Using Segmented Files

The SAM-QFS environment supports segmented files. Segmenting files improves tape storage retrieval speed, access, and manageability for very large files. A segmented file can be larger than the physical disk cache. In this case, only part of a segmented file resides on the disk cache at any one time.

The segment command enables you to specify the segment size. You cannot set a segment size that is smaller than the current file size.

Segmented files support tape striping. After a file is segmented, it can be striped simultaneously over multiple tape devices, which significantly reduces the time needed to store the file segments. Data access is accelerated by allowing users to retrieve only the desired file segments rather than the entire file.

Segmentation can enhance archiving efficiency because only changed portions of a file are re-archived. Segments of a file can be archived in parallel, and segmented files can be staged in parallel. This increases performance during archiving and retrieving.

Segmentation can be enabled on a file, directory, or entire file system. Segmented files support all other SAM-QFS capabilities.

The following sections describe how segmented files differ from nonsegmented files. For more information about segmented files, see segment(1) in Sun QFS and Sun Storage Archive Manager Reference Manual or the sam_segment(3) in Sun QFS and Sun Storage Archive Manager Reference Manual.

Archiving Segmented File

For a segmented file, the archivable unit is the segment itself, not the file. All archiving properties and priorities apply to the individual segment and not to the file.

You can stripe a segment by specifying both the - drives and -drivemin parameters for the archive set in the archiver.cmd file. For example, assume that a 100-megabyte segmented file in the file system has segment size of 10 megabytes. If the archiver.cmd file defines an archive set with a -drives 2 directive, this file is archived to two drives in parallel. Segments 1, 3, 5, 7, and 9 are archived using the first drive, and segments 2, 4, 6, 8, and 10 are archived using the second drive.

Only segments that have been modified are archived. Up to four archive copies can be made for each segment. SAM-QFS also supports volume overflow for segments.