Oracle Database Tips by Donald Burleson

IBM Shared-Nothing Configuration

IBM has traditionally used the shared nothing disk architecture for all versions of its DB2 shared database except OS/390 (see Figure 14.6).      

The shared nothing configuration utilizes isolated disk assets for each server and the database (in the IBM implementation) is hashed across all of the servers. The database image is perceived by the users to be a single database and each machine has access to its own data. However, this implementation of the parallel database server falls prey to what is known as the "convoy effect". In a convoy of trucks or ships the speed of the convoy is limited by the speed of the convoys slowest member. It is the same with a database that is spread across isolated disk assets by hashing, the speed of database processing is limited by the speed of the slowest member of the cluster.

Another problem with the hashed spread of a database across multiple isolated disk assets is that the loss of one of the servers means loss of the entire database until the server or its disk assets are rebuilt and/or reassigned.      

In addition, due to the way the data is hashed across the servers and disks standard methods for maintaining data integrity won't work. This indicates that for a hashed storage database cluster much, if not all of the data integrity constraints must be programmed into the applications using complex two-phase commit algorithms instead of relying on built in constraint mechanisms.      

The final problem with the shared nothing approach used by IBM is that to add a new cluster member the data must be re-hashed across all members requiring database down time.

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