Mass Storage Management.
Figure 0-1 Options for Data Storage
Mass Storage Management (MSM) describes various alternatives for data storage using
the best possible and most appropriate method. Its objectives are logical and its
structure is self-explanatory.
Figure 0-1 above shows the main components in a mass storage system.
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On-line storage contains the most frequently accessed materials. On-line servers
and storage facilities can provide small amounts of data (generally 5-10% of
the total) very quickly and efficiently to many users. The data stored is often
highly compressed to optimise storage and delivery efficiency. To use a video
library analogy the on-line storage consists of the shelf containing multiple
copies of the most popular videos requested. The on-line storage and servers
are the most expensive form of data storage.
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Near-line storage contains larger amounts of less commonly accessed material.
The rate at which the near-line material can be transferred is typically much
lower than that of near-line storage data. In many setups data is transferred
from the near-line storage system to the on-line system as required. That way
the on-line system can keep up to date with the most commonly requested material.
Data stored in near-line systems is also typically heavily compressed. The near-line
storage is slower, has greater capacity and is cheaper than the on-line storage.
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Archive storage systems are simply large stores of data. While near-line and
on-line systems for multimedia usually employ heavy compression, archive systems
usually have little (light) or no compression. The purpose of such systems is
usually as a store of huge amounts of data that are unlikely to be used very
often (such as original film footage or soundtracks), and also, in the case of
multimedia, to store assets in a form such that the information is as close to
the original as possible, unlike lossy compressed video and audio, which have
artefacts introduced by the compression system that makes them unsuitable for
further processing. The archive systems are generally massive and transferring
to or from them is a tedious and expensive process. The storage system must be
very cheap.
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Off-line storage is usually employed as a backup to the other three types of
storage. As an example for a small-scale system, a fire-safe filled with DAT
tapes is an appropriate form of off-line storage for a workstation environment.
The two latter types of storage (offline and archive) are sometimes also known
as tertiary storage library, some components of which are shown in Figure 0-2.
Figure 0-2 Components for a Tertiary Storage Library
Archive storage holds copy of the materials in lossless compressed format. It provides
the safety net if all other storage types are damaged. It can perform automated
checking and maintenance of the tapes with built in error detection and correction
as shown below in Figure 0-3.
Figure 0-3 Archive Data Management