libvirt users - Mar 2017 - Cache / IO Setting for qcow2 disks with prealloc=metadata?

Hello,
i´ve done 2 vm with qcow2 disks and created them with qemu-img with 
prealloc=metadata.
Virt-Manager shows me under the performance settings for the Storage,  
Cache= Hypervisor Default  IO= Hypervisor Default
Is this the best setting? Or do i choose "native" IO on 
prealloc=metadata created disks? And do i have to change Cache too?
The Virtualisation Host is gento linux, the qcow2 files are stored on a 
xfs partition, qemu is build with "xfs" Flag.

thanks for hints and tipps

marko

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