IBM eServer zSeries 900 Technical Guide

142 IBM eServer zSeries 900 Technical Guide

3.8 External Time Reference

There is a long-standing requirement for accurate time and date information in data

processing. As single operating systems have been replaced by multiple, coupled operating

systems on multiple servers, this need has evolved into a requirement for both accurate and

consistent clocks among these systems. Clocks are said to be consistent when the difference

or offset between them is sufficiently small. An accurate clock is consistent with a standard

time source.

The IBM z/Architecture and S/390 Architecture External Time Reference (ETR) architecture

facilitates the synchronization of server time-of-day (TOD) clocks to ensure consistent time

stamp data across multiple servers and operating systems.

The ETR architecture provides a means of synchronizing TOD clocks in different servers with

a centralized time reference, which in turn may be set accurately on the basis of an

international time standard (External Time Source). The architecture defines a time-signal

protocol and a distribution network, called the ETR network, that permits accurate setting and

maintenance of consistency of TOD clocks.

ETR time

In defining an architecture to meet z/Architecture and S/390 Architecture time-coordination

requirements, it was necessary to introduce a new kind of time, sometimes called ETR time,

that reflects the evolution of international time standards, yet remains consistent with the

original TOD definition. Until the advent of the ETR architecture, the server TOD clock value

had been entered manually, and the occurrence of leap seconds had been essentially

ignored. Introduction of the ETR architecture has provided a means whereby TOD clocks can

be set and stepped very accurately, on the basis of an external Universal Time Coordinate

(UTC) time source.

Sysplex Timer attachment

The IBM Sysplex Timer synchronizes a server’s time-of-day clock with multiple servers in a

sysplex. A server’s Oscillator/External Time Reference (OSC/ETR) card provides the

interface to the IBM Sysplex Timer. Two OSC/ETR cards are standard on the z900 server.

Sysplex Timer synchronization

As server and Coupling Facility link technologies have improved over the years, the

synchronization tolerance between operating systems in a Parallel Sysplex has become more

rigorous. In order to ensure that any exchanges of timestamped information between

operating systems in a sysplex involving the Coupling Facility observe the correct time

ordering, timestamps are now included in the message-transfer protocol between the server

operating systems and the Coupling Facility.

Therefore, when a Coupling Facility is configured as an ICF on any z900 server model 2C1

through 216, the Coupling Facility will require connectivity to the same 9037 Sysplex Timer

that the operating systems in its Parallel Sysplex are using for the time synchronization. If the

ICF is on the same z900 server as a member of its Parallel Sysplex, no additional Sysplex

Timer connectivity is required, since the z900 server already has connectivity to the Sysplex

Timer. However, when an ICF is configured on any z900 server model 2C1 through 216 which

does not host any operating systems in the same Parallel Sysplex, it is necessary to attach

the server to the 9037 Sysplex Timer.