Re: [Qemu-devel] [PATCH 2/3] block-nbd: fix use of protocols in backing files and nbd probing

[Anthony Liguori]

On 09/11/2010 11:53 AM, Stefan Hajnoczi wrote:
> On Sat, Sep 11, 2010 at 3:04 PM, Anthony Liguori<address@hidden>  wrote:
>    
> > Additionally, there's a spurious read when using an nbd protocol that can be
> > quite destructive when using copy-on-read.  Potentially, this can lead to
> > probing an image file over top of NBD but this is completely wrong as NBD
> > devices are not growable.
> >      
> Can you describe the copy-on-read scenario where the 2 KB probe read
> is a problem?
>
> Accessing a fixed size image file over NBD is probably uncommon, but
> I'm not sure if there's a reason to forbid it.
>    

I think the better solution is to explicitly specific raw with nbd.  
IOW, I think -drive file=nbd:localhost:1026,format=raw should work the 
same way.  I still feel slightly weird about probing happening with 
nbd.  It seems like it could only result in badness.

The specific scenario is migration.  I'm using a copy-on-read file on 
the destination and I want to be sure that I don't read any blocks 
(since they're copied) until the source stops execution.

Regards,

Anthony Liguori

> Stefan
>
>