Question - How Large Is It Preferred To Configure The Io Buffer Cache?
Answer -
The database parameter CACHE_SIZE or (as of Version 7.7.03) the database parameter CacheMemorySize should be used to configure the IO buffer cache.
The converter cache and the data cache of an SAP MaxDB database are included in the IO buffer cache.
The database performance is greatly influenced by the size of the IO buffer cache. The larger the dimensions of the IO buffer cache, the fewer time-consuming hard disk accesses have to be executed.
The data volume that is to be processed in day-to-day business and the application determine the size of the IO buffer cache that is to be set.
It is generally recommended to process all the data in a system that is up and running, without accessing the hard disk which is in the data cache. However it is often not possible in the BI environment and OLTP environment.
It should be possible to hold all data that is to be processed in the IO buffer cache, while using the SAP liveCache technology. Generally, the results of the Quick Sizer should be used to configure the IO buffer cache for SAP liveCache technology.
The following applies to the IO buffer cache: the larger, the better (provided that sufficient physical memory is available).
It should be noted that a heap memory is also allocated by the database in addition to the IO buffer cache. The overall memory consumption of an SAP MaxDB database can be determined using the information from the system table MEMORYALLOKATORSTATISTICS.
The number of successful and failed accesses to the data cache determines the data cache hit ratio. The hit ratio indicates whether the size of the data cache is well configured. However, if exceptional applications are running at the time of the analysis then the data cache hit ratio does not provide sufficient information. For example, during year-end closing the hit ratio may deteriorate because this data does not have to be held in the cache permanently. After directly restarting the database, it is not indicated by the data cache hit ratio that whether the system is well configured, because all data must first be loaded into the cache.
The settings for the size of the IO buffer cache which has been tried and tested by many OLTP customers and BW customers for SAP systems are as follows:
OLTP NON-UNICODE: 1% of the data volume
OLTP UNICODE: 2% of the data volume
BW NON-UNICODE: 2% of the data volume
BW UNICODE: 4% of the data volume