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Re: [Xen-devel] [PATCH][TOOLS] Reducing impact ofdomainsave/restore/dump on Dom0


I also read somewhere (either the man page or in the code) that the sync is necessary for exactly these reasons.


On Feb 21, 2007, at 07:22, Graham, Simon wrote:

Do you need the fsync at all? It's possible that the kernel will
launder-then-discard the affected pages automatically, just from the
fadvise() alone.

To be honest, I'm not completely sure; the implementation of the fadvise call triggers a write back and then explicitely discards any clean pages
-- I don't _think_ it will discard clean pages once the write back
completes (but it's not entirely clear to me how much of the write back
is done synchronously).

However, it definitely makes a perf difference if you explicitely fsync before each call to fadvise -- therefore I believe that the fadvise call
is definitely not cleaning all the pages inline (which would be
equivalent to the fsync()).

So -- my belief is that without the fsync, there will be some (clean)
pages for the file still in the cache when this is done; my goal was to
remove ALL the pages, but perhaps leaving a few lying around is OK..


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