From: "Peter W. Draper" <p.w.draper@durham.ac.uk>
Subject: Re: Re: Using mmap result in data corruption
Date: Tue, 8 Mar 2005 16:17:53 +0000 (GMT)
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To: Trond Myklebust <trond.myklebust@fys.uio.no>
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On Tue, 8 Mar 2005, Trond Myklebust wrote:

> ty den 08.03.2005 Klokka 14:41 (+0000) skreiv Peter W. Draper:
> 
> > Following Trond's advice I've changed to using an msync() call before
> > munmap() and that fixes the problem.  However, I'd really like to avoid
> > the potential performance impact of using an msync(MS_SYNC) by using an
> > msync(MS_ASYNC) call instead, as the files I access can be mmap'd many
> > times, plus can be quite large, but, when I switch to msync(MS_ASYNC) the
> > data corruption returns (I am only interested in the correctness of the
> > data when my applications exit).
> > 
> > Surely that is incorrect behaviour? Otherwise what is the point of
> > msync(MS_ASYNC)?
> 
> The Single UNIX Spec v3 says that
> 
>         MS_ASYNC Perform asynchronous writes.
>         MS_SYNC  Perform synchronous writes.
> 
>         When MS_ASYNC is specified, msync() shall return immediately
>         once all the write operations are initiated or queued for
>         servicing; when MS_SYNC is specified, msync() shall not return
>         until all write operations are completed as defined for
>         synchronized I/O data integrity completion. Either MS_ASYNC or
>         MS_SYNC is specified, but not both.
> 
> So on Linux, msync(MS_ASYNC) just marks the pages as dirty (see the
> comment in mm/msync.c). It is then up to the application to call fsync()
> if it wants to flush those asynchronous writes to disk at some specific
> time.

Hi Trond,

thanks for the explanation, calling fsync() solves my problem. One fact
you probably don't want to know is that just calling fsync() works
regardless of whether I have called msync(MS_ASYNC) or not.

Cheers,

Peter.




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