On 10/06/2017 09:09 AM, Jan Heidbrink wrote:
> Public bug reported:
> 
> When running 'qemu-img info test.qcow2' while test.qcow2 is currently
> used by a Qemu process, I get the error
> 
> qemu-img: Could not open 'test.qcow2': Failed to get shared "write"
> lock.
> 
> 
> Why does displaying information about a disk image need a write lock for the file?

Because there is a risk (albeit rather slight) that what you read from
the disk is inconsistent due to being an intermediate state in-between
separate non-atomic write actions by the other process that has it open
for write.

If you are willing to ignore the risk, then use:

qemu-img info -U test.qcow2

which says that you are okay reading the image while it is shared with a
concurrent writer, even if the read fails spectacularly in the unlikely
case that it sees inconsistent information.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org