Working Memory Areas

To prevent that the database system accesses the permanent storage (volumes) more often than necessary, read and write operations are buffered in the working memory.

The unit used for reading from and writing to the working memory is a page (8 KB).

The database system divides the working memory into the following areas:

I/O buffer cache, which mainly consists of:
- Data cache (largest part)
- Converter
- Internal file directory
Catalog cache
Shared SQL cache
Sequence cache
Log queues
For SAP liveCache databases only: OMS heap

Working Memory Areas
Working Memory Area	Database Parameters	Description
I/O Buffer Cache	`CacheMemorySize` More information: Database Administration, General Database Parameters	The database system uses the I/O buffer cache to manage the working memory that is available for I/O operations.
Data Cache	It is not possible to configure the size of the data cache and the converter directly; instead, you can configure them indirectly through the size of the I/O buffer cache.	The database system uses the data cache to store the pages that it has recently read or write-accessed in the data area. This includes, for example, tables, indexes, LOB values and undo log entries. The database system always searches for data in the data cache before accessing the data area. 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, on a computer with a small working memory, the data cache is configured too large, this can cause the system to swap pages from the working memory to the hard disk, which considerably reduces performance.
Converter Cache	It is not possible to configure the size of the data cache and the converter directly; instead, you can configure them indirectly through the size of the I/O buffer cache.	The converter is where the database system stores information about which logical page number is saved at what physical position (block address). When the database starts, the database system imports the complete converter into the working memory. If the converter grows while the database is running, and requires more pages, the database system gives it more pages from the I/O buffer cache. More information: Converter
Catalog Cache	`TaskSpecificCatalogCacheMinSize` More information: Database Administration, Special Database Parameters	The database system stores the following in the catalog cache: Information from the database catalog Information on SQL statements that have already been executed by individual database users (user-specific, only if the `SHAREDSQL` database parameter has the value `NO`). If more than one database user executes the same SQL statement, the system stores the statement in the catalog cache more than once. Once the catalog cache is full, the database system moves part of it to the data cache. The database system assigns a catalog cache area to each user task at the start of the database session; this catalog cache is then released after the database session has ended. We recommend a hit rate > 85 % for the catalog cache. The lower limit for the hit rate depends largely on the application.
Shared SQL Cache	`SharedSQLCommandCacheSize` More information: Database Administration, Special Database Parameters	If the `UseSharedSQL` database parameter has the value YES, the database system uses the shared SQL cache. The shared SQL cache is where the database system stores SQL statements that have already been executed, together with their execution plans. The shared SQL cache is shared by all database users, so the system saves the same SQL statement only once. Note that if the `UseSharedSQL` database parameter has the value `NO`, the database system saves the executed SQL statements for each database user individually in the catalog cache.
Sequence Cache	`SequenceCacheSize` More information: Database Administration, Special Database Parameters	The database system stores all information about sequences in the sequence cache.
Log Queues	`LogQueueSize` `LogQueues` More information: Database Administration, Special Database Parameters	The database system assigns each transaction a log queue, where the transaction writes its redo log entries. More than one transaction can write to the same log queue.
Working Memory for the Internal File Directory	-	When the database starts, the database system imports the complete internal file directory into the working memory. The database system stores information for logically accessing database objects, such as the assignments of the root pages of B* trees to table IDs, in the internal file directory.
OMS Heap (only for SAP liveCache Databases)	You can configure the maximum size of the OMS heap using the `OmsMaxHeapSize` SAP liveCache database parameter. More information: Database Administration, liveCache Database Parameters	The OMS heap is a specific working memory area only used by SAP liveCache databases. The OMS heap contains the following data: Local copies of OMS data When an object within a consistent view is accessed for the first time, the database system writes the corresponding data to the OMS heap. Local application data The database system copies the data of each OMS version to the OMS heap when it is read. To access a persistent object, the database system searches first in the OMS heap. If it does not find the object there, it searches in the data cache. If it does not find the object there either, the database system copies the object from the data area to the data cache, and from there to the OMS heap. The database system then makes all changes to the object in the OMS heap. Any objects that are read but not changed within a consistent view are deleted from the OMS heap by the database system. When a transaction ends (`COMMIT`), the database system writes the OMS data from the OMS heap to the data cache. If the application requires additional working memory, the OMS heap grows.

This graphic is explained in the accompanying text.

Working Memory Areas

Example: Working Memory Areas
Working Memory Area	Database Parameters	Maximum Size
I/O Buffer Cache	`CacheMemorySize` = 2500	19.53 MB
Catalog Cache	`TaskSpecificCatalogCacheMinSize` = 1632	12.75 MB
Shared SQL Cache	`SharedSQLCommandCacheSize` = 262144	256 MB (initial 16 MB)
Sequence Cache	`SequenceCacheSize` = 1	0.01 MB
Log Queues	`LogQueueSize` = 50 (size of a log queue) `LogQueues` = 1 (number of log queues)	0.4 MB

How Does the Database System Synchronize Access to Caches?

A stripe is a logical part of a cache (data cache or converter). To synchronize access of competing tasks, the database system uses special synchronization means such as regions and reader-writer locks. A stripe contains several critical sections.

You can override the access restrictions calculated by the system using the special database parameters DataCacheStripes and ConverterStripes.

Special Synchronization Mechanisms
Name	Used for	Description
Region	Data Cache	`EXCLUSIVE` When a task accesses a critical section, the associated region blocks this critical section for all other tasks.
Reader-writer lock (share lock)	Catalog Cache Shared SQL Cache	`EXCLUSIVE` or `SHARED` If a task write-accesses a critical section, the associated reader-writer lock (share lock) blocks this critical section for all other write-accesses but allows other read-accesses. If a task read-accesses a critical section, the associated reader-writer lock (share lock) blocks this critical section for all other write-accesses but allows other read-accesses.

More Information

Database Parameters

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