Oracle RAC block size & cache size Configuration tips Oracle Tips by Burleson Consulting January 27, 2004 - Updated January 22, 2008

All Oracle DEFAULT caches and block sizes in a RAC environment (as defined by db_block_size) must be identical, but there are other important differences between the configuration of a RAC database and a single-instance Oracle database, especially when using grid server blades.

Beginning with Oracle9i RAC Oracle allowed sub-caches to be configured that have different blocksizes than the default db_block_size, allowing db_2K_block_size, db_4k_block_size, db_8k_block_size, db_16k_block_size and db_32k_block_size (not available on all platforms) as sub-caches.

 For example, if your default db_block_size is 4K, you could define a db_2k_cache_size to hold additional objects.  However, all RAC nodes must have the sub-cache defined.

We must remember that the rules for caching in a RAC instance are very different than for a standard Oracle system.  For example, numerous studies have shown that Oracle indexes often benefit from using the largest supported blocksize.  This is not always true in RAC systems.

With Oracle system now employing RAM-SAN instead of disk, and systems having giant db_cache_size regions, it is tempting to configure RAC with large data buffers.  However, we must remember that multi-instance Oracle is very different from a single-instance system:

• Blade clusters have small buffer caches – Most blade servers have only 2-gig or 4-gig of RAM.  Hence, each RAC node is limited in the available db_cache_size.
   

• RAC likes small blocksizes – Because of the inter-instances block transfer via Cache Fusion, smaller block sizes minimize pinging of blocks between instances.  Most RAC DBA’s will define the default db_block_buffers to 2k and then add a 4k buffer ton isolate data objects.
   

• RAC scales by the sum of RAM caches – Unlike a single Oracle database with a 32 gigabyte RAM data cache, RAC system achieve high caching by the sum of the individual RAM caches.  Hence, a 32 node RAC cluster with a 2 gigabyte RAM cache on each node would effectively have a total cache of 128 gigabytes.

See my related notes on RAC caching and block sizes:

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