I'm trying to figure out the correct way to get the 'real world' write performance of my NFS cluster.
The NFS export is mount on a XEN hypervisor, and I'm running 'dd' from within a VM.
What I can't decide is should I use
O_DIRECT/O_SYNC or not.
If I won't use it, the kernel will do page caching to RAM, and I'll will end up measuring RAM throughput, not disk.
On the other hand, most if not all server applications will use RAM caching anyway(am I wrong with that assumption?).
So I wonder where is the balance point, what will really happen in the 'real world'?