The maturation of virtualisation solutions in the x86 and UNIX realm has opened the door to an endless array of choices. More choices offer organisations greater flexibility, but they can also introduce confusion and complexity.
Every virtualisation technology operates in a slightly different manner. This is compounded by the fact that every IT environment is vastly different, with its own unique operating patterns, technical compositions, and business constraints. Because of this, there’s probably never going to be one ideal virtualisation technology for every IT scenario. So it’s better to focus resources on choosing the right technology for a specific situation instead of finding the ever-elusive “perfect” solution.
There are several factors that influence the choice of virtualisation software.
Mobility and motioning
Motioning enables applications to move between physical servers without disruption. Available on VMware’s VMotion, XenMotion, and IBM P6 LPARs, motioning has the potential to transform capacity management.
However, it’s not without its problems. Motioning can introduce volatility, and create vexing challenges for management groups tasked with incident management and compliance issues. To gauge whether motioning is a good option in the environment, organisations first need to analyse maintenance windows, consistency of workload patterns, and disaster recovery strategies.
When combined on a single physical platform, maintenance windows become intermingled. This can easily create scenarios where there is no window of time available for hardware maintenance. The same problem arises for software freezes.
The ability to motion virtual machines can alleviate this problem by allowing servers to be moved offline for scheduled maintenance or software updates. Alternatively, without motioning in place, the proper initial placement of applications on virtual hosts is extremely important. In either case, making the right placement decisions is critical, since the mere act of motioning may constitute a change that violates a software freeze.
Consistency of workload patterns
The advantages of motioning may vary widely depending on the level of volatility in workload patterns. It can be very useful to leverage space capacity in highly volatile workloads. However, those benefits diminish in low-volatility scenarios.
Organisations can analyse the ideal placements based on the variations in utilisation patterns for each day or the week or both. If patterns do not vary widely from day to day, a static placement may be sufficient and the volatility of motioning avoided. If the patterns are significantly different from day to day, a more dynamic solution is warranted.
Disaster recovery strategy
If application-level replication or hot spares are part of the disaster recovery plan, motioning may undermine these efforts. For example, one might inadvertently place a production server in the same locale as its disaster recovery counterpart. To avoid such pitfalls, organisations undergo a detailed analysis of disaster recovery strategies, roles, cluster strategies, cluster roles, and replication structures.
Overhead and scalability
There are numerous aspects of an operational model that may affect the success of virtualisation. These include the way I/O is handled, the maximum number of CPUs per VM, as well as the way vendors license their software on the platform. Organisations can bypass these overhead and scalability concerns by considering the following factors.