DBA Tips Archive for Oracle


Why does RAC require raw devices?

by Jeff Hunter, Sr. Database Administrator


  1. Overview
  2. OS Vendors and Cluster Aware File Systems


Oracle requires raw devices for various reasons. Take for example, Microsoft and its family of operating systems over the years. Nodes cannot mount the NTFS or FAT file systems on more than one node of a cluster.

In Unix, the operating system uses a buffer cache. When a process such as the DBWR issues a unix write system call, the OS does not actually write to the storage system. It buffers the blocks in the file system buffer cache, occasionally synchronizing the storage system. In an environment when processes on multiple nodes of a cluster need to be able to write and read database blocks at will, the Oracle server assumes that when a node has issued a write that it has actually happened, not that it is in a buffer waiting to happen. This is usually known as direct IO.

OS Vendors and Cluster Aware File Systems

Some OS vendors offer certified cluster aware file systems.

For a complete article on this issue, see MetaLink Doc ID: Note:183408.1, "Raw Devices and Cluster Filesystems With Real Application Clusters".

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Last modified on
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