3.3.1. Removable media

There are many factors in choosing the right hardware for mass storage for our needs. Some of the key issues are: volume, easy access, easy maintenance, reliability, cost of media and the system as whole, I/O speed, durability, and lifetime of the technology. In Table 2, high density media most commonly used for mass storage system are evaluated in terms of these criteria.

Tapes: Since tapes utilize sequential access, the data transfer rate is generally slow. The 8-mm exabyte and DAT tapes have been one standard storage medium for astronomical data. The tape auto-exchanger, or library can store many 100 or 1000 Gbytes of the data. While these media are still very good for transporting and backing up the data, these are not suitable for long term storage because of the limited lifetime of the media. A recent development in tape storage technology is DLT (Digital Linear Tape). DLT is a cartridge tape, data are recorded in longitudinal multi-tracks versus the slanted stripes of helical scan technology which significantly increases data accessing rate. DLT is a durable (media lifetime $\sim$30 years) and has very large capacity ($\sim$10 - 80G[compressed] per volume), with a low cost. The DLT libraries are currently available from many vendors that can hold up to few tens of Tbytes. Super DLT is the newest technology which may be available on the market in the future, with about ten times capacity on a single cartridge than DLT. There are other variants of high capacity tapes but the longevity of the such technologies may be in question.

MO: Magneto-optical (MO) disks are also a popular choice among the high density media with reasonable storage capacity (5.2GBytes on a single MO disk). Juke boxes to store up to $\sim$ 1 TBytes are available.

WORM: Write-Once-Read-Many(WORM) optical disks and the juke boxes have been used in the astronomical data archive such as the HST. The capacity can go up to 12 Gbytes per medium. But as CD/DVD becomes more popular, this technology is now obsolete. The media and system are more expensive than others.

CD-R: The ``write-once recordable'' CD (CD-R) is inexpensive and CD juke boxes have been available for quite sometime. But a single CD-R can hold only 0.65 Gbytes while our data at maximum rate can produce a single FITS file of a single observing run of a few G bytes.

DVD-R: The ``write-once recordable'' digital versatile disk (DVD-R), which has been considered to replace CD, are now starting to appear on the market. Current single-sided DVD-R holds 4.7 Gbytes. The storage capacity will doubled when double-sided disks become available in future. DVD-R drives in juke box configuration (capacity $\sim$4 T bytes) are just appearing on the market. These juke boxes(drives) are often backward compatible as the devices can handle mixture of CDs and DVDs. The ESO is considering the DVD-R juke boxes for their mass storage system for the VLT (Pirenne 1999).