From: Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default
Date: Wed, 26 Aug 2009 21:36:46 +0930
Message-ID: <200908262136.46570.rusty@rustcorp.com.au>
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To: Christoph Hellwig <hch@lst.de>
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On Tue, 25 Aug 2009 11:46:08 pm Christoph Hellwig wrote:
> On Tue, Aug 25, 2009 at 11:41:37PM +0930, Rusty Russell wrote:
> > On Fri, 21 Aug 2009 06:26:16 am Christoph Hellwig wrote:
> > > Currently virtio-blk doesn't set any QUEUE_ORDERED_ flag by default, which
> > > means it does not allow filesystems to use barriers.  But the typical use
> > > case for virtio-blk is to use a backed that uses synchronous I/O
> > 
> > Really?  Does qemu open with O_SYNC?
> > 
> > I'm definitely no block expert, but this seems strange...
> > Rusty.
> 
> Qemu can open it various ways, but the only one that is fully safe
> is O_SYNC (cache=writethrough).

(Rusty goes away and reads the qemu man page).

	By default, if no explicit caching is specified for a qcow2 disk image,
	cache=writeback will be used.

Are you claiming qcow2 is unusual?  I can believe snapshot is less common,
though I use it all the time.

You'd normally have to add a feature for something like this.  I don't
think this is different.

Sorry,
Rusty.