21.1 About Database File System Links

DBFS Links allows storing SecureFiles LOBs transparently in a location separate from the segment where the LOB is normally stored. Instead, you store a link to the LOB in the segment.

The link in the segment must reference a path that uses DBFS Content API to locate the LOB when accessed. This means that the LOB could be stored on another file system, on a tape system, in the cloud, or any other location that can be accessed using DBFS Content API.

When a user or application tries to access a SecureFiles LOB that has been stored outside the segment using a DBFS Link, the behavior can vary depending on the attempted operation and the characteristics of the DBFS store that holds the LOB:

• Read:

  If the LOB is not already cached in a local area in the database, then it can be read directly from the DBFS content store that holds it, if the content store allows streaming access based on the setting of the PROPNAME_STREAMABLE parameter. If the content store does not allow streaming access, then the entire LOB will first be read into a local area in the database, where it will be stored for a period of time for future access.

• Write:

  If the LOB is not already cached in a local area in the database, then it will first be read into the database, modified as needed, and then written back to the DBFS content store defined in the DBFS Link for the LOB in question.

• Delete:

  When a SecureFiles LOB that is stored through a DBFS Link is deleted, the DBFS Link is deleted from the table, but the LOB itself is NOT deleted from the DBFS content store. Or it is more complex, based on the characteristics/settings, of the DBFS content store in question.

DBFS Links enable the use of SecureFiles LOBs to implement Hierarchical Storage Management (HSM) in conjunction with the DBFS Hierarchical Store (DBFS HS). HSM is a process by which the database moves rarely used or unused data from faster, more expensive, and smaller storage to slower, cheaper, and higher capacity storage.

Figure 21-1 Database File System Link

Description of "Figure 21-1 Database File System Link"