.6.37.y.git;a=blob;f=mm/fadvise.c;hb=refs/heads/master#l118
To me as an end user is irrelevant what method is chosen to avoid filling up
the cache.

The LD_PRELOAD library wrapper solution looks like a more logic approach;
however besides the mentioned blogspot article, are there out successful usage
reports based on it?

Second, if the answer to the previous question is "yes", how can I
enable it
remotely ? - My backup solution is a "pull" type - to avoid a bunch of
servers
potentially abuse the backup machine, only the backup machine connects to
servers. So if I on the backup machine would be able to run rsync with
LD_PRELOAD wrapper, how do I run the remote '-e ssh' rsync with the
LD_PRELOAD
wrapper?

Thank you.

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--- Comment #7 from Tomasz Chmielewski <mangoo at wpkg.org> 2011-06-15
14:47:25 UTC ---
There is also this tool by Andrew Morton, which can be used with any arbitrary
application:

http://lwn.net/Articles/224653/

http://code.google.com/p/pagecache-mangagement/



Other than that, it would be of course useful to have such a
"--direct" switch
in rsync, as using LD_PRELOAD with remote servers may not be feasible.

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