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Purpose

This page describes the meaning behind the "Performance warning: Database "database" has a page size of X that does not match maximum of Y set for server, causing inefficient use of cache" database console log message.

Overview

Each running database engine uses a single, fixed page size that it uses for its cache across all databases. This cache page size is set when the database engine is started, and has a minimum value of 2048 bytes.

The server cache page size must be at least as big as the largest database page size

Each running database engine uses a single, fixed page size that it uses for its cache. This cache page size is set when the database engine is started, and has a minimum value of 2048 bytes (2K).

Each database uses a fixed page size, which is set when the database is created. This database page size also has a minimum value of 2048 bytes (2K).

By default, when an engine is started with a single database, the cache page size matches that of the database. If more than one database is started on the database engine command line, the cache page size is set by default to the maximum of the database page sizes.

The cache page size must be at least as big as the page size of any database you wish to run. If you wish to set a page size for your engine explicitly, to a value larger than the databases being loaded, it can be set with the -gp engine command-line switch. You may want to do this to allow a database with a bigger page size to be started dynamically during the operation of the engine (e.g. using START DATABASE).

Mismatched page sizes

If a database engine is started with a cache page size larger than the database page size, it will report the following warning message in the database console log:

e.g.

   dbeng16 -gp 8192 demo.db

      Performance warning: Database "demo" has a page size of 4096 that does not match maximum of 8192 set for server, causing inefficient use of cache.

Only one database page per cache page

At only one database page can be put into one cache page. Thus if you start two databases, one with a page size of 2K and one with a page size of 4K, the server cache pages used for the 2K page size database pages will be only half full. For this reason, it is recommended to use a common page size across all concurrently started databases on one database server. 

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