This article is from the storage FAQ part2, by Rodney D. Van Meter with numerous contributions by others.
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. (firstname.lastname@example.org) 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.