Background documentationHow Databases Store Data and Log Entries Locate this document in the navigation structure


Physical View

If a database user changes data in the database, the database system first stores the changed data in the data cache in the working memory and logs the changes in redo log entries in the log queue (also in the working memory).

More information: Working Memory Areas

In the following situations, the Pager tasks of the database system write the data changes from the data cache to the data area in permanent storage:

  • If the data cache is too full

    This depends on, among other things, the ratio between changed and read pages in the data cache.

  • At savepoints

In the following situations, the Log Writer task of the database system writes the redo log entries from a log queue to the log area:

  • If the log queue is full

  • If the transaction was completed with a COMMIT

  • At savepoints

Log pages in the log queue that were not full at the time of writing remain in the log queue. The system fills these log queues and writes them to the log volumes again during the next write process.

More information:

The unit that the database system uses to write information to and read it from the permanent storage is a page of 8 KB. A page corresponds to a certain number of operating system blocks. Pages are often the unit used to specify memory and disk space.

This graphic is explained in the accompanying text.

Working Memory and Permanent Storage

Logical View

The database system uses the following organizational structures to store data in the database:

  • Page chains

  • Pages in B* trees

More information: Logical Access Structures

This graphic is explained in the accompanying text.

Logical Access Structures

How Is Logical Information Assigned to Physical Pages?

The database system stores the assignment of logical data pages to physical addresses in the converter.

More information: Converter