Managing multi-block Oracle reads

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: 

For complete details, see my master book "Oracle Tuning: The Definitive Reference":

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:

- AIX - Look for a "dio" option.

- 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).

Next, you need to set db_file_multiblock_read_count: 


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