The VMware ESX core hypervisor, has also been upgraded, and provided no upgrade drama at all; vCenter Update Manager upgraded our hypervisors to 4.1 level.
Memory management changes
VMware claims that memory use for VM instances is more efficient in 4.1 compared to 4.0. We tried to test overhead and found that 4.1 decreases memory overhead, but only for 64-bit instances. We couldn't find any real change for 32-bit virtual machine memory use over the same time cycles (many minutes).
Memory can also be compressed so that VMs don't go to disk for virtual memory as often, which saves access speed. We found that the amount of actual time savings from compression could be high, and is likely to be useful in constructions where VMs are crammed and somewhat over-subscribed, resource-wise.
CPU use changes
Unlike most other hypervisors, VMware 4.1's ESX uses processor sleep instructions to save power when idle. Most hypervisor kernels thwart processor sleep in a quest to be responsive to the random requests of hosted VMs, and keep CPUs at full speed all the time.
Sadly, there's not a hardware compatibility list (HCL) that notes which servers can use this feature, and while we were able to set power management policies on our Dell 1950 VMware ESX 4.1 host, we weren't able to deploy it on our HP servers. And on all three mainstream machines we tested, were unable to view power history, power consumption, or the power cap information. According to a VMware support person, "the graphs are supported on systems that have built-in power meters that we know how to read". We hope more can be read, soon.
Virtual CPU changes can now be seen by guests as the number of sockets and number of cores, where previously guests could only see sockets (and one socket equals one core). Now it's possible to do such things as have the VM detect two sockets with two cores for a total of four CPUs - with advance settings. Instructions on this aren't particularly clear. Sadly, there's still a limit of eight vCPUs per VM, as well.
VMware's vCenter also tracks more storage statistics. We found more console info and tracking on throughput and latency for both hosts and VMs. And to our happiness, vCenter now tracks NFS stats as well.
VMware's vNetwork Distributed Switch (available only in the vSphere Enterprise Plus edition) adds network control, by protocol throttling in the switch. Similarly, storage I/O can be throttled as well, per VM, but this also requires the Enterprise Plus Edition.
As an example, we could limit iSCSI, FT (fault tolerance mirroring), NFS, as well as vMotion traffic. The control over both networking and storage resources allows administrative control over each VM. With vCenter, behavior becomes more molecularly confined, and therefore predictable so that machines can be further optimized with VMs.
VMware says that vCenter can now manage up to 1,000 hosts, and 10,000 powered on VMs. VMware also increased the number of VMs per cluster, from 1,280 to 3,000.