On Wed, 26 Aug 2009 09:58:13 pm Avi Kivity wrote: > 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". Well, that was the intent of the virtio barrier feature; *don't* lie to the guest, make it aware of the limitations. Of course, having read Christoph's excellent summary of the situation, it's clear I failed. > > 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? Because cache=writeback should *not* lie to the guest? Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
