Oracle db_cache_size

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The 64-bit Oracle database now allow for far larger SGA regions. Sadly a 32-bit word size can only address (2 to the 32nd power), or about 4 gigabytes of RAM. All 64-bit servers have a larger word size (2 to the 64th power) that allows for up to 18 billion gigabytes (18 exabytes).

Many Oracle DBAs are running SGAs larger than 30 gigabytes, most of it dedicated to the data buffer caches such as db_cache_size and use the v$db_cache_advice view to tune their db_cache_size to cache the working set of frequently used objects. 

Also note that the Oracle 10g dba_hist_db_cache_advice tables in AWR now all Oracle 10g Automatic memory management to automatically adjust the db_cache_size.

It is important to remember that there are downsides to having a super-large db_cache_size. While direct access to data is done with hashing, there are times when the database must examine all of the blocks in the RAM cache.

• Systems with high Invalidations - Whenever a program issues a truncate table, uses temporary tables or runs a large data purge, Oracle must sweep all of the blocks in the db_cache_size to remove dirty blocks. This can cause excessive overhead for system with a db_cache_size greater than 10 gigabytes.
   
• High Update Systems - The database writer (DBWR) process must sweep all of the blocks in db_cache_size when performing an asynchronous write. Having a huge db_cache_size can cause excessive work and most DBA's segregate high-update and truncate objects into a separate, smaller data buffer such as db_2k_cache_size.
   
• RAC systems - Systems using Oracle9i RAC may experience high cross-instance call when using a large db_cache_size in multiple RAC instances. This inter-instance ?pinging? can cause excessive overhead, and that is why RAC DBA's try to segregate RAC instances to access specific areas of the database.

If your system has any of these characteristics, you will need to perform special operations to reduce the stress on the db_cache_size.  Click here for details and scripts for automatically sizing your db_cache_size:

Advanced Oracle Monitoring and Tuning Script Collection