Re: [Qemu-devel] need to resurrect no-lock option?

[Stefan Hajnoczi]

On Wed, Sep 20, 2017 at 11:26:11AM +0200, Christian Ehrhardt wrote:
> Hi,
> this might have been discussed in the wake of the lock changes that took
> place in 2.10 but I can't find anything clear enough to follow in the
> current case.
> There was an old submission [1] by Fam to make it possible to no-lock
> qemu-img and others if needed. But it seems nothing like it made it along
> to the locking we have in 2.10.
> 
> One (maybe more) case where missing this causes pain is e.g. running an
> info check on a running guest.
> In general info shouldn't need a write lock anyway, but without --no-lock
> that use case is broken.

It's still an invalid use case.  There is no guarantee that qemu-img
will see a consistent version of the image file.  Metadata could change
underneath qemu-img because QEMU may still write to it.  QEMU may also
have some metadata cached so there's no guarantee that qemu-img sees an
up-to-date image.

Why do you need to run qemu-img on a disk image that is in use?

> Repro of the case is rather simple:
> $ qemu-img create -f qcow2 /tmp/test.qcow2 1M
> $ qemu-system-x86_64 -hda /tmp/test.qcow2 -enable-kvm -nodefaults
> -nographic &
> $ qemu-img info /tmp/test.qcow2
> qemu-img: Could not open '/tmp/test.qcow2': Failed to get shared "write"
> lock
> Is another process using the image?
> 
> [1]: https://lists.gnu.org/archive/html/qemu-block/2016-04/msg00349.html
> 
> -- 
> Christian Ehrhardt
> Software Engineer, Ubuntu Server
> Canonical Ltd