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Oracle Block size tips

Oracle block size tips by Burleson Consulting

Update: Oracle is always changing, and for the latest consensus on using multiple blocksizes in Oracle, see the latest research on multiple blocksizes.

The Oracle Database Administrator's Reference 10g Release 2 (10.2) for UNIX-Based Operating Systems notes these guidelines for choosing blocksizes in AIX:

"Oracle recommends smaller Oracle Database block sizes (2 KB or 4 KB) for online transaction processing (OLTP) or mixed workload environments and larger block sizes (8 KB, 16 KB, or 32 KB) for decision support system (DSS) workload environments."

The Oracle 11.2 Database Performance Tuning Guide notes the advantages and disadvantages of different blocksizes:

Block Size Advantages

Smaller blocksize:

-  Good for small rows with lots of random access.
-  Reduces block contention.
Larger  blocksize:

-  Has lower overhead, so there is more room to store data.
-  Permits reading several rows into the buffer cache with a single I/O (depending on row size and block size).
- Good for sequential access or very large rows (such as LOB data).
Block Size Disadvantages

Smaller  blocksize:

- Has relatively large space overhead due to metadata (that is, block header).
- Not recommended for large rows. There might only be a few rows stored for each block, or worse, row chaining if a single row does not fit into a block.

Larger blocksize:

- Wastes space in the buffer cache, if you are doing random access to small rows and have a large block size. For example, with an 8 KB block size and 50 byte row size, you waste 7,950 bytes in the buffer cache when doing random access.
- Not good for index blocks used in an OLTP environment, because they increase block contention on the index leaf blocks.

WARNING:  Using multiple blocksizes effectively 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.

For large mission-critical Oracle databases, using multiple blocksizes can improve Oracle performance and manageability 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.

  • Reduced row chaining - Placing large object rows (BLOB, CLOB) into a tablespace space with a larger blocksize can greatly reduce row chaining and improve I/O.
  • 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 2K blocksizes, greatly reducing cache fusion overhead.

  • Less disk space waste - When using Oracle 11g advanced compression, testing shows that a 32k blocksize to maximize compression and minimize waste.
  • 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.
  • Minimize redo generation - Some experts recommend 2K blocksizes for bitmap indexes, to minimize redo generation during bitmap index rebuilds.
  • Faster scans - Tables and indexes that require full scans can see faster performance when placed in a large blocksize

There are many factors in choosing the best Oracle block size, and I have my notes on Oracle block size here:

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