This is upstream in nbdkit now: https://gitlab.com/nbdkit/nbdkit/-/tree/master/plugins/blkio

Another question:

(6) vhost-user + "read-only" property acts a bit strangely. After opening virtio-blk-vhost-user it throws an EROFS error if you try to "blkio_start" it. However if you set the "read-only" property to true then it's OK to start it, and subsequent read operations work too.

I would expect that any device can be started without needing to set this property, and in general I don't understand why vhost-user is r/o since everything I read about it seems to indicate it's a general purpose writable protocol.

read-only has POSIX file open-like semantics: if the device is itself read only but the read-only property is false, blkio_start() fails. With qemu-storage-daemon, you must set writable=on on the vhost-user-blk export for the device to be writable. Alberto