Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
>> I think I made my point poorly.  Consider the following:
>>
>> qemu-img create -f raw base.img 10G
>> qemu-img create -f qcow2 -b base.img cow1.img
>> qemu-img create -f qcow2 -b base.img cow2.img
>>
>> qemu -drive file=cow1.img,lock=exclusive
>> qemu -drive file=cow2.img,lock=exclusive
>>
>> With the current patch, the second command will fail and it's 
>> impossible to invoke correctly.  That's because flags are passed down 
>> to backing devices directly.  You really need to be much smarter here 
>> in how you handle locking.
>
> Oh, that's just an implementation bug.  Obviously there's no need to 
> open a file for exclusive access if it will only be accessed for read 
> (when merging a snapshot we'd need to upgrade the lock to exclusive 
> while that takes place).  I thought you objected to the whole concept.

I'm not sure whether it's best to enable it by default because, as I 
said earlier, I'm not comfortable with the lack of correctness wrt 
advisory vs. mandatory locking.  Only qemu can implement this level of 
locking though so it's definitely something we ought to support.

However, from a UI and implementation perspective, I think we need 
significantly different semantics.  You either want locking or you 
don't.  I don't think the selection of none|shared|exclusive really 
makes sense.

Regards,

Anthony Liguori