Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default

[Avi Kivity]

On 08/26/2009 03:06 PM, Rusty Russell wrote:
> 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.
>    

It's now switched to writethrough.  In any case, cache=writeback means 
"lie to the guest, we don't care about integrity".

> 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.
>    

Why do we need to add a feature for this?

--
error compiling committee.c: too many arguments to function

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/