qemu 2.10+: consider adding --force-share flag #4024

ISSUE TYPE

• Enhancement Request

Investigate a possible usage of "--force-share" parameter when accessing qcow2 files, due to "Qemu 2.10 added the requirement of a --force-share flag to qemu-img info when reading information about a disk that is in use by a guest."

Not sure if this is applicable to qemu itself or just qemu-img (nothing to do here...) - but the way to reproduce the issue where this would help is:

• qemu 2.10+, NFS v3 (v4 is not affected)
• kill (power off) the host hosting a running VM
• "qe0mu-img info" from another host can not access the qcow2 file on a shared NFS storage and also the VM can't be started - so VM HA actions are affected (this is in contrast to when you kill the qemu process only - then the lock is immediately releases, so this is not a valid test) - i.e. killing a host doesn't release a lock

Adding the --force-share option might help in VM HA cases, where NFSv3 is used, so that the VM can be started on another host. This is equivalent (afaik) to qemu-img info -U xxx.qcow2 which does allow access to the qcow2 file which is locked by the dead host.