Near-line Storage
Near-line storage refers to material that is stored in a manner that is more cost
effective than on-line storage, but at the expense of a time delay required to
retrieve it. This means, for a Video On Demand system for example, that if a user
requests a video title that is not stored within the on-line storage facility the
data is transferred from the much larger near-line storage unit to the on-line
storage unit, and is then delivered to the user. The delay depends on the type
of system used, and can be as little as a matter of minutes for a robotic system.
The near-line storage facility must be significantly cheaper than the on-line storage
unit as it is used to archive much more material. Generally less than 10% of material
would be stored on-line and the remaining 90% of the collection would be stored
near-line.
For such huge storage requirements a jukebox style setup or robotic tape library
is usually employed with the data stored on CD ROM, DAT tape, Digital Linear Tape
medium (DLT) or other tape format. In the future, the Digital Versatile Disk (DVD)
will also be used.
The disadvantages of magnetic tape are
- there can be no random access to the data because of the serial nature of the medium
- the rates can be quite slow
The advantages of magnetic tape are that
- they are a relatively cheap storage medium
- each tape can hold a significant amount of data
CD ROMs have the advantage that they are random access devices but the disadvantage
is that each CD cannot hold enough material to make it viable. A CD ROM can hold
about 640 Mbytes of data, a one hour movie encoded at 2Mbit/s requires 900Mbytes
of storage. CDs are also write-once devices which makes them considerably less
flexible than tape. Newer technologies will address this problem, however, and
read-write DVD-RAMs are supposed to become available in 1998 with capacity of
about 3 Gigabytes. Read only DVDs with capacity up to 15.9G are becoming available
now.
Table 1 summarises the speed and capacity specifications for various offline data
archiving media.
| Device | Capacity | Data Access Speed | Media Lifetime | Write once or Write many |
| DAT DDS2 | 4-8 Gbyte | 510 Kbyte/s | 10-25 Yrs | WM |
| DAT DDS3 | 12-24 Gbyte | 1 Mbyte/s | 10-25 Yrs | WM |
| CD-ROM | 640 Mbyte | X times 1.5 Mbits/s to Read | 10 Yrs Plus | WO |
| CD-RW | 640 Mbyte | X times 1.5 Mbits/s to Read | 10 Yrs Plus | WM |
| Exabyte | 20-40 Gbyte | 3-6 Mbyte/s | 10-25 Yrs | WM |
| DLT | 35 Gbyte | 5 MByte/s | 30 Yrs | WM |
| DVD | up to 15Gbyte | Not Known | Not Known | WO |
| DTF | 42Gbyte | 12 Mbyte/s | 10-25 Yrs | WM |
| Data D3 | 50 Gbyte | 12 Mbyte/s | 10-25 Yrs | WM |
| DVD-RAM | up to 3 Gbyte | Not Known | Not Known | WM |
| Magneto-optical | 2.6-5.6 Gbyte | Not Known | Not Known | WM |
Table 1 Device Parameters