Storage Management (HSM)
Although “storage” is generally thought
of in terms of memory, disk, tape, etc., there is a more general concept
of storage which has nothing to do with constraints imposed by physical
media. Storage in this broad sense, simply holds objects. Mass storage
systems based on the idea of a storage hierarchy are an implementation
of this general concept. The figure ‘Characteristics of Network Magnetic
Disk File System’ shows usage patterns of files in a network. It is seen
clearly that some files are not used for long periods and proportionately
few files are accessed frequently. This type of usage suggests the storing
of files in a storage hierarchy, such that the frequently used files are
stored on fast-access (expensive) media and the infrequently used files
are stored on slow-access (cheap) media. The usual hierarchy is disk (expensive),
tape library (cheap) and shelf storage (cheaper).
the last few years, the cost/capacity of disk storage has decreased tremendously.
At the same time, also, the speeds and capacities of magnetic tapes have
increased by equally impressive margins. Although these facts have not
changed the storage hierarchy, they have influenced the point at which
HSM systems are deployed: in many cases the disk cache can be extended
to levels that were previously cost-prohibitive.
simplest kind of HSM system is often known as the Virtual Disk. It provides
a method of expanding the online storage of a single (usually large and
fast) computer. The well-known counterpart of virtual disk, virtual memory,
operates on the basic mechanism of a page fault and provides an infinitely
large address space for programs executing in a host computer. The virtual
disk, likewise, operates on the principle of a file fault and provides
an infinitely large file space. A virtual disk is most useful for a single
computer whose existing disk space is becoming exhausted and where it
is impractical or uneconomical to keep adding more and more disk. Special
‘hooks’ in the host’s operating system kernel detect a ‘disk full’ condition
and activate other software routines to clear disk space according to
a selectable algorithm. This algorithm is similar in principle to page
turning algorithms of virtual memory systems.
Virtual Disk extends the virtual disk concept to clients of a file server.
Studies have shown that over 80% of the data in a given system may be
removed from expensive online storage without significantly sacrificing
performance. This principle, which is true of one machine and its data,
is also true of networked machines and their data. Therefore, much of
the data on individual network nodes can be removed from local disks and
placed in a common networked storage node.