January 11, 2008, 5:26 PM — Fewer and fewer IT organizations are asking whether they should virtualize
systems. The focus is now on how should they leverage virtualization
in their environment. The maturation of virtualization solutions in the x86
and UNIX realm has opened the door to an endless array of choices. More choices
offer organizations greater flexibility, but they can also introduce confusion
and complexity. Every virtualization 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 virtualization
technology for every IT scenario. Thus, it's better to focus resources on choosing
the right technology for a specific situation.
Following are six factors to consider when evaluating virtualization software.
1. 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, organizations
first need to analyze maintenance windows, consistency of workload patterns,
and disaster recovery strategies.
Maintenance windows - When combined on a single physical
platform, maintenance windows become intermingled. This can easy 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.
Organizations can analyze the ideal placements based on the variations in utilization
patterns for a day or a 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,
organizations undergo a detailed analysis of disaster recovery strategies, roles,
cluster strategies, cluster roles, and replication structures.
2. Overhead and Scalability
There are numerous aspects of an operational model that may impact the success
of virtualization. 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.
Organizations can bypass these overhead and scalability concerns by considering
the following factors.
I/O rates - Software components such as database servers
that are I/O intensive may be better suited to virtualization technologies that
don't use virtual device drivers, as these device drivers "tax" the
CPUs with every I/O transaction they perform, causing the system to hit its
limits before it otherwise would. Techniques such as VMware's raw device mapping
also provide higher efficiency in this area, but the use of such features prevents
motioning.
To determine the best approach, organizations can use a strategy-specific overhead
model that adds up CPU utilization numbers based on the I/O activity on the
physical servers. This is an easy way to catch any workload types that are unsuitable
for a given virtualization solution.
