The Hierarchy of Network Storage

Hierarchical Storage Management (HSM) provides on-line storage of frequently used files and near-line storage of other files. It automatically and transparently moves fi les to and from near-line storage as it extends the storage capacity of file servers. A person at a LAN workstation who accesses a migrated file incurs a slight delay lasting a few seconds to half a minute while the HSM software ``demigrates'' a file. HSM is particularly well suited for situations involving many large files (e.g., images of documents) when only a subset of those files needs to be on-line at any one time.

These products are for serious LANs. Installation and setup time is hefty, and two of the reviewed products require multiple file servers with multiple volumes on each server. You might even need to install additional RAM in your file servers; NetWare doesn't offer virtual memory management, relying on physical RAM to hold all the programs running on the server.

HSM promises near bottomless pits of storage for modern computer networks. It automatically migrates files between more expensive, higher-speed magnetic disk and less costly, but slower jukebox-based optical disc and magnetic tape units. Medium-speed, randomly erasable media, like rewriteable optical discs serve as a secondary level of HSM storage, holding files most likely to be needed by users. Lower-speed media that can't be erased randomly, like tape or write-once optical disc, support a tertiary level of HSM storage where little-used files reside.

HSM's goal is to store network data on the lowest-cost device that meets performance requirements. At the top of the hierarchy are hard disks, with their fast access times, high per-megabyte costs, and relatively low capacities. HSM products automate the process of migrating less active files down the hierarchy to larger, slower, less expensive media, such as optical disks and tape drives.

Peripheral Strategies has identified five levels of HSM that are widely accepted as guidelines by the HSM industry. Level 1 is simple automatic migration with trans parent retrieval. Level 2 adds real-time, dynamic load balancing of free disk space based on predefined thresholds. Level 2 also can manage two or more layers of near-line storage (e.g., an optical jukebox and magnetic tape library). Level 3 provides for the management of three or more layers of storage hierarchy and dynamically balances the consumption of available space in each layer. Level 4 HSM products can migrate files based on data type and other criteria, through the use of policies (rules). Products that conform to level 4 preserve ownership, attribute, and location information about files, thus allowing multiplatform (DOS, Macintosh, OS/2) HSM. Finally, level 5 identifies HSM products that can work with database manager software, such as DB2/2, NT Server, or Oracle, to migrate portions of a database (rather than an entire file) to and from secondary storage. There are, as yet, no level 5 HSM products; the most advanced of the three reviewed here implements level 4 features.

Migrating old or infrequently used files onto inexpensive media such as removable optical disks or tape not only frees up primary storage space for more current files but also reduces the average cost of storage. Thus, instead of expanding your LAN in an on-line fashion, you can use HSM to begin expanding it in a more controlled, near-line manner. HSM can also increase overall network performance by optimizing access times for the data you're most likely to need. In a complex HSM setup, you might have ultrafast cached hard drives layered above slower 10-GB single-spindle drives, which are layered on top of a 20- or 40-GB optical jukebox for near-line storage, in turn layered above a 192-GB tape library. Click here to view the HSM Overview