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.

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