Oracle Tips by Burleson

10g Grid Computing with RAC
Preparing Shared Storage

Overview of the Files and Directories Required

Bottom line is that all the participating instances of an Oracle RAC database must have access to the shared data files, redo log files and control files. At the same time, the physical host level cluster consisting of physical nodes needs to have shared access to the quorum disk, voting disk and OCR file. If you are using the vendor provided cluster manager, you may still need the appropriate quorum disk as prescribed in the cluster manager software. The Oracle provided Cluster Ready Services (CRS) need voting disks and OCR files. There are other files, which may need to be shared. They include the files supporting the external tables. It is also recommended to have the archive log files destination on a shared file system.

The Following Diagram (Fig. 5.2) shows a collection of all the components that make up the RAC File structures. This diagram will provide us a road map for discussion as to how we can prepare these structures before we embark on creating the database and launch the database instances.

Figure 5.2 All the Physical File Structures of the RAC Database

Of the files that we have shown above, most of them need to be on shared storage. There are some files that can be on a local file system, but it is recommended that they are placed on a shared file system. There are some file structures, which can very well reside on a local file system.

Let’s look at these three combinations:

1.  The following files reside on Shared Storage and are accessible by all the RAC instances at the same time.

• Data Files, which make up the main data storage file structures
   

• Control Files
   

• Parameter File (SPFILE) - Maintains a common spfile located on shared file system that is accessible to all the instances of the RAC database.
   

• Password File that maintains the authentication privileges
   

• Voting Disk File and OCR files that are used by the cluster ready services.

2.  The following files reside on Shared Storage and are accessible by a specific instance. However, under certain conditions like recovery etc., the files need to be available for other instances also.

• Redo Log Files that record the instance specific transaction changes.
   

• Files supporting the Undo Tablespace, used by a specific instance to maintain the read consistency
   

• Archive Log Files that are the saved redo log files
   

• Files supporting the External Tables – External Tables are logical database objects that are accessible to all the instances.

3.  The following are file structures, which are created, accessed and managed by a specific instance.

• Alert Log File - keeps a running log of database changes and events.
   

• Trace Files that provide detailed information about the database events.
   

• Oracle Executables (Oracle Home Files).
   

• Oracle Cluster Ready Services (CRS) Home, which has all the binaries supporting the CRS.

However, note that with the use of the cluster file system it is possible to use a Shared Oracle Home and shared CRS Home. When using the cluster file system such as HP’s Tru64 CFS, Veritas CFS, Polyserve Matrix Server, you can maintain a single Oracle Home and share it among all the nodes of the cluster.

While talking about the shared file structures, there are four combinations as shown in the Fig. 5.3.

Figure 5.3 Shared Storage Inputs

• They can be on raw devices.
   

• They can be on a Cluster File System (CFS). CFSs concurrently mounted on all the participating nodes of the cluster.
   

• They can be accessed via the Automatic Storage Management (ASM) instance.
   

• They can be located on Network File System.

Even though we have listed three types of shared storage structures, it is also possible to mix them so that some shared files can be on raw devices, some can locate on CFS and some can come out of ASM.

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