Your machine may have more real memory than the maximum amount of addressable memory (for example, addressable memory is usually between 2 GB and 4 GB on most platforms). You can configure any additional memory beyond addressable memory as an extended storage cache. Such an extended storage cache can be used by any of the defined buffer pools and should improve the performance of the database manager. The extended storage cache is defined in terms of memory segments.
DB2 makes use of addressable memory in your machine with buffer pools (see "Managing the Database Buffer Pool"). The extended storage cache is used by the buffer pools as a secondary level of caching (with the buffer pools performing the first level of caching). Ideally buffer pools can hold the data that is most frequently accessed, while the extended storage cache can hold data that is accessed, but less frequently.
The following database configuration parameters influence the amount and the size of the memory available for extended storage:
Because an extended storage cache is an extension to a buffer pool, it must always be associated with one or more specific buffer pools. Therefore, you must declare which buffer pools can take advantage of a cache once it is created. The CREATE and ALTER BUFFERPOOL statements have the attributes NOT EXTENDED STORAGE and EXTENDED STORAGE that control cache usage. By default neither IBMDEFAULTBP nor any newly created buffer pool will use extended storage.
The database manager cannot directly manipulate data that resides in the extended storage cache. However, it can transfer data from the extended storage cache to the buffer pool much faster than from disk storage.
When a row of data is needed from a page in an extended storage cache, the entire page is read into the corresponding buffer pool. If the row is changed while in the buffer pool, the page is not written back to the extended storage cache until it has been written to disk storage. The extended storage cache holds only pages that have been read into the buffer pool and have been discarded; they are kept in case they are needed again.
A buffer pool and its associated extended storage cache, if defined, are created when a database is activated or first connected to.
| Note: | Use of the extended storage cache is not allowed if objects using an 8K page size exist in the database. |