Call now: 252-767-6166  
Oracle Training Oracle Support Development Oracle Apps

 E-mail Us
 Oracle Articles
New Oracle Articles

 Oracle Training
 Oracle Tips

 Oracle Forum
 Class Catalog

 Remote DBA
 Oracle Tuning
 Emergency 911
 RAC Support
 Apps Support
 Oracle Support

 SQL Tuning

 Oracle UNIX
 Oracle Linux
 Remote s
 Remote plans
 Application Server

 Oracle Forms
 Oracle Portal
 App Upgrades
 SQL Server
 Oracle Concepts
 Software Support

 Remote S


 Consulting Staff
 Consulting Prices
 Help Wanted!


 Oracle Posters
 Oracle Books

 Oracle Scripts

Don Burleson Blog 







Oracle server Internal page cache vs. JFS cache

Oracle Database Tips by Donald BurlesonApril 25, 2015

Question:  What is the difference between the UNIX internal page cache and the UNIX Journaled Filesystem (JFS) cache?  Under what circumstances do I turn-off each cache for Oracle databases?  I was under the impression that "raw" I/O bypasses the JFS, while direct I/O bypasses the internal page cache?
Answer:  UNIX has the problem of duplicate caches, with caching at the disk layer, server layer and again at the Oracle database layer.  Basically UNIX/Linux will take unused memory and use it to buffer reads and direct I/O bypasses the internal page cache.

See these important notes on Oracle physical file caching.  Note that Oracle AIX I/O is different because Concurrent I/O (CIO) is not available on JFS.

The basic difference between JFS and the internal filesystem page cache is that the Journaled file system (JFS) is for caching writes, while the internal filesystem cache caches incoming data blocks from disk at the OS layer.

Internal file cache (direct I/O):  At disk read time, UNIX/Linux moves the requested data block into the internal file cache, and again transfers the file into the Oracle SGA.  This double-caching can be alleviated by using Oracle with direct I/O.

JFS file cache (raw I/O): The JFS cache is only used for updates, and the Oracle redo logs should be on a JFS filesystem.  Traditionally, moving to a "raw" device will bypass the JFS buffer and only "external" updated files such as archived redo logs should always be on JFS because of the protection for writes.

Col. Garmany notes:

"There is no performance penalty for using a JFS and JFS only makes small writes a little bit faster.  Only UNIX filesystems support JFS, and raw devices are outside of JFS.  On Linux, ext2 is not a JSF and ext3 is JSF and you can easily switch between ext2 and ext3 without moving files.  Only a test will show if raw devices are faster than JFS, and many current file systems are close to the performance of raw.  I would recommend using JFS unless you have a high end array that provides a protected cache for writes."

Get the Complete
Oracle SQL Tuning Information 

The landmark book "Advanced Oracle SQL Tuning  The Definitive Reference"  is filled with valuable information on Oracle SQL Tuning. This book includes scripts and tools to hypercharge Oracle 11g performance and you can buy it for 30% off directly from the publisher.



Burleson is the American Team

Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

Verify experience! Anyone considering using the services of an Oracle support expert should independently investigate their credentials and experience, and not rely on advertisements and self-proclaimed expertise. All legitimate Oracle experts publish their Oracle qualifications.

Errata?  Oracle technology is changing and we strive to update our BC Oracle support information.  If you find an error or have a suggestion for improving our content, we would appreciate your feedback.  Just  e-mail:  

and include the URL for the page.


Burleson Consulting

The Oracle of Database Support

Oracle Performance Tuning

Remote DBA Services


Copyright © 1996 -  2020

All rights reserved by Burleson

Oracle ® is the registered trademark of Oracle Corporation.



Oracle Training at Sea
oracle dba poster

Follow us on Twitter 
Oracle performance tuning software 
Oracle Linux poster