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Background documentation Data Cache Locate the document in the library structure

The database system uses the data cache to retain the pages that it has recently read or write-accessed in the data area. This includes, for example, tables, indexes, long values, undo log entries and OMS data.

Note

Since the data in an SAP liveCache database instance consists mainly of OMS data, you should configure the data cache for SAP liveCache database instances to be large enough to store all the OMS data, if possible.

The database system does not store converter information in the data cache; this information is stored in the converter.

All concurrently logged on users use the data cache jointly.

The data cache is dynamically dimensioned by the database system, i.e. you cannot configure the size of the data cache directly, but can only influence it implicitly by configuring the I/O buffer cache. The database system takes the pages required for the data cache from the I/O buffer cache. If the other I/O buffer cache user, the converter, grows in size, the database system decreases the size of the data cache, if necessary.

The size of the data cache has the following effect on the performance of the database system:

·        A large data cache makes a high hit rate possible. We recommend a hit rate of at least 98%.

·        If the data cache is too large for a small main memory, this can lead to operating system swapping with a very high level of I/O activity.

The pagers write changed data from the data cache to I/O queues. Later, special threads write the data asynchronously and in sufficiently large units onto the data area. If a user task cannot find any free pages in the data cache, then, as an exception, it writes the data from the data cache onto the data area itself.

See also:

Database Manager GUI, Displaying Cache Information

Database Manager CLI, Displaying Information About the Database Instance

Database Administration in CCMS: MaxDB, Structure linkCaches

Database Administration in CCMS: SAP liveCache Technology, Structure linkCaches

 

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