19.2 Ways to Create Database File System Links

Database File System Links require the creation of a Database File System through the use of the DBFS Content package, DBMS_DBFS_CONTENT.

Oracle provides several methods for creating a DBFS Link:

• Move SecureFiles LOB data into a specified DBFS pathname and store the reference to the new location in the LOB.

  Call DBMS_LOB.MOVE_TO_DBFS_LINK()with LOB and DBFS path name arguments, and the system creates the specified DBFS HSM Store if it does not exist, copies data from the SecureFiles LOB into the specified DBFS HSM Store, removes data from the SecureFiles LOB, and stores the file path name for subsequent access through this LOB.

• Copy or create a reference to an existing file.

  Call DBMS_LOB.COPY_DBFS_LINK() to copy a link from an existing DBFS Link. If there is any data in the destination SecureFiles LOB, the system removes this data and stores a copy of the reference to the link in the destination SecureFiles LOB.

• Call DBMS_LOB.SET_DBFS_LINK(), which assumes that the data for the link is stored in the specified DBFS path name.

  The system removes data in the specified SecureFiles LOB and stores the link to the DBFS path name.

Creating a DBFS Link impacts which operations may be performed and how. Any DBMS_LOB operations that modify the contents of a LOB will throw an exception if the underlying LOB has been moved into a DBFS Link. The application must explicitly replace the DBFS Link with a LOB by calling DBMS_LOB.COPY_FROM_LINK() before making these calls.

When it is completed, the application can move the updated LOB back to DBFS using DBMS_LOB.MOVE_TO_DBFS_LINK(), if needed. Other DBMS_LOB operations that existed before Oracle Database 11g Release 2 work transparently if the DBFS Link is in a file system that supports streaming. Note that these operations fail if streaming is either not supported or disabled.

If the DBFS Link file is modified through DBFS interfaces directly, the change is reflected in subsequent reads of the SecureFiles LOB. If the file is deleted through DBFS interfaces, then an exception occurs on subsequent reads.

For the database, it is also possible that a DBA may not want to store all of the data stored in a SecureFiles LOB HSM during export and import. Oracle has the ability to export and import only the Database File System Links. The links are fully qualified identifiers that provide access to the stored data, when entered into a SecureFiles LOB or registered on a SecureFiles LOB in a different database. This ability to export and import a link is similar to the common file system functionality of symbolic links.

The newly imported link is only available as long as the source, the stored data, is available, or until the first retrieval occurs on the imported system. The application is responsible for stored data retention. If the application system removes data from the store that still has a reference to it, the database throws an exception when the referencing SecureFiles LOB(s) attempt to access the data. Oracle also supports continuing to keep the data in the database after migration out to a DBFS store as a cached copy. It is up to the application to purge these copies in compliance with its retention policies.