October 19, 2001, 11:32 AM — Performance monitoring and capacity planning are
infamous subjects to managers of mainframe-based data centers. In
brief, an Information Technology manager must decide how much computing
resources an organization needs, when they are needed, and answer why
the "how much" estimation always changes.
Managers of distributed, client/server environments wrestle with
this problem too. But they must endure the added complexity of dealing
with multiple sites.
In supporting the functions of capacity planning and performance
monitoring, IT organizations are really assessing the service levels they provide, which are decided by customer expectations:
- What service will IT provide?
The customers define this.
- What service can a given configuration provide?
Provided by the IT organization, these are the goals for user performance,
factoring in the budgetary restrictions.
As those associated with the mainframe data centers know, this is not
easy, and even old-hands guess wrong. The difficulties arise from
several reasons:
- Predicting the future is hard.
We are dealing with future hardware and software and
how this vaporware may work together, future users and their
requirements, and the organization's future mission. (Pop quiz: How much
CPU capacity did you plan for your public Web server this year? Oh,
you didn't even plan a Web server?)
- Users can't predict the future either.
Smart people always find new ways of using all of the computing
resources available. (Pop quiz: Ask your marketing manager how
many Web pages your organization will publish next April.)
- Distributed client/server environments are more complex
than mainframe data centers.
There is now a wider
variety of potential configurations, and each new generation of
hardware and software will introduce new capabilities and costs.
(Pop quiz: Ask your Webmaster what Java- and VRML-enabled Web pages
mean to your organization's network load next April.)
- The rate of change in new technologies continues to
accelerate.
Very often, by the time a person
understands and feels comfortable with a new technology it is
obsolete. Both as an institution and as individuals, IT staffs
must keep abreast of new technologies or risk being left behind.
(Pop quiz: Did your IT department spearhead your Web server
development and deployment?)
- Organizations are complex.
Complexity breeds specialization. Experts in one field are rarely
experts in another. When planning computer and network use
it helps speak the language of your constituency.
While some view capacity planning as an art form, you can approach it scientifically. Keep in mind it is easier to find that a
configuration will not support a specific service level than
to predict it will. For example, it is easy to determine that a
system with a single disk drive cannot achieve random access throughput
rates of 130 accesses per second if that one disk can handle only 65.
However, a system with two disks (each of which can handle 65 accesses
per second) may or may not handle the same load, because the
bottleneck may not be in the disk subsystem.
Also, actual use follows Parkinson's Law and will eventually
employ all available resources. (This is most distressful when customers
devour an over-configured system by piling on unplanned tasks.)
To properly analyze a distributed environment, view it as a series
of connected components (computers, peripherals, software, networks,
etc.), and break these down into their individual, measurable
components. For example, client machines are composed of CPUs, memory,
software, busses, and peripheral devices. Each of these can be
monitored as to its effects on the entire configuration's total
performance. As with the old saying about a chain being as strong as
its weakest link, a distributed environment's performance is often set
by its weakest component.
