Re: [Qemu-devel] bdrv_flush for qemu block drivers nbd, rbd and sheepdog

[Anthony Liguori]

On 10/22/2010 03:29 AM, Kevin Wolf wrote:
> > I agree.
> >      
> Of course, as Laurent said a while ago, there is no specification for
> NBD, so it's hard to say what the intended semantics is.
>
> However, I did have a look at the nbdserver code and it looks as if it
> implements something similar to writethrough (namely fsync after each
> write) only if configured this way on the server side. qemu-nbd defaults
> to writethrough, but can be configured to use cache=none. So with either
> server qemu as a client can't tell whether the data is safe on disk or not.
>
> In my book this is a strong argument for refusing to open nbd
> connections with anything but cache=unsafe.
>    

On a physical system, if you don't have a battery backed disk and you 
enable the WC on your disk, then even with cache=writethrough we're unsafe.

Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe.

QEMU can't guarantee safety.  The underlying storage needs to be 
configured correctly.  As long as we're not introducing caching within 
QEMU, I don't think we should assume we're unsafe.

Do we have any place where we can add docs on a per-block format basis?  
It would be good to at least mention for each block device how the 
backing storage needed to be configured for safety.

Regards,

Anthony Liguori

> Kevin
>