man qemu-nbd (on Centos 7.6) says:
Allow up to num clients to share the device (default 1)
I see that in qemu-img 4.1 there is a note about consistency with writers:
Allow up to num clients to share the device (default 1). Safe for readers, but for now, consistency is not guaranteed between multiple writers.
But it is not clear what are the consistency guarantees.
Supporting multiple writers is important. oVirt is giving the user a URL (since 4.3), and the user
can use multiple connections using the same URL, each having a connection to the same qemu-nbd
socket. I know that some backup vendors tried to use multiple connections to speed up backups, and
they may try to do this also for restore.
An interesting use case would be using multiple connections on client side to write in parallel to
same image, when every client is writing different ranges.
Do we have real issue in qemu-nbd serving multiple clients writing to different parts of