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

[Anthony Liguori]

Richard W.M. Jones wrote:
> So to be clear, the use case is that all the other VMs must be shut
> down, then the VM which wants to commit will upgrade its lock and
> commit, and then all the other VMs will restart?  I agree this should
> avoid corruption, although it sounds like something which is fairly
> unlikely to be done in practice.
>   

Commit is an extremely uncommon use-case.  There's really no correct way 
to use it.  That said, assuming people used it, here would be a 
reasonable flow:

1) user starts up with base.img -> cow.img
2) user makes some changes, realized it would be good for the golden 
master so issues a commit to base.img
3) user does not shut down this machine
4) user tries to start another vm with a base.img -> cow2.img

This is a valid scenario and temporarily promoting to write lock during 
commit allows this scenario without introducing a window for data 
corruption.

Regards,

Anthony Liguori