* Use writeback caching for virtual disks instead of writethrough

caching.  This makes a huge performance difference (e.g. from 4 MB/s
  `dd' throughput to 140 MB/s on the Hydra machines).  As the QEMU
  manual says: "Some block drivers perform badly with
  ‘cache=writethrough’, most notably, qcow2."

svn path=/nixos/branches/boot-order/; revision=22248

modules/virtualisation/qemu-vm.nix

@@ -128,7 +128,7 @@ let
           -m ${toString config.virtualisation.memorySize} \
           -no-kvm-irqchip \
           -net nic,vlan=0,model=virtio -net user,vlan=0 -smb / \
-          -drive file=$NIX_DISK_IMAGE,if=virtio,boot=on,werror=report \
+          -drive file=$NIX_DISK_IMAGE,if=virtio,boot=on,cache=writeback,werror=report \
           -kernel ${config.system.build.toplevel}/kernel \
           -initrd ${config.system.build.toplevel}/initrd \
           ${qemuGraphics} \