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From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] [PATCH] ide.c make write cacheing controllable by
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To: qemu-devel@nongnu.org

Ian Jackson wrote:
> > Ideally, the host would provide variation of fdatasync() which flushes
> > data to hard storage in the same way that kernel filesystem journal
> > writes can do, and Qemu would use that.
> 
> Another question arises: do we want bdrv_flush to call (eventually)
> fsync or fdatasync ?  If the latter we need to make sure that we call
> fsync instead when necessary, for example when a cow file is extended.

I'm imagining that fdatasync() will flush the necessary metadata,
including file size, when a file is extended.  As would O_DSYNC.

I could be wrong, but I think it's expected to do that, as I recall
VxFS doing something like that with O_DSYNC.

Then again, even if it's meant to, that doesn't mean it does...

-- Jamie