Comment 11 for bug 1816170

Answering on commend #9 to be complete.
    #1 creating a snapshot should be safe, and in fact an qemu-img create works fine on a base file
       that currently has a running VM associated

    #2 running multiple VMs with images referring to the same backing file is safe as well

    Yes if each has an own disk (.qcow) and own .iso then the locks will be necessary but it should
    be able to get the logs as nothing else holds them.

    Also yes the patch for qemu-img is in Bionic by 2.11.1

Ok in comment #10 you came to the same conclusions (\o/) and identified an issue in the concurrent pull&convert of the initial image - that would make sense as the convert will need the write lock (which also denies further read locks) and the concurrent actions potentially corrupt the file anyway.

So yeah, I'd conclude similarly to you and would assume that any "image synchronize + convert" actions by Juju should be mutually exclusive to each other at least per target-img file.

You already assigned to Rick for Juju which I think is the right next step, thanks!