IBM Offerings

IBM offers DB2 databases both for a shared-nothing (for UNIX, Linux, and Windows) and a shared-disk (on the mainframe only) approach. At a very high level view, the architecture of IBM Parallel Sysplex and Oracle RAC looks very similar. However, they differ heavily in their implementation methodology. Both of these products follow the shared architecture. Oracle implementation provides an open system approach and it is possible to implement it by utilizing commodity-style components. Parallel Sysplex implantation depends on the special hardware components provided by IBM. Again, looking at the UNIX based UDB EEE parallel database, it is more of a partitioned database than a parallel database. It is parallel to the extent you don’t care where you query or access. However, the processing is done by a function shipping method when a query needs to access data attached to another node.

IBM Parallel SysPlex Architecture

The S/390 Parallel Sysplex follows the shared-disk model approach with all nodes (or MVS systems) accessing the disks concurrently. The cluster is built in with a ‘coupling facility’, which addresses the global concurrency issues involved in the shared-data scheme. The coupling facility is a special hardware component with proprietary micro code.

To share data within the parallel cluster, DB2 subsystems must belong to a predefined data-sharing group. All members of a data-sharing group use the same-shared DB2 catalog and directory. A DB2 data sharing implementation [Fig 3.13] in a Parallel Sysplex environment usually consists of:

• At least one coupling facility — a component that manages the shared resources of the connected central processor complexes (CPCs).
• At least one Sysplex Timer, which keeps the processor timestamps synchronized in the data-sharing group.
• A connection to shared DASD (Direct access storage device), where user data, system catalog, directory data, and MVS catalog data all reside.
• One or more CPCs, consisting of main storage, central processors, timers, and channels that can attach to a coupling facility.

Fig 3.13 Parallel SysPlex System

The coupling facility is a microprocessor unit. High bandwidth fiber optic links, called channels, provide connectivity between CF and nodes/systems. DB2 uses the coupling facility to provide for inter-node communications. The coupling facility ensures data availability, while maintaining data integrity across the connected DB2 subsystems. The coupling facility provides core services such as data locking, data consistency, and buffering.

The coupling facility uses three structures to synchronize the activities of the data-sharing group members:

• Cache structure - Supplies a mechanism called buffer invalidation to ensure consistency of cached data. The cache structure can also be used as a high-speed buffer for storing shared data with common read/write access.
• List structure - Enables 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.
• Lock structure - Supplies shared and exclusive locking capability for serialization of shared resources down to a very small unit of data.

Thus the coupling facility manages all locking, contention, and concurrency issues. It does liaison with local buffers of the nodes. It is the heart of a data sharing system.

While it provides a high performance parallel cluster for database support, it comes with a very high price. This system is also a proprietary solution requiring higher costs in terms of administration and manageability.