Previous versions of ESX emulated BusLogic and LSI Logic storage adapters within the virtual machine. The virtual machine would load a BusLogic or LSI Logic driver within the guest to access the virtual storage adapter. The advantage of this full virtualization is that most operating systems ship drivers for these devices. ESX 4.0 includes a new paravirtualized storage adapter called PVSCSI. This extends to the storage stack the kind of performance gains associated with other paravirtualized devices, such as the VMXNET network adapter available in earlier versions of ESX.
As with other paravirtualized devices, PVSCSI emulation improves efficiency by:
- Reducing the cost of virtual interrupts
- Batching the processing of I/O requests
- Batching I/O completion interrupts
A further optimization, which is specific to virtual environments, reduces the number of context switches between the guest and Virtual Machine Monitor.