Virtualization emerged as an area of interest in IT earlier this decade and is now unarguably one of the hottest trends in IT at the moment.  Prior posts explored the technical details of how processor virtualization and I/O virtualization function.  But what are the uses of virtualization that are driving all this attention and interest from IT, vendors and the press? 

The most important use today for virtualization is server consolidation.  During the late 1990’s and early part of this decade, small rack-mounted x86 servers proliferated rapidly.  In part, this was due to the low cost of x86 servers using Linux or Windows, compared to their UNIX predecessors.  But this was also driven by the flakiness of the software stacks on those same servers.  The software and operating systems were not nearly as robust as they are today and it was problematic enough that a lot of IT staff considered isolating applications from one another a best practice – even if it lead to low utilization.  There were plenty of peculiar bugs that resulted from a combination of corner cases in the OS and different software packages and using a server dedicated to a specific application avoids that problem entirely.

The proliferation of small, underutilized servers was slowed down by the dot-com bust in 2001, but also by constraints on data center space, cooling and power.  Cooling and power were particularly sensitive, because while Moore’s Law means that you can get more computing power in a chip over time – it also means that the power density (whether it is W/cm² or W/cm³) increases substantially.  So a data center designed for 1995 servers is wholly inadequate for those in 2003.  Worse yet, data centers are incredibly expensive to build or redesign – for many companies there is a bit of pain as your data center approaches 80% capacity and the cooling or power system gets stretched.  But typically, building another data center is far too painful and expensive to be an acceptable solution.

In 2002, VMware’s first server products came out and started the trend towards virtualization by providing the isolation that IT departments needed, but within the context of a single server.  From there, it became a simple equation as server consolidation reduced power costs, freed up data center space and reduced future expenditures on servers.  The overall poor economic environment is accelerating the trend towards virtualization as the cost of a virtualization project can easily be outweighed by the savings over a short period of time, thus enhancing the bottom line.

But above and beyond consolidating multiple physical servers onto a single server, where can virtualization go?   Snapshots and rollbacks are a spectacularly useful feature, even for an individual user, as opposed to a corporate IT department.  Since virtualization maintains all the state for each instance separately, it is very easy to save a copy of the state of a virtual machine at a given point in time, creating a snapshot.  At any point later in time, the user can rollback the VM to the state recorded by the snapshot, which has the practical effect of creating an ‘undo’ button for anything on a PC.  This is equally handy to corporate IT departments, which can create snapshots before making any substantial changes.

Migrating a virtual machine is a close cousin of the rollback technique described above.  Instead of rolling back the same VM to a prior state, the user can send the state to a different VM, on a different physical computer system even.  Often when a VM is migrated, it can be done ‘live’, without noticeably interrupting service; a paper from the research group behind the Xen hypervisor documented migrating a Quake server with under 60ms of downtime.  This is a boon to IT staff, who can use live migration for load balancing by moving VMs from an over-crowded server to a new or underutilized server. 

Another key use for virtualization is providing backwards compatibility for older applications or operating systems.  I can still vividly remember my first job in 2000, where I learned that the engineers were all still using Windows 95 because one of their engineering applications was still DOS only!  With virtualization that is no problem at all – and you don’t get stuck with an ancient and defunct OS.  Apple popularized the use of virtualization with binary translation to enable backwards compability for older PowerPC applications on the newer x86-based Macs, via Rosetta and later Boot Camp.

The last application for virtualization is testing and validation.  For many software or hardware companies, testing to ensure compatibility is a major undertaking.  Before the advent of virtualization, systems had to be configured with different combinations of real test hardware and software.  With virtualization, dozens of configurations can be tested on a single server simultaneously.

While a catalogue of the various uses of virtualization is interesting and provides some insight to the value that virtualization can provide, the implications for server design are in some cases more fascinating, and that will be the subject of my next post.