thanks Andryi
We us round robin when using NBD , but to be fair, it does not change the performance a lot in most of the case. The concurrency settings ( multiple connection to the same file ) is helping when there is a high latency between XO and the host.
SO , @fcgo if you have thousand of VMs , you should enable NBD it will consume less resource on the DOM0 and XO , and it will be spread on all the possible hosts.
