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Server Time Protocol (STP) - when the operating system doesn't support it!



Document Author:

Gregory Hutchison


Document ID:

FLASH10631


Doc. Organization:

Advanced Technical Sales


Document Revised:

12/18/2013


Product(s) covered:

z9 BC; z9 EC; zSeries 890; zSeries 990







Abstract: STP is intended to accurately coordinate time among many z/OS operating systems on System z servers. Customers are increasingly implementing STP to provide an accurate timestamp to operating systems that do not have STP support. The following document includes important information that customers need to know when implementing STP if they are using an operating system which does not have STP support.


STP is intended to accurately coordinate time among many z/OS operating systems on System z servers. Customers are increasingly implementing STP to provide an accurate timestamp to operating systems that do not have STP support. The following document includes important information that customers need to know when implementing STP if they are using an operating system which does not have STP support.

Note: In the text below, OS-xyz applies to any operating system that does not support STP or is not configured for STP.

External Time Source (ETS)

In order for STP to access an External Time Source (ETS), the CTN must be an STP-only CTN.

If an ETS is configured for an STP-only CTN, the time of the STP-configured Current Time Server (CTS) – Stratum 1 server – can be set to the accurate time provided by the ETS device. However, after a little while, the server time will drift a bit from the time provided by the ETS. STP can "steer out" this drift automatically as long as the delta is less than 60 seconds. The CTS will periodically access the ETS to maintain time accuracy. NTP server access is driven from the CTS, once every 10 minutes, with an adjust time occurring once an hour. Once the CTS obtains an accurate time, all servers in the STP-only CTN also have the accurate time, because they are time synchronized.

All active LPARs on servers in the STP-only CTN can benefit from the time accuracy provided by using the ETS function, even if the operating systems hasn't specified for STP or if they don't support STP. All LPARs (OS-xyz included) can still maintain time within 100 ms of the External Time Source. In addition to the drift associated with oscillators (which can be corrected as stated above), there are a few other instances that will cause a larger difference between the CEC time and the ETS time that OS-xyz running on a STP configured server will not be able to handle. The limitations for OS-xyz are listed below.

1. Sync Checks: Sometimes, clocks will differ by more than can be steered by STP due to unexpected activity or events like someone unplugging a LAN cable. This difference may be something between a few seconds and half a minute, maybe even more. If this happens, the server and all STP configured operating systems may be able to correct the delta, but OS-xyz will not know that the delta exists. In order to remedy this for OS-xyz, you would need to deactivate the LPAR, activate it again, and re-IPL OS-xyz.

2. Leap Seconds: If using leap seconds, the OS-xyz system will not be able to recognize them when STP time is initialized because OS-xyz does not accept leap seconds. The following steps are a means of remedying this: When STP time is initialized, Leap seconds should be set to 0. When the ETS is used, it will already include leap seconds. At this point, the server time will match the time provided by the ETS. However, the next time a leap second occurs, the user should NOT schedule for it, meaning that the next dial out or NTP server access should steer out the difference in time.

3. Messages: Messages from STP will not be seen by OS-xyz. However, the HMC will still post messages from the firmware or microcode, for example "ETS access successful/unsuccessful." The fact that STP messages will not be posted by OS-xyz images will only impact a customer’s environment if they depend on monitoring the system with messages from the operating system, as they will not see any messages from STP until they are running an operating system configured for STP.



Classification:

Hardware

Category:

Installation and Migration




Platform(s):

IBM System z Family



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Keywords:

STP, Server Time Protocol

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