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by Burleson Consulting

What about BOB, Oracle 64 bit SGA?

The Oracle 64 bit option has two enhanced features, big Oracle blocks (BOB), and Large SGA (LSGA). BOB is probably one of the most important features of Oracle on 64-bit machines. Because block sizes can reach 32k an entire track of data can be read with a single I/O, and system performance improves dramatically. This is especially important in data warehouses that scan large ranges of tables. When used with the init.ora parameter db_multiblock_read_count, physical I/O for table scan operations will be greatly reduced. BOB is also very useful if you are storing non-tabular data in your Oracle database, as is the case with Oracle's spatial data option. In these cases, binary large objects (BLOBs) such as images can be accessed with a single physical disk I/O.

LSGA is something that is even more important to the future of Oracle data warehouse systems. As the cost of RAM memory falls we will be able to see huge Oracle SGAs, some of which will be able to cache an entire database. In some 64-bit Oracle warehouses, large tables are read into Oracle data buffer using the table CACHE option. For example, assume that we have a 10 gigabyte data warehouse with 32K blocksizes and db_block_buffers parameter set to 10,000, for a total buffer size of 320 megabytes. Here, we have an SGA large enough to hold the entire database, which can be read at startup where it will remain for the entire processing day without any disk I/O (assuming, of course, that we are in query mode). Since accessing RAM takes 50 nanoseconds when compared to a disk I/O that takes 50 milliseconds, we see that I/O will proceed one million times faster in a system that does not perform disk I/O.

Sixty-four bit architectures also remove many of the traditional barriers of UNIX systems. File sizes can now exceed 2 gigabytes, SGA regions can reach up to 14 gigabytes, and database block sizes can now be made 32K, the same as Oracle's mainframe cousins.

This is an excerpt from "High Performance Data Warehousing". To learn more about Oracle, try "Oracle Tuning: The Definitive Reference", by Donald K. Burleson.  You can buy it direct from the publisher at 30% off here:


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