* 64 bit dom0 freed us from the limitations of dreaded Linux low memory, but also allows us to use modern drivers and work better with modern servers. From personal experience, when I took alpha.2 and installed it on some of my test Dell servers, it automatically detected my hardware RAID without my having to jump through any driver disk hoops. That was huge for me. * The move to a 3.10 kernel from kernel.org meant that we were out of the business of having a completely custom kernel and corresponding patch queue. Upstream is goodness. * The move to the Xen Project hypervisor 4.4 meant that we're now consuming the most stable version of the core hypervisor available to us. * We've updated to an ovs 2.10 virtual switch giving us improved network stability when the virtual switch is under serious pressure. While we introduced the ovs way back in December of 2010, there remained cases where the legacy Linux bridge worked best. With Creedence, those situations should be very few and far between * A thread per vif model was introduced to better ensure network hogs didn't impact adjacent VM performance * Network datapath optimizations allow us to drive line rate for 10Gbps NICs, and we're doing pretty well with 40Gbps NICs. * Storage was improved through an update to tapdisk3, and Felipe did a fantastic job of engaging with the community to provide performance details. Overall we've seen very significant improvements in aggregate disk throughput, and when you're virtualizing it's the aggregate which matters more than the single VM case.