This is not a Linux NFS issue (or a GlusterFS issue) at all. It's the underlying filesystem issue.

If you use NFS or GlusterFS on top of XFS, XFS needs special mount options to create qcow2 files quickly. Here is my fstab:

/dev/md2 /export/nfs/RAID-6 xfs noatime,inode64,nodiratime,allocsize=64k 0 2

The trick being allocsize. 

XFS changed it's default pre-allocation size: http://serverfault.com/questions/406...of-sparse-file

I now have a working system!

Gerald