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Reducing Disk I/O Contention

The XDB Server allows a single database to span multiple physical devices, as long as no single object of the database spans more than one physical device. Thus, while no database object (table or index) can exceed the size of its physical storage device, objects can be spread among many devices to reduce I/O contention.

For example:

The guiding principle in determining object location is to spread the workload evenly among the drive controllers and disk heads. If disk head movement can be kept to a minimum and drive controller usage evenly dispersed, optimal performance can be obtained. Table and index caching can improve performance even more. See the section Improving Production Processing for more information.

Given that the server stores database objects in separate files, tuning of database object locations can be done at any time. By simply copying the database object file to a different physical device and updating the stogroup or environment variable pathname for that particular database object, different space allocation strategies can be tested.

Paths for stogroups are maintained in the SYSIBM.SYSVOLUMES table. If a table space was created without specifying a stogroup, the table space's path will be that of the database (either ..\loc-path\dbname or the path of the stogroup that was used to create the database). For greatest flexibility, each table should be associated with its own stogroup (drive and path) when it is created.

Paths for indexes and other objects can be specified in environment variables in the server machine's configuration file. (See the chapter Environment Variables for more information.)

Guidelines on how to position tables, indexes and temporary files are provided on the following pages.

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