About hierarchical storage

The Hierarchical Storage Management system (hereinafter referred to as the "HSM system") allows data to be relocated between fast local drives and slow long-term mass storage devices. Despite the obvious advantages of fast mass storage devices, they tend to be too expensive for most organizations. HSM systems transfer unused data to inexpensive remote mass storage devices thus minimizing corporate expenses.

HSM systems store some data in remote storage areas and restore the information if necessary. HSM systems constantly monitor file accesses to detect which files can safely be moved to remote storage and which should be stored locally. Files are relocated to remote storage if no access attempts are made for a specified time period. If a user accesses a file stored remotely, the file is transferred back to the local drive. This approach ensures that users can quickly access a volume of data considerably larger than available local disk space.

While moving a file from a local drive to remote storage, the HSM system saves a reference to the actual location of the file. Whenever the corresponding file is accessed, the system determines its location on the backup device. Replacing files with references to their remote storage locations makes it possible to create storage areas of practically unlimited size.

Some HSM systems support local storage of part of a file. In this case, a large part of file is transferred to remote storage while local storage retains just a small part of the original file.

HSM systems use two methods to access the data in hierarchical storage:

• Reparse points.
• Extended file attributes.