Posted on18 February 2011.

All organizations are becoming increasingly reliant upon their computer systems. The
availability of those systems can be the difference between the organization succeeding
and failing. A commercial organization that fails is out of business with the consequences
rippling out to suppliers, customers, and the community.

This series will examine how we can configure our Windows Server 2008 environments to
provide the level of availability our organizations need. The topics we cover will comprise:

• The Art of High Availability—What do we mean by high availability? Why do we
need it, and how do we achieve it?

• Windows Server 2008 Native Technologies—What does Windows Server 2008
bring to the high?availability game, and how can we best use it?

• Non?Native Options for High Availability—Are there other ways of achieving high
availability, and how can we integrate these solutions into our environments?

The first question we need to consider is why we need highly available systems.

Why Do We Need It?
This question can be turned on its head by asking “Do all of our systems need to be highly
available?” The answer for many, if not most, organizations is no. The art of high
availability comes in deciding which systems need to be made highly available and how this
is going to be achieved. When thinking about these systems, we need to consider the effects
of the systems not being available.

Downtime Hurts
Downtime is when the computer system is unavailable to the user or customer and the business
process cannot be completed. If the server is up and the database is online but a network
problem prevents access, the system is suffering downtime. Availability is an end?to?end
activity. Downtime hurts in two ways: If a system is unavailable, the business process it
supports cannot be completed and there is an immediate loss of revenue. This could be due
to:

Customer orders not being placed or being lostStaff not workingOrders not being processed

The second way that downtime hurts is loss of reputation. This loss can be even more
damaging in the long term if customers decide that your organization cannot be trusted to
deliver and they turn to a competitor. The ability to gain business increases with ease of
communication and access. The converse is that the ability to lose business increases just
as fast if not faster.

Mission Critical Systems on Microsoft Windows
Critical business systems are hosted on the Microsoft Windows platform. These can be customer
facing or internal, but without them, the business grinds to a halt. Email may not seem to be
a critical system, but it is essential to the modern business. More than 60% of person to person
communication is via email in most businesses. This includes internal and external
communications. If a company is non?responsive to communications, it is judged, perhaps
harshly, as being out of business. This can become reality if it progresses too long.

24 × 7 Business Culture
The “Global Village” concept has been accelerated by the adoption of the Internet for
business purposes. Globalization in this case means that business can come from anywhere
in the world—not necessarily your own time zone. If your business competes at this level,
high availability isn’t an option, it’s a necessity.

Legislation
Industries such as the financial services and health sector have a requirement to protect
the data they store. This requirement can involve the availability of the data. In other cases,
the systems must be highly available to meet safety requirements.

Once you know why you need it, you need to define what is meant by high availability.

What Is High Availability?
High availability is usually expressed in terms of a number of “9”s. Four nines is 99.99%
availability. The ultimate goal is often expressed as 5 “9”s availability (99.999%), which
equates to five and a quarter minutes of downtime per year. The more nines we need, the
greater the cost to achieve that level of protection.

One common argument is scheduled downtime. If downtime is scheduled, for example, for
application of a service pack, does that mean the system is unavailable? If the system is
counted as unavailable, any Service Level Agreements (SLAs) on downtime will probably
be broken. In hosting or outsourcing scenarios, this could lead to financial penalties.
However, if scheduled downtime doesn’t mean the system is counted as unavailable,
impressive availability figures can be achieved—but are they a true reflection of
availability to the users? There is no simple answer to these questions, but all systems
require preventative maintenance or they will fail. The disruption to service can be
minimized (for example, the patching nodes of a cluster in sequence) but cannot be
completely eliminated. Probably the best that can be achieved is to ensure that
maintenance windows are negotiated into the SLA.

These measurements are normally taken against the servers hosting the system. As we
have seen, the server being available doesn’t necessarily mean the system is available. We
have to extend our definition of highly available from protecting the server to also include
protecting the data.

The Server Clustering Service built?in to Microsoft Windows is often our first thought for protecting the
server. In the event of failure, the service automatically fails over to a standby server, and
the business system remains available. However, this doesn’t protect the data in that a
failure in the disk system, or even network failures, can make the system unavailable.

Do We Still Need to Back Up our server and data?
One common question is “Do I still need to take a backup?” The only possible answer is
YES! High availability is not, and never can be, a substitute for a well?planned backup
regimen. Backup is your ultimate “get out of jail card.” When all else fails, you can always
restore from backup. However, this pre supposes a few points.

