From: Hierarchical Storage Management

HSM systems transparently migrate files from disk to optical disk 
and/or magnetic tape, usually robotically accessible.  Then when files are 
accessed by a user, they transparently move them back to disk.

Watch for maximum file size limitations, sometimes limited by the 
size of the media used, sometimes by the server's OS, and sometimes
neither.

Some offer integrated backup. Some will manage multiple copies of
files for data reliability.

Some offer integrated migration from other systems (ie, file servers 
and/or workstations) to the central location disks, then to the central 
location robotics. This generally requires changes to the on-disk file
system format on the migration clients.

An item to watch for is that the file management may be exactly like
Unix -- that is, all files appear to be online, and once they're
deleted, they're gone forever, even though the data may still be on
tape.

All of the subsections here are Unix-compatible (in various flavors)
unless indicated otherwise.

Additional Information:

See also _DEC Professional_, February 1993, Page 40 and _Client/Server
Today_, Dec. '94, p. 60.

The System-Managed Storage Guide by Howard W. Miller, $225 for first
copy, $75 for additional copies for same company available from The
Information Technology Institute, 136 Orchard Street, Byfield,
Massachusetts, 01922-1605.

        (stith@fnal.gov)

Thomas Woodrow did an evaluation of NAStore, FileServ, DMF and Unitree
in 1993. It can be obtained through
http://www.nas.nasa.gov/NAS/TechReports/RNDreports/RND-93-014/RND-93-014.html
or the Proc. 3rd NASA Goddard Conference on Mass Storage Systems and
Technologies, Oct. 1993, pp. 187--216. Somewhat dated now but
excellent methodology for comparing HSMs.