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Oracle blocksize tips

Oracle Database Tips by Donald Burleson

The benefits of multiple Oracle blocksize are most important for large OLTP databases and the benefits of multiple Oracle block sizes are very well documented.  Oracle blocksize I/O performance depends on optimizing your db_file_multiblock_read_count for your Oracle blocksize.

WARNING:  Using multiple blocksizes effectively is not simple.  It requires expert-level Oracle skills and an intimate knowledge of your I/O landscape.  While deploying multiple blocksizes can greatly reduce I/O and improve response time, it can also wreak havoc in the hands of inexperienced DBA's.  Using non-standard blocksizes is not recommended for beginners.

In general, different blocksizes can improve performance in a variety of ways:

  • Contention reduction - small rows in a large block perform worse under heavy DML than large rows in a small blocksize.
  • Faster updates - Heavy insert/update tables can see faster performance when segregated into another blocksize which is mapped to a small data buffer cache.  Smaller data buffer caches often see faster throughput performance.
  • Reduced Pinging - RAC can perform far faster with smaller blocksizes, reducing cache fusion overhead.
  • Less RAM waste - Moving random access small row tables to a smaller blocksize (with a corresponding small blocksize buffer) will reduce buffer waste and improve the chance that other data blocks will remain in the cache.
  • Faster scans - Tables and indexes that require full scans can see faster performance when placed in a large blocksize.

Here, Oracle expert Robin Schumacher proves that Oracle indexes build with less levels in a 32k Oracle blocksize and that a large blocksize will reduce physical I/O latency.  Here are details on setting the blocksize for index data files.

Properly implementing your Oracle blocksize will also reduce waste in the data buffer cache and allow multi-block reads to perform more efficiently. For example, a table where 500-byte rows are fetched in a random fashion will have less buffer waste when placed in a 2k blocksize, improving overall data caching.  Here are benchmark results to show the benefits of large Oracle blocksize and Oracle performance.

See these related notes on tuning the Oracle I/O subsystem:




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