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

[Richard W.M. Jones]

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).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://libguestfs.org