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 Oracle Files for Loading into External Tables
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Files for Loading into External Tables

External tables allow direct access of data located in the operating system level files by using the SQL interface within the database. It is a way of reading and writing files into and out of the database. Note that one cannot write to an existing external table.  Data stored in operating system level files (ASCII filer) can be accessed as if they are a table with rows and columns. Joins and views can be constructed with data in the O/S file and logical database tables.


For all practical purposes, external tables act the same as the usual tables; however, the data is not stored with the Oracle data files. External tables are a great way to load the data into a database and do data processing. Inserts, updates, and index creation are not allowed on an external table.  A DROP TABLE statement issued against an external table removes the table metadata only; the operating system file is not removed.


There is no restriction as to where the external table data file has to be located. In a RAC database system, it can be located on the local file system or on shared disk. For the sake of concurrent access, it becomes more meaningful to keep the external table file on a shared storage. This allows for transparent access to the external table so that any instance in the RAC database should be able to read it at the operating system level (ASCII). This is possible only if the external table file is located on a shared disk.

Oracle Cluster Registry (OCR)

The OCR contains cluster and database configuration information for RAC and Cluster Ready Services (CRS) such as the cluster node list, instance to node mapping, and CRS application resource profiles.


The OCR is a shared file located in a cluster file system. When not using a cluster file system, the OCR file can be located on a shared raw device in UNIX-based systems or a shared logical partition in Windows environments. If more than one database is created on the cluster, they all share the same Oracle cluster registry.

Voting Disk

The voting disk file must reside on shared storage.  The voting disk file manages information for node membership.  Oracle recommends using multiple voting disk files.

ORACLE_HOME Files (Oracle Binaries)

Typically, every instance in Oracle 11g RAC will have its own ORACLE_HOME and a set of exclusive binaries. However, the Oracle binaries are located either on a local file system or on a clustered file system. Locating the Oracle home (binaries) on a clustered file system provides easier management by keeping a single copy of Oracle home supporting all the instances.


A common Oracle home for multiple instances has some advantages because it helps to easily expand the nodes and shrink the nodes as needed. It helps the dynamic addition and expansion of nodes without bothering with a fresh install of the Oracle binaries for the new instance. This feature is useful for large clusters, and it fits into the grid strategy of easier addition and reduction of computing resources.

UNDO Tablespace Files

UNDO tablespaces are special tablespaces that have system undo segments. They contain before images of blocks involved in uncommitted transactions. As such, they are the primary support structure allowing a transaction to rollback if the decision is made to not commit the transaction.  Tables and indexes cannot be created in the undo tablespace.  A database can have more than one undo tablespace, but only one can be used at one time. Automatic undo management is the default mode for an 11g database.  DBCA will automatically create an undo tablespace named UNDOTBS1.


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