Achieving the Highest Levels of Parallel Sysplex Availability

Chapter 2. Hardware 41

2.4 9037 Sysplex Timers considerations

The 9037 provides the synchronization for the Time-of-Day (TOD) clocks of multiple CPCs,

and thereby allows events started by different CPCs to be properly sequenced in time. When

multiple CPCs update the same database and database recovery is necessary, all updates

are required to be time-stamped in proper sequence.

In a sysplex environment, the allowable differences between TOD clocks in different CPCs is

limited to the inter-CPC signalling time, which is very small. Some environments require that

TOD clocks be accurately set to an international time standard. The Sysplex Timer and the

Sysplex Timer attachment feature enable these requirements to be met by providing an

accurate clock-setting process, a common clock-stepping signal, and an optional capability

for attaching to an external time source.

It is a requirement that all systems in a multi-CPC sysplex are connected to the same set of

Sysplex Timers. If one of these systems loses access to the signals from

both timers, it will go

into either a disabled or an enabled wait state. It is for this reason that the availability of the

Sysplex Timers are

so important in a sysplex.

q To minimize the risk of having no functioning Sysplex Timer, you should always have two

Sysplex Timers. In addition, if your sysplex is spread over two sites, you should place one

timer in each site.

2.4.1 Sysplex Timer® Models

The 9037-001 Sysplex Timer ran out of service on December 31, 2003, and therefore will not

be discussed further in this document.

q Any customers still using 9037-001 timers should plan on upgrading to 9037-002s as soon

as possible. Note that the upgrade is disruptive—you should only plan on being able to

keep one CPC running during the migration. An updated migration procedure entitled

“Migration Planning for the IBM 9037 Model 2 Sysplex Timer” is available on the Web at:

http://www.redbooks.ibm.com/redpapers/pdfs/redp3666.pdf

q The recommended configuration in a Parallel Sysplex environment is the 9037 Expanded

Availability configuration. This configuration is fault-tolerant to single points of failure and

minimizes the possibility that a failure can cause a loss of time synchronization information

to the attached CPCs.

In an Expanded Availability configuration, the TOD clocks in the two 9037s are

synchronized using the hardware on the Control Link Oscillator (CLO) card and the CLO

links between the 9037s. Both 9037s simultaneously transmit the same time

synchronization information to all attached CPCs. The connections between 9037 units

are duplicated to provide redundancy, and critical information is exchanged between the

two 9037s every 1.048576 seconds, so if one of the 9037 units fails, the other will continue

transmitting to the attached CPCs. Figure 2-7 on page 42 contains an example of an

Expanded Availability Configuration. Note that this example only supports a maximum of 3

kms between Sysplex Timers. For greater distances, some form of repeater or DWDM

must be used.