Despite the publicity around Parallels' flagship Mac product, Parallels Desktop (which we've contributed to in some small part), there are some other interesting tools in the stable of Parallels' parent company, SWsoft -- which is now being rebranded as Parallels, Inc. just to keep us on our toes. One of those products is Parallels Virtuozzo Containers (PVC), which launches version 4.0 today in a webcast event at 11 am (Eastern). While PVC is not a Mac-compatible server (yet), the technology choices Parallels made in developing it may illuminate some future paths for the yet-unreleased Parallels Server product for Mac OS X.
Virtualization products like Parallels Desktop, VMware Fusion, and Microsoft's Virtual Server work via a hypervisor model, virtualizing the hardware platform so that each virtual machine runs in its own full-OS environment. This approach does a great job of insulating the VMs from the underlying physical hardware and provides for OS and build diversity (Windows, Linux and eventually Mac OS X Server running on the same virtualization platform) but it has some drawbacks: each virtual machine consumes a full helping of RAM, disk space and CPU resources while running, as though it was a physical machine. VMware in particular is respected for its expertise in optimizing these resource demands with its ESX enterprise-level server products, but the baseline requirements for system resources can't be fully alleviated.
PVC does OS virtualization, which tackles the problem of resource allocation for virtual machines in a different way. Virtuozzo VMs, or "containers," are created as overlays on the base OS that runs on the physical server, like a piece of transparent acetate over an animation backdrop. All the basic OS processes, files and libraries are present in the base server OS instance, and the container holds the differential changes that allow the VMs their individual characteristics and configurations. While you sacrifice one big feature of hypervisor systems, the ability to diversify OSes -- since the base OS is fixed as either Windows or Linux, all the containers have to be built on top of that OS -- what you gain is substantial, as each additional running VM takes only a small chunk of RAM and a comparatively tiny swath of hard drive space to work in. Virtuozzo servers can handle high VM loading on relatively modest hardware without taking major performance hits, which is a big plus if your server budget is constrained.
In the OS X virtualization session at Macworld Expo, the Parallels folks suggested that we may see both hypervisor and OS virtualization approaches integrated in the Parallels Server product, as the engineering teams from PVC and Server have the opportunity to put their heads together. Parallels has also said that the management tools for PVC and Parallels Server will be integrated as development moves forward. Even though PVC doesn't support the Mac directly, if you're interested in virtualization you might pop into the webcast and check it out.