Basic Sysplex vs. a Parallel Sysplex

In September 1990, IBM introduced the SYStems comPLEX, or sysplex, to help solve the complexity of managing multiple z/OS systems. A Sysplex is a collection of z/OS systems that cooperate using hardware and software functions to process work, providing improved growth potential and higher level of availability.

The Sysplex architecture established the groundwork for simplified multi-system management through the Cross-System Coupling Facility (XCF) component of z/OS. XCF services allow authorized applications on one system to communicate with applications on the same system or on other systems. In a base sysplex, connectivity and communication between images is provided by high-speed interconnects called channel-to-channel (CTC) links. The disk couple data set, which is shared between all of the images, holds control information and provides a mechanism for monitoring the status of the images. When more than one server is involved, a common time source such as a Sysplex Timer or STP is required to synchronize the time on all systems. From a functionality perspective, the base sysplex did not change the clustering storage model. Data can be shared at the file level (data set) using z/OS Global Resource Serialization (GRS) locking services.

What sysplex added was the following:

The basic Sysplex laid a foundation for communications between components and subsystems on the participating z/OS member images, but the basic Sysplex did not provide the support for cache buffer coherency required for high-performance database data sharing. In addition, the existing GRS implementation could not provide the required performance for sharing high-volume transactional systems across a large number of systems.

The Parallel Sysplex technology builds on the base sysplex capability and allows an increased number of operating system images that can directly share work (up to 32). With the introduction of Parallel Sysplex technology in 1994, high-performance data sharing through a new coupling technology, the Coupling Facility provided high-performance multi-system data sharing capability to authorized applications, such as system components and subsystems. The Coupling Facility (CF) is used by subsystems, such as IMS/DB DB2 and VSAM/RLS, to ensure the integrity and consistency of data throughout the entire Parallel Sysplex. This enables high performance, multi-system data sharing across all the systems.

z/OS Components

XCF is a component of z/OS. It provides the signaling services in a sysplex. Authorized applications can call XCF services to pass messages to other LPARs in the sysplex. XCF uses signaling paths over high-speed fibre interconnects between servers or through CF list structures and for maintaining the status of members of the sysplex.

XES is another component of z/OS. It provides the Parallel Sysplex services that applications and subsystems use to communicate with the Coupling Facility. These services support the sharing of cached data and common queues while maintaining data integrity and consistency. Any subsystem, such as DB2, that needs to perform activity against the CF structures uses XES services.

One means of implementing a CF is in a Logical Partition (LP) just like a z/OS image but it runs its own highly specialized operating system, Coupling Facility Control Code (CFCC). The LPAR can also run in a specialized processor called the Integrated Coupling Facility (ICF). The Coupling Facility connects to all the z/OS images in a Parallel Sysplex by high-speed fibre or copper links.

The design of the CF ensures that requests are queued in a logical order that is not dependent on the speed of the connected servers. This removes the unpredictable response time that is inherent in conventional devices such as DASD. This capability of linking together many systems and provides multi-system data sharing making the Parallel Sysplex platform ideal for parallel processing, particularly for online transaction processing and decision support.

As well as supporting a data sharing environment for OLTP applications, the Coupling Facility has been used by other products to help with performance or system management. This is known as "Resource Sharing."

The following is what Parallel Sysplex added:

In the CF, data is organized into distinct objects called structures. CF structures are used by authorized programs to implement data sharing and high-speed serialization. There are a number of different types of CF structures that can be created: cache, list, and lock, each providing a specific function to the application. In addition, some storage in the Coupling Facility can also be allocated as a dedicated dump space for capturing structured information for diagnostic purposes.

Cache structure supplies a mechanism called buffer invalidation to ensure consistency of cached data. The cache structure can be used as a high-speed buffer for storing shared data with common read/write access. It is used by subsystems like database managers to provide shared buffers and maintain buffer coherency across multiple systems.

Lock structures supply shared and exclusive locking capability for serialization of shared resources down to a very small unit of data. The main users of lock structures are GRS (for sharing any type of resource) and the data sharing lock managers (IRLM and VSAM/Record Level Sharing: RLS). Associated with the lock structures are XES lock services, which come into play in case of lock contention. The user of the request can specify shared or exclusive access. Normally, shared access is associated with reads and exclusive access with a write, but it depends on the program issuing the request. As long as no one has exclusive access to the resource, shared access requests are always granted by the Coupling Facility. NOTE: If there is anyone with exclusive access, the allow/disallow decision must be made by the resource serialization manager for that resource. For example, IRLM with DB2 can allow multiple “Intent to Update” (IX) lock requests against the same table.

List structures are the most general purpose type of structure and as such are not easy to define in a concise manner. List structures enable authorized applications to share data that is organized in a set of lists for implementing functions such as shared work queues and shared status information. List entries can be processed as FIFO or LIFO, and access to list entries can be serialized or unserialized. List structures provide the most flexibility and functionality of all the CF structuretypes.

Coupling Facility Links (CF links) are high speed fibre or copper interconnects used to communicate between CF LPARs and z/OS LPARs. In the case of System Managed structure duplexing, CF links are also used for CF LPAR to CF LPAR communication. There are a number of types of CF links.

Timers: The presence of the timers is due to 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. Time synchronization between all systems in a sysplex is required because there are many instances where events that happen on different systems need to be aggregated for processing. An example is a database restart that needs to apply log records from several systems in the correct time sequence. Common time source capability has been provided by the Sysplex Timer device (IBM 9037) and by its follow-on and preferred the Server Time Protocol (STP).

STP is designed for servers that are configured in a Parallel Sysplex or a standard sysplex without a Coupling Facility, as well as servers that are not in a sysplex but need to be time-synchronized. STP supports a multi-site timing network of up to100 Km (62 miles) over fiber optic cabling (longer with RPQ). STP is a message-based protocol in which time keeping information is passed over data links between servers. Capabilities of STP include being able to provide a common time source between the System z and non-System z timing network using NTP.

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