Choosing the right virtualization technology

by Andrew Hillier

January 11, 2008 —

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.

Non-compute intensive applications - Theoretically, it
is possible to place many non-compute intensive applications together on a virtual
host. However, there are many factors that may limit the scalability of this
scenario. Moreover, pinpointing which factors are constraining the environment
can be tricky.

The first step is to apply a CPU "quantization" model. If a technology
uses a virtual-CPU-per-physical-CPU model that is rigid in nature, the number
of virtual systems is limited by the number of CPUs. While this issue is fading
away as new fractional models become available to allow for allocation of fractions
of physical resources, it is still prudent to be aware of this constraint to
prevent unpleasant surprises.

Memory is a more complex part of the equation. Applications that aren't doing
very much will often utilize the same amount of memory as similar applications
that are more active. Combining even a small number of these applications can
quickly tax the memory capacity of the target system while making very little
impact on CPU utilization.

The scalability of the underlying architecture also complicates matters. Some
applications will crash when running too many images, regardless of what they
are doing. Others leverage robust backplane interconnects, caching models, and
high context switching capacity to allow for a maximum number of virtual machines
without compromising reliability. To determine if moving to "fat nodes"
makes sense, organizations factor in platform scalability and the workload "bend".

3. Software Licensing Models

Some applications are not supported on specific virtualization technology. Even
if support isn't an issue, however, software licensing models may play a major
role in the ROI gained from virtualization. If, for example, applications are
licensed per physical server, this drastically minimizes any hopes for gains
from virtualization endeavors. This leaves businesses seeking a physical configuration
that will support the workload, typically requiring abandonment of vertically
scaled infrastructures in favor of smaller, commoditized servers deployed in
a horizontally scaled fashion.

4. Security

Organizations often have little guidance for ensuring security in a physical-to-virtual
transition, as there are no published guidebooks or best practices for securing
a virtual environment. The following are key considerations to minimize risk.

Security zones - Mixing security zones in a virtual environment
is a bad idea, since most virtualization technologies don't provide for strong
enough security isolation models. For example, it wouldn't make sense to place
systems that are connected to sensitive internal networks on the same physical
host as those connected to a DMZ.

In addition, many virtualization solutions have administrator-level roles that
allow for viewing the disk images of all the virtual images. This introduces
huge vulnerabilities by allowing sensitive security zones to be bridged. This
problem is exacerbated with virtualization solutions that have internal network
switches that control traffic between VMs on the same physical host. These technologies
allow virtual systems to completely bypass all established port-level firewall
filters, deep packet sniffers, and QoS rules governing traffic in the environment.
This opens up the entire environment to threats that cannot be detected by network-level
security tools.

Information privacy - Many virtualization technologies
allow access to information stored in offline virtual images by simply mounting
them as disk images. While this gives users added convenience, it also introduces
considerable liabilities, making it easy for someone to walk off with a hard
drive. In addition, this makes it all the more important to be careful when
virtualizing any application that leaves residual data in temp files or other
local storage.

5. Financial Differences

Savvy businesses undergo "what if" scenarios to determine the most
lucrative virtualization solution for their environment. This includes taking
into consideration licensing costs, implementation expenses, and hardware/software
savings. For example, the cost of implementation varies widely based on the
cost of transitioning to next-generation servers and storage, application engineering,
etc. Moreover, if application-level changes are required, costs can skyrocket.
As a general rule, if functional or user acceptance level testing is involved,
ROI quickly disappears.

Another consideration is the type of hardware in use. Virtualizing high-density
physical infrastructures such as blades vastly reduces the server footprint,
but the cost of associated cooling systems may outweigh the benefits. Likewise,
the use of fat nodes and large vertically scaled servers offers high scalability
and efficiency but come with higher up-front costs. Commoditized rackmounted
servers are simple and easy to deploy, but since they share fewer hardware components
they may be less effective in offering economies of scale.

6. Chargeback Models

An important and often unexpected issue arising from virtualization is the lack
of viable chargeback models. This means virtual environments must be designed
so that they do not cross departments. If chargebacks are in place, the solution
must provide a way to obtain accurate utilization information to ensure equitable
billing for compute resources.

Conclusion

The sheer volume of virtualization offerings makes choosing the right solution
a confusing and often overwhelming task. Having the foresight to analyze key
factors impacting the virtualization effort enables businesses to avoid critical
missteps. Moreover, conducting "what if" scenarios that analyze business
and technical constraints-from security to workloads-drives more informed decision
making that will ultimately mean the difference between success and failure
in this complex era.

CiRBA