[ovirt-users] cloning a VM or creating a template speed is so so slow

I have been trying to figure out why cloning a VM and creating a template from ovirt is so slow. I am using ovirt 4.3.10 over NFS. My NFS server is running NFS 4 over RAID10 with SSD disks over a 10G network and 9000 MTU Therocially I should be writing a 50GB file in around 1m30s a direct copy from the SPM host server of an image to another image on the same host takes 6m34s a cloning from ovirt takes around 29m So quite a big difference. Therefore I started investigating and found that ovirt launches a qemu-img process with no source and target cache. Therefore thinking that could be the issue, I change the cache mode to writeback and was able to run the exact command in 8m14s. Over 3 times faster. I haven't tried yet other parameters line -o preallocation=metadata but was wondering why no cache was selected and how to change it to use cache writeback command launched by ovirt: /usr/bin/qemu-img convert -p -t none -T none -f qcow2 /rhev/data-center/mnt/nas1.bfit:_home_VMS/8e6bea49-9c62-4e31-a3c9-0be09c2fcdbf/images/21f438fb-0c0e-4bdc-abb3-64a7e033cff6/c256a972-4328-4833-984d-fa8e62f76be8 -O qcow2 -o compat=1.1 /rhev/data-center/mnt/nas1.bfit:_home_VMS/8e6bea49-9c62-4e31-a3c9-0be09c2fcdbf/images/5a90515c-066d-43fb-9313-5c7742f68146/ed6dc60d-1d6f-48b6-aa6e-0e7fb1ad96b9