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==Sysplex==
In 1990, [[IBM]] [[mainframe computer]]s introduced the concept of a '''Systems Complex''', commonly called a '''Sysplex''', with [[MVS]]/ESA SPV4.1. This allows authorized components in up to eight
Components of a Sysplex include:
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==Parallel Sysplex==
[[File:GDPS.svg|thumb|300px|Schematic representation of a Parallel Sysplex]]
The Parallel Sysplex was introduced with the addition of the [[Coupling Facility]] (CF) with coupling links for high speed communication, with MVS/ESA V5.1 operating system support, together with the mainframe models in April 1994.<ref>{{cite
| title = System/390 Parallel Sysplex Performance
| id = SG24-4356-03
| date = December 1998
| edition = Fourth
| url = http://www.redbooks.ibm.com/redbooks/pdfs/sg244356.pdf
| publisher = International Business Machines Corporation
| access-date = 2007-09-17
| url-status = dead
| archive-url = https://web.archive.org/web/20110518132944/http://www.redbooks.ibm.com/redbooks/pdfs/sg244356.pdf
| archive-date = 2011-05-18
}}
</ref>
The Coupling Facility (CF) may reside on a dedicated stand-alone server configured with processors that can run Coupling Facility control code (CFCC), as integral processors on the mainframes themselves configured as ICFs (Internal Coupling Facilities), or less common, as normal LPARs. The CF contains Lock, List, and Cache structures to help with serialization, message passing, and buffer consistency between multiple LPARs.<ref>
| title = | author = David Raften
| date = November 2019
| publisher = IBM
| url = http://www.ibm.com/common/ssi/fcgi-bin/ssialias?infotype=SA&subtype=WH&attachment=ZSW01971USEN.PDF&appname=STGE_ZS_ZS_USEN&htmlfid=ZSW01971USEN
}}
</ref>
The primary goal of a Parallel Sysplex is to provide data sharing capabilities, allowing multiple databases for direct reads and writes to shared data. This can provide benefits of
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==Server Time Protocol==
Maintaining accurate time is important in computer systems. For example, in a transaction-processing system the recovery process reconstructs the transaction data from log files. If time stamps are used for transaction-data logging, and the time stamps of two related transactions are transposed from the actual sequence, then the reconstruction of the transaction database may not match the state before the recovery process.
Server Time Protocol (STP) can be used to provide a single time source between multiple servers. Based on Network Time Protocol concepts, one of the System z servers is designated by the HMC as the primary time source (Stratum 1). It then sends timing signals to the Stratum 2 servers through use of coupling links. The Stratum 2 servers in turn send timing signals to the Stratum 3 servers. To provide availability, one of the servers can be designated as a backup time source, and a third server can be designated as an
STP has been available on System z servers since 2005.
More information on STP is available in “Server Time Protocol Planning Guide”.<ref>
| title = Server Time Protocol Planning Guide
| id = SG24-7280-03
| date = June 2013
| edition = Fourth
| work = Redbooks
| publisher = International Business Machines Corporation
| url = http://www.redbooks.ibm.com/redbooks/pdfs/sg247280.pdf
}}
</ref>
==Geographically Dispersed Parallel Sysplex==
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