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

Rusty Russell
2009-08-26T12:06:48Z

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.