Test restores have been performed against the backup media. The last place you
want to be is explaining why a business?critical system cannot be restored because
the tapes cannot be read.A plan exists to perform the restore that has been tested and practiced. Again, you
don’t want to be performing recoveries where the systems and steps necessary for
recovery are not understood.

Backup also forms an essential part of your disaster recovery planning.

Disaster Recovery vs. High Availability
These two topics, high availability and disaster recovery, are often thought of as being the
same thing. They are related but separate topics. High availability can be best summed up
as “keeping the lights on.” It is involved with keeping our business processes working and
dealing with day?to?day issues. Disaster recovery is the process and procedures required to
recover the critical infrastructure after a natural or man?made disaster. The important
point of disaster recovery planning is restoring the systems that are critical to the business
in the shortest possible time.

Traditionally, these are two separate subjects, but the technologies are converging. One
common disaster recovery technique is replicating the data to a standby data center. In the
event of a disaster, this center is brought online and business continues. There are some
applications, such as relational database systems and email systems, that can manage the
data replication to another location. At one end of the scale, we have a simple data
replication technique with a manual procedure required to bring the standby data online in
place of the primary data source. This can range up to full database mirroring where
transactions are committed to both the primary and mirror databases and fail over to the
mirror can be automatically triggered in the event of applications losing access to the
primary. In a geographically dispersed organization where systems are accessed over the
WAN, these techniques can supply both high availability and disaster recovery.

We have seen why we need high availability and what it is. We will now consider how we
are going to achieve the required level of high availability.

Achieving High Availability
When high availability is discussed, the usual assumption is that we are talking about
clustering Windows systems. In fact, technology is one of three areas that need to be in
place before high availability works properly:

People and Processes
These are the two points that are commonly overlooked. I have often heard people say that
clustering is hard or that they had a cluster for the application but still had a failure. More
often than not, these issues come down to a failure of the people and processes rather than
the technology.

The first question that should be asked is “Who owns the system?” The simple answer is
that IT owns the system. This is incorrect. There should be an established business owner
for all critical systems. They are the people who make decisions regarding the system from
a business perspective—especially decisions concerning potential downtime. A technical
owner may also be established. If there is no technical owner, multiple people try to make
decisions that are often conflicting. This can have a serious impact on availability.
Ownership implies responsibility and accountability. With these in place, it becomes
someone’s job to ensure the system remains available.

A second major issue is the skills and knowledge of the people administering highly
available systems. Do they really understand the technologies they are administering?
Unfortunately, the answer is often that they don’t. We wouldn’t make an untrained or
unskilled administrator responsible for a mainframe or a large UNIX system. We should
ensure the same standards are applied to our highly available Windows systems. I once
worked on a large Exchange 5.5 to Exchange 2003 migration. This involved a number of
multi?node server clusters, each running several instances of Microsoft Exchange. One of the Microsoft Exchange
administrators asked me “Why do I need to know anything about Active Directory?” Given
the tight integration between Exchange and Active Directory (AD), I found this an
incredible question. This was definitely a case of untrained and unskilled network administrator.

Last, but very definitely not least, we need to consider the processes around our high availability
systems. In particular, two questions need to be answered:

Do we have a change control system?

If the answer to either of these is no, our system won’t be highly available for very long. In
addition, all procedures we perform on our systems should be documented and tested.
They should always be performed as documented.

Technology
Technology will be the major focus of the next two articles, but for now, we need to
consider the wider implications of high availability. We normally concentrate on the
servers and ensure that the hardware has the maximum levels of resiliency. On top of this,
we need to consider other factors:

Network—Do we have redundant paths from client to server? Does this include
LAN, WAN, and Internet access?Does the storage introduce a single point of failure?Has the operating system (OS) been hardened to the correct levels? Is there a
procedure to ensure it remains hardened?Does our infrastructure in terms of AD, DNS, and DHCP support high availability?Does the application function in a high?availability environment?

Costs
Highly?available systems explicitly mean higher costs due to the technology and people we
need to utilize. The more availability we want, the higher the costs will rise. A business
decision must be made regarding the cost of implementing the highly?available system
when compared against the risk to the business of the system not being available.

This calculation should include the cost of downtime internally together with potential loss
of business and reputation. When a system is unavailable and people can’t work, the final
costs can be huge leading to the question “We lost how much?”

Summary
You need high availability data solutions to ensure your business processes keep functioning. This ensures
your revenue streams and your business reputation are protected. We help you achieve high availability
through the correct mixture of people, processes, and technology.

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