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Donald K. Burleson

Oracle Tips  

Oracle SGA Issues

As discussed previously, in UNIX, the parameters controlling shared memory usage are the limiting factor. In any case, before you create the database, serious thought has to be given to how much you expect the SGA to grow over the next year. Overspecifying the shared memory parameters on a UNIX platform will not do any harm and may save you and the system administrator some headaches.

The size of the SGA is controlled by buffer sizes, and the buffer sizes are controlled by the database block size, which is specified at database creation and cannot be changed without rebuilding the database in Oracle8i but can vary on Oracle. This usually defaults to 8 KB. I usually suggest at least 8 KB, although in some cases 16 KB works best. If in doubt, set the block size to the largest supported on your system.

The five major components of the SGA are the database buffers, log buffers, LARGE POOL, Java pool, and the shared pool. The SGA also contains the redo log buffers. The ideal situation would be to size the SGA to hold the entire database in memory. For small systems, this may be a real situation; for most, it is not feasible. However, with the new 64-bit architectures 16 exabytes of storage can be directly addressed; only a few exabytes can hold all of the world’s printed, videotaped, and recorded data, so in the future entire databases will reside easily in memory. Therefore, you must decide how much to allocate.

This is an excerpt by Mike Ault’s book “Oracle Administration & Management”.  If you want more current Oracle tips by Mike Ault, check out his new book “Mike Ault’s Oracle Internals Monitoring & Tuning Scripts” or Ault’s Oracle Scripts Download.




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