Oracle Linux: File System's Buffer Cache versus Direct I/O
(Doc ID 462072.1)
Last updated on OCTOBER 13, 2022
Applies to:
Linux OS - Version Oracle Linux 4.4 and laterOracle Database - Enterprise Edition - Version 9.2.0.1 to 11.1.0.6 [Release 9.2 to 11.1]
Oracle Database Cloud Schema Service - Version N/A and later
Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Linux x86-64
Linux x86
Linux Kernel - Version: 2.4.9 to 2.6.18-128.1.10.0.1
Oracle Server Enterprise Edition - Version: 9.2.0.1 to 11.1.0.6
Purpose
The the File System's Buffer Cache & the Direct I/O are not much commonly and comprehensively understood concepts. This article aims to clarify the difference among these concepts and options.
Scope
This article will introduce the File System's Buffer Cache and the Direct I/O concept on Linux platform and it will not discuss about the Asynchronous I/O which is a totally different topic. For that topic you may check the following:
- <Note 432854.1> Asynchronous I/O Support on OCFS/OCFS2 and Related Settings: filesystemio_options, disk_asynch_io
- <Note 225751.1> Asynchronous I/O (aio) on RedHat Advanced Server 2.1 and RedHat Enterprise Linux 3
Details
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In this Document
| Purpose |
| Scope |
| Details |
| 1. Introduction to the File System's Buffer Cache |
| 2. Introduction to the Direct I/O capability |
| 3. File System's Buffer Cache versus Direct I/O |
| 4. Enabling and Using Direct I/O |
| 5. Measuring the Direct IO Advantages & Disadvantages |
| Example of Direct I/O is causing Full Table Scans to Run Slower |
| References |