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Context Dependent Symbolic Link (CDSL)

Oracle RAC Cluster Tips by Burleson Consulting

This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters.  To get immediate access to the code depot of working RAC scripts, buy it directly from the publisher and save more than 30%.

Since Tru64 CFS provides a single namespace, there is a single copy of most configuration files. With a few exceptions, the directory structure of a cluster is identical to that of a stand-alone system. However, wherever files are not to be shared (like /etc/passwd, /etcsysconfigtab), the context dependent symbolic link (CDSL) manages the copies for cluster members.

Node-specific Oracle files like tnsnames.ora, listener.ora, and sqlnet.ora can also be created using the CDSL system. CDSL(s) contain a variable whose value is determined only during pathname resolution. The {memb} variable is used to access member-specific files in a cluster.

The following example shows the CDSL for /etc/rc.config:

/etc/rc.config -> ../cluster/members/{memb} /etc/rc.config

When resolving a CDSL pathname, the kernel replaces the {memb} variable with the string membern, where n is the member ID of the current member.

CDSL(s) are useful when running multiple instances of an application on different cluster members, where each member operates on a different set of data. The mkcdsl command lets system administrators create CDSL(s) and update a CDSL inventory file. The cdslinvchk command verifies the current CDSL inventory.

Oracle Cluster File System (OCFS)

Oracle Cluster File System (OCFS) is a shared file system designed specifically for Oracle Real Application Clusters. OCFS eliminates the requirement for Oracle database files to be located on the raw devices in Linux RAC cluster and Windows RAC cluster.

OCFS is released under the GNU General Public License (GPL). The source code for the binaries shipped with the RPM's are directly available in the source RPMS (.src.rpm) for reference and compliance.  Oracle will only formally support the binary RPM's compiled by Oracle and downloadable for Red Hat Advanced Server or United Linux. Oracle provides OCFS product support to customers that already have an Oracle support license.

For Linux and Windows

With active support from open source community, Oracle has developed the cluster file system (OCFS) for use in Red Hat Linux and United Linux, and released it as an open source. It is available in the form of RPM packages. Users can download it for free. OCFS only supports Oracle data files, redo log files, and control files. It is not a general-purpose file system. It does not support a shared Oracle Home at least not until Version 2 is released.

OCFS requires the use of the operating system?s o_direct compile-time flag to get direct I/O. I/O must be performed through aligned buffers with 512 byte (or multiples thereof) buffer offsets. These are details the Oracle server takes care of. The Oracle RAC database manages all the difficult concurrency issues and maintains the data integrity of the application just as it does when using raw partitions.

The OCFS version for Windows NT/2000 does support a shared Oracle Home.

OCFS2 is the latest version of the Oracle Cluster File System software. While OCFS Version 1 was designed specifically for Oracle Database files, OCFS Ver 2 supports a shared ORACLE_HOME installation also.

New features of the OCFS Version 2 include:


* Improved performance of metadata operations (space allocation, locking, etc).

* Improved metadata caching.

* Improved data caching (for files such as oracle binaries, libraries, etc)

* Network based DLM is used by default.

* Improved journaling / node recovery - we now use the Linux Kernel JBD subsystem

* Keep the same performance for Oracle data files as OCFS1.

* CDSL for node specific files

PolyServe Matrix Server (MxS)

MxS is a general-purpose cluster file system that runs on a Linux or Windows cluster. It is used for Oracle RAC and other HA solutions. MxS supports all Oracle data files and redo logs. It further supports advanced Oracle functionality including external tables, export/import from text files, and archived redo log compression. MxS also supports a shared Oracle Home, which simplifies installation, configuration, and maintenance.

Other CFS solutions:

Network Appliance Filers offers CFS-like functionality via NFS to the server machines. These file systems are mounted using special mount options.

IBM's General Parallel File System (GPFS) allows users shared access to files that may span multiple disk drives on multiple nodes. GPFS provides access to all data from all nodes of the cluster. GPFS is only supported with HACMP/ES in a RAC configuration.


This is an excerpt from the bestselling book Oracle Grid & Real Application Clusters, Rampant TechPress, by Mike Ault and Madhu Tumma.

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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