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Tuning multi-block I/O in Oracle

Oracle Tips by Burleson Consulting

Multi-block reads (index range scans, full-table scans, index fast-full scans) are very common in Oracle, and the DBA must understand how to tune their disk I/O subsystem for multi-block reads. 

The first step is ensuring that your Oracle database is properly configured for direct I/O.


10gr2 Note:  Starting in Oracle 10g release2, Oracle recommends not setting the db_file_multiblock_read_count parameter, allowing Oracle to empirically determine the optimal setting.  For more details, see my notes on 10gR2 automatically tuned multi-block reads.


Direct I/O and Async I/O note:  While most Oracle shops employ direct I/O, there can be compatibility issues when using direct I/O with a server that also supports async I/O.  Kevin Closson has many great notes on this issue.



Methods for configuring the OS will vary depending on the operating system and file system in use.  Here are some examples of quick checks that anyone can perform to ensure that you are using direct I/O:

?         Solaris - Look for a "forcedirectio" option.  Oracle DBAs claim this option makes a huge difference in I/O speed for Sun servers.  Glen Faucett also notes tips for setting direct I/O on Sun Solaris Oracle servers using filesystemio_options=setall and forcedirectio:  Here is the Sun documentation:  

?         AIX - Look for a "dio" option.  Here is a great link for AIX direct I/O:  

?         Veritas VxFS - (including HP-UX, Solaris and AIX), look for "convosync=direct".  It is also possible to enable direct I/O on a   per-file basis using Veritas QIO; refer to the "qiostat" command and corresponding man page for hints.  For HPUX, see Oracle on HP-UX ? Best Practices.   

?         Linux - Linux systems support direct I/O on a per-filehandle basis (which is much more flexible), and I believe Oracle enables this feature automatically.  Someone should verify at what release Oracle started to support this feature (it is called O_DIRECT). See Kernel Asynchronous I/O (AIO) Support for Linux  and this great OTN article: Talking Linux: OCFS Update.

Next, you need to set db_file_multiblock_read_count:


Remember, the parameter db_file_multiblock_read_count is only applicable for tables/indexes that are full scanned, but it also effects the SQL optimizer in its calculation of the cost of a full-table scan.

According to Oracle, this is the formula for setting db_file_multiblock_read_count:


      max I/O chunk size
     ------------------------- = db_file_multiblock_read_count


But how do we know the value of the max I/O chunk size? 

The maximum effective setting for db_file_multiblock_read_count is OS and disk dependant. Steve Adams, an independent Oracle performance consultant (see ), has published a helpful script to assist you in setting an appropriate level.  This script conducts a test and sample actual I/O chunk sizes on your server to aid you in setting db_file_multiblock_read_count:


If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.



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