From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH, RFC] virtio_blk: add cache flush command
Date: Mon, 11 May 2009 17:40:46 +0200
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Cc: Christoph Hellwig <hch@lst.de>,
	Rusty Russell <rusty@rustcorp.com.au>, kvm@vger.kernel.org
To: Anthony Liguori <anthony@codemonkey.ws>
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On Mon, May 11, 2009 at 09:51:40AM -0500, Anthony Liguori wrote:
> What typically triggers a flush operation?

fsync.

> I would assume an fsync would, but would a flush happen after every 
> O_DIRECT write?

Right now it doesn't, but it probably should.

> If the backend implementation of T_FLUSH is fsync, I would think that 
> this would result in rather poor performance for O_DIRECT operations in 
> the guest.

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.