Re: adding proper O_SYNC/O_DSYNC, was Re: O_DIRECT and barriers

From: Trond Myklebust
Date: Fri Aug 28 2009 - 17:43:27 EST

On Fri, 2009-08-28 at 17:29 -0400, Christoph Hellwig wrote:
> On Fri, Aug 28, 2009 at 05:16:14PM -0400, Trond Myklebust wrote:
> > On Fri, 2009-08-28 at 17:08 -0400, Christoph Hellwig wrote:
> > > #define O_SYNC (O_FULLSYNC|O_DSYNC)
> > >
> > > - during the normal merge window I will add a real implementation for
> > > for O_FULLSYNC and O_RSYNC
> > >
> > > P.S. better naming suggestions for O_FULLSYNC welcome
> >
> > Basically you are just ensuring that the metadata changes are being
> > synced together with the data changes, so how about O_ISYNC (inode
> > sync)?
> Yeah. Thinking about this a bit more we should define this flag
> much more clearly. In the obvious implementation it would not actually
> do anything if it's set on it's own. We would only check it if O_DSYNC
> is already set to decided if we want to set the datasync argument to
> ->fsync to 0 or 1 for the generic filesystems (and similar things for
> filesystems not using the generic helper).
> If we deem that this is too unsafe we could make sure O_DSYNC always
> gets set on this fag in ->open, but if we make sure O_SYNC is defined
> like the one above in the kernel headers and glibc we should be fine.
> Although in that case a name that doesn't suggest that it actually does
> something useful would be better.

If you are going to automatically set O_DSYNC in open(), then
fcntl(F_SETFL) might get a bit nasty.

Imagine using it after the open in order to clear the O_ISYNC flag;
you'll still be left with the O_DSYNC (which you never set in the first
place). That would be confusing...


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