[ovirt-users] oVirt slow backups from ISCSI storage domain to nfs target

Hi I am using oVirt 4.4.10.7 with vms on iscsi Lenovo DE-6600F(nvme interfaced ssds). Backing up thin vms performance is fine, but with preallocated disks, backups to nfs storage with 200Gb interface is only transferring qcow2 image to nfs at 140MiB per sec. Why is qemu-nbd so slow with preallocated images? Does anyone have any issues related to this? Any help would be appreciated. I have made iscsid.conf changes, linux kernel boot changes, and still the same performance. Thanks Don