Re: [PATCH, RFC] virtio_blk: add cache flush command

[Christoph Hellwig <hch@lst.de>]

On Mon, May 11, 2009 at 06:45:50PM +0300, Avi Kivity wrote:
> >Right now it's fsync.  By the time I'll submit the backend change it
> >will still be fsync, but at least called from the posix-aio-compat
> >thread pool.
> >  
> 
> I think if we have cache=writeback we should ignore this.

It's only needed for cache=writeback, because without that there is no
reason to flush a write cache.

> For cache=none and cache=writethrough we don't really need fsync, but we 
> do need to flush the inflight commands.

What we do need for those modes is the basic barrier support because
we can currently re-order requests.  The next version of my patch will
implement a barriers without cache flush mode, although I don't think
a fdatasync without any outstanding dirty data should cause problems.
(Or maybe ext3 actually is stupid enough to flush the whole fs even for
that case)