On 04/12/2018 05:24 AM, Richard W.M. Jones wrote:

> I don't think we have nbd-server in RHEL, and in any case wouldn't it
> be better to use qemu-nbd?
>
> You just start a new qemu-nbd process instead of faffing around with
> configuration files, kill the qemu-nbd process when you're done, and

> qemu-nbd supports qcow2 already.

That, and qemu-nbd supports extensions such as NBD_CMD_BLOCK_STATUS and
NBD_OPT_STRUCTURED_REPLY that nbd-server has not implemented yet; a qemu
NBD client talking to a qemu-nbd server is thus going to be able to take
advantage of those extensions for better performance that would not be
possible with a qemu NBD client talking to an nbd-server instance (at
least, not without someone implementing the new features there).  And
this is no different from the situation where nbdkit as the server lacks
several features; the current rhv-upload patches use a python plugin to
nbdkit, which is implemented as serializing all requests; while using
qemu-nbd as the server would allow parallel requests to be in flight
simultaneously.