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Exclusive Article by
Gus Kassimis
Senior Software
Engineer,
IBM's Enterprise Networking Solutions
 Optimizing
external IP load balancing in a Sysplex environment
GUS KASSIMIS: a synopsis
Gus
Kassimis is a Senior Software Engineer in IBM’s Enterprise
Networking Solutions strategy and design group.
During the past 17 years he has held various leadership positions in
OS/390 and z/OS design and development areas, such service aids,
System Resource Manager (SRM), Workload Management (WLM), Workflow
for MVS, and the z/OS Communications Server. He is currently a
designer for z/OS Communications Server with a focus on TCP/IP
technologies on z/OS. He is also the lead designer for TCP/IP
sysplex technologies on z/OS and a frequent speaker at SHARE.
IBM’s Parallel Sysplex®
clustering technology on z/OS®
and OS/390®
has long provided for an environment for deploying highly scalable
and highly available workloads. Applications can be cloned and
deployed across multiple system images, while accessing the same
data by virtue of the zSeries Coupling Facility.
Need more capacity? Besides the normal capacity upgrade path
(i.e. processor upgrades or Logical Partition adjustments) you can
also scale out by starting additional cloned application instances
on underutilized systems or add another system to the sysplex.
How about high availability? By spreading workload over
multiple application and system instances you can insulate end users
from outages (whether planned or unplanned) to individual
application instances or systems.
One of the key solution components required to realize the benefits
of the sysplex environment is a load balancing solution.
Effective load balancing solutions allow the entire sysplex to be
viewed as single system from an end user perspective, delivering
work requests to the application instance that can best service that
work request. This can be accomplished if detailed information
regarding the current state of applications, systems, and capacity
in the sysplex is available. For example, the Sysplex
Distributor, an IP load balancing solution that is part of the
z/OS Communications Server, by virtue of its proximity (it resides
within the sysplex), has all this information at its disposal and
can optimize its load balancing decisions.
But what if you have decided to deploy an external load balancing
solution for some of your z/OS sysplex workloads? Does that
mean that you have to forgo all the benefits that an internal load
balancing solution, such as Sysplex Distributor, would have
provided? Until quite recently, the answer to this question
would quite often be yes.
However, a new function recently introduced by IBM, the z/OS Load
Balancing Advisor, can help change this.
The z/OS Load Balancing Advisor
Lo and behold!
The recommendations provided by the Advisor are dynamic in nature;
they may change as the conditions of the applications and the
systems in the sysplex change. These recommendations consist of the following components:
1.
State of the target applications and systems
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This includes an indicator on whether the target application and/or
target system is currently active. This allows the load balancer to exclude systems that are not active or do not
have the desired application running. |
2. z/OS WLM recommendations
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WLM recommendations provide a relative measure of a target system's
ability to handle new workload as compared to other target systems in the sysplex. For example, if a
system in the sysplex in underutilized, the weights returned for that system will be higher than the weights of the other
systems. Even in scenarios where all systems in the sysplex are highly utilized, differentiation is still possible –
WLM will favor systems that have a larger proportion of lower importance work (as designated by the user’s WLM policy)
since that work can be displaced by higher importance workload. |
3.
Server application health, from a TCP/IP perspective
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TCP/IP statistics for target applications are monitored to determine
if specific server applications are encountering problems keeping up with the current workload. For example, is a
target TCP server application keeping up with TCP connection requests? Or are requests being rejected because
the TCP backlog queue is full? When these types of problems are detected, recommendations are adjusted
appropriately so that the load balancer can direct less connections to any applications that experiencing these problems. |
Server/Application State Protocol
The
Server/Application State Protocol (SASP) defines the
interface that is used between the external load balancers and the
z/OS Load Balancing Advisor. SASP was introduced recently as
part of the IBM’s Enterprise Workload Manager (EWLM) solution
that is part of the IBM Virtualization Engine and is available on
several platforms. For more details on EWLM refer to:
http://www-1.ibm.com/servers/eserver/about/virtualization/suiteforservers/
As a result, load balancing solutions that exploit SASP, can work
with both the EWLM and the z/OS Load Balancing Advisor solution.
SASP is an efficient protocol that is quite flexible, it provides
support for both TCP and UDP applications, and also supports both
IPv4 and IPv6 network configurations.
z/OS
Load Balancing Advisor solution overview
The graphic above depicts the high level structure of the z/OS Load
Balancing Advisor. A single Advisor must be active within one
of the systems in the sysplex. Using SASP, the external load
balancer(s) contact the Advisor to register their interest in
specific TCP and UDP applications executing within the z/OS sysplex.
The Advisor communicates this information to Load Balancing Agents
that are executing on every system in the sysplex that is considered
a target for load balancing. The Agents dynamically determine what
applications are active on the local system, derive the load
balancing recommendations, and then communicate those back to the
Advisor who provides them to the load balancer(s).
Server/Application State Protocol
In summary, with the new z/OS Load Balancing Advisor and SASP,
external load balancing solutions can now optimize workload
balancing for your z/OS Sysplex environment. If you are currently
using or planning to use an external IP load balancing solution, and
are interested in these optimizations, you should contact your
vendor to determine whether their solution supports SASP. SASP is
an open protocol that is available for use by any vendor. At the
time this article was written, it is an individual
Internet Draft submitted to and published by the IETF (Internet
Engineering Task Force). It is also worth noting, that CISCO
Systems, Inc.™, has already announced support for SASP in their
CISCO®
Content Switching Module (CSM).
The z/OS Load Balancing Advisor function is available on z/OS
release V1R4 right now via APAR PQ90032. Support for z/OS release
V1R5 and V1R6 will follow shortly via APAR PQ96293. For more
details related to this new function, refer to the APARs mentioned
above and the following web site:
http://www-1.ibm.com/support/docview.wss?uid=swg27005585
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