Re: [Qemu-block] [Qemu-devel] [PATCH v4 00/27] block: Lock images when opening

[Kevin Wolf]

Am 11.05.2016 um 11:28 hat Richard W.M. Jones geschrieben:
> 
> On Wed, May 11, 2016 at 04:04:40PM +0800, Fam Zheng wrote:
> > On Tue, 05/10 13:22, Daniel P. Berrange wrote:
> > > On Tue, May 10, 2016 at 01:11:30PM +0100, Richard W.M. Jones wrote:
> > > > At no point did I say that it was safe to use libguestfs on live VMs
> > > > or that you would always get consistent data out.
> > > > 
> > > > But the fact that it can fail is understood, the chance of failure is
> > > > really tiny (it has literally only happened twice that I've read
> > > > corrupted data, in years of daily use), and the operation is very
> > > > useful.
> > > > 
> > > > So I think this patch series should either not lock r/o VMs, or should
> > > > add a nolock flag to override the locking (which libguestfs will
> > > > always use).
> > 
> > It sounds you are happy with either way but actually this series does both. 
> > So,
> > would it be okay for libguestfs if we go for "lock r/o VMs by default and
> > provide nolock flag"? It would then have the best default for non-libguestfs
> > users.
> 
> Yes, and libguestfs will always pass the nolock flag for readonly disks.
> 
> Another question is whether doing:
> 
>   qemu-img create -b disk.img -f qcow2 overlay.qcow2
> 
> is permitted, since that is the first command that libguestfs issues
> when opening a disk read-only (where disk.img could be a live VM in
> the case we're talking about).

I think qemu-img has a few special cases where the lock can be ignored
as well. Opening the backing file to query its size could be one of
them; but if you pass a size, qemu-img doesn't even need to open it.
'qemu-img info' could be another thing that is reasonable enough on an
image that is in use.

Kevin