Archiving Online Data to Optical Disc
ARCHIVING ONLINE DATA TO OPTICAL DISK
By J. L. Porter, J. L. Kiesler, and D. A. Stedfast
ARCHIVING ONLINE DATA TO OPTICAL DISK
The U.S. Geological Survey's Distributed Information System Program Office is currently (1990) assessing optical disk storage as an alternative means of archiving unit-value data. Optical disks have a longer shelf life than magnetic tapes, and retrieval of archived data is substantially easier. The cost for data storage on writeonce/read-many optical disks is comparable to that of magnetic tape and only a fraction of the cost for fixed magnetic disk (hard disk) data storage. In a test study, unit-value data was archived using one optical disk drive connected to a microcomputer and a second drive connected to a minicomputer. The cost of data storage, alternative forms of data storage, and specifications for data retrieval are described.
(1) U.S. Geological Survey, Reston, Va.
To evaluate the feasibility of using optical media for archiving hydrologic data, unit-value data from a USGS office was archived on both a microcomputer and minicomputer optical storage system. This paper (1) describes types of optical storage, (2) illustrates information on storage media costs and alternative forms of data storage, (3) compares paper, microfiche, write-once/read-many (WORM), compact disk read-only memory (CD ROM), and magnetic tape and fixed magnetic disk (hard disk) media, (4) documents benefits of optical storage, and (5) presents the archival procedures and results.
TYPES OF OPTICAL STORAGE
1. Read-only media such as the audio compact disk (CD) and CD ROM disks are fabricated by a pressing process, like phonograph records, where data are permanently embedded onto the disk. These disks are used to distribute large volumes of data, which can neither be altered nor erased.
2. WORM disks are supplied with no information written on the surface. The write once disk allows the user to write data to the disk, where it cannot be altered or erased, but can be read many times.
3. Rewritable optical disks can be written to, read from, altered, and erased. Because rewritable optical disks can be altered, they are inappropriate for applications involving data archiving.
STORAGE MEDIA COSTS AND ALTERNATIVE MEDIA USED FOR DATA ARCHIVAL
The average long-term cost of storing the media required to archive 100 MB is shown in figure 2. Paper, by far the most expensive, costs approximately $175 per year to store 100 MB. To store 100 MB of data on an 8-mm tape cartridge costs less than $0.01. The cost of storing on write-once optical media is comparable to that for 8-mm tape cartridges.
Another storage method is the Bernoulli Boxl, with removable cartridges. One 5 1/4-inch Bernoulli cartridge drive holding 20 MB costs about $2,500; each data cartridge costs about $100. Even these units require 5 to 20 cartridges to store as much as one optical cartridge holds (Bican, 1988).
COMPARISONS OF STORAGE MEDIA
(1) Use of trade names or trademarks in this report is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.
Compact Disk Read-Only Memory Compared to Write-Once/Read Many Media
The main advantages of CD ROM media are as follows:
WORM disk drives are able to read and write data. A 5.25-inch WORM drive for a microcomputer ranges in cost from $3,000 to $6,000. A 5.25-inch WORM cartridge ranges in capacity from 400 MB to 1.2 GB (gigabytes). A 12-inch WORM drive for a minicomputer or workstation ranges in cost from $15,000 to $20,000. A 12-inch WORM cartridge ranges in capacity from 1.2 to 2.4 GB. The cost and capacity of the disk drive and media vary from manufacturer to manufacturer.
WORM drives have the following advantages:
One problem with the current WORM media is the lack of a common standard. Although outwardly similar, the optical format and physical dimensions of each manufacturer's products are significantly different from those of its competitors (Bican, 1988). Consequently, one manufacturer's cartridge cannot be used in another manufacturer's drive, or conversely. Standards are evolving slowly and are being addressed first for the media. Another problem is that of legality. Because a document stored on optical disk (CD ROM or WORM) has not been used as evidence in a court of law, agencies are using this as a reason to not "trust" their documents to optical media (Association for Information and Image Management, 1990).
Erasable Compared to Write-Once/Read-Many Media
Paper and Microfiche Compared to Optical Media
The disadvantages of paper and microfiche are:
ADVANTAGES OF WRITE-ONCE/READ-MANY OPTICAL STORAGE
ARCHIVAL PROCEDURE AND RESULTS
The first test was conducted on an optical-disk subsystem, Model 525WC, Information Storage, Inc. (ISI), connected to a COMPAQ 286 microcomputer. The optical-disk subsystem emulates a magnetic disk, and uses a 5.25-inch disk with a capacity of 115 MB per side. A Fortran program was written and executed on the Prime to create an index file (fig. 4) of the 400 unit-value data files. Each record in the index file contains general information about an individual unit-value data file including an alternate filename, the original UV_ARCHIVE file name, station name, station number, station type, location, type of unit-value data collected, amount of record collected, latitude, longitude, and so forth.
To store the unit-value data files on an optical disk, the file names had to be modified to correspond to MS-DOS naming conventions. A Prime Command Procedure Language (CPL) program was developed to rename the archived data files on the Prime using the alternate file name in the index file. The archived data files were transferred to the microcomputer magnetic disk by a program called FTP (File Transfer Protocol). This program provides a simple way to transfer files between a local computer and a remote computer. The MS-DOS copy command was used to transfer the archived files from the microcomputer magnetic disk to the optical disk.
The second test was conducted on an optical disk subsystem, Model 8502, Dallastone, Inc., connected to a Prime 6550 minicomputer. The optical disk subsystem emulates a tape drive, and uses a 12-inch optical disk with a capacity of 1 GB per side. The same Fortran program was executed to create the index file. To store the unit-value data files on the optical disk, a CPL program that wrote the unit value data files to the Dallastone optical disk using the Prime Operating System (PRIMOS) tape utility MAGSAV was executed. The time to transfer the data from the magnetic disk to the optical disk (in both tests) was comparable to writing files to a magnetic tape.
Because write-once disks cannot be erased, they are more suitable for applications involving data archiving and as backup storage devices than magnetic, CD ROM, and erasable optical media. Magnetic media are easily damaged by static electricity, stray magnetic fields, or excessive temperature or humidity. The CD ROM media is designed for the dissemination of information. Information stored on erasable optical media can be easily altered.
Paper and microfiche are alternative media choices that are currently being used to archive data. However, the cost of these media and the cost to maintain them indefinitely is substantially greater than for optical media. One key factor in favor of data stored on optical disk is that these data are readily accessible by computers, whereas data stored on paper or microfiche are accessed manually.
One year of hydrologic data was archived on two different optical-disk subsystems. One was a disk emulation system and the other was a tape emulation system. Both systems performed satisfactorily. Files on the disk emulation system, however, were accessed in seconds, whereas it took 7 minutes to access the 400-hundreth file on the tape emulation system.
Although no media for data storage can be considered as permanent, the high data-storage capacities, long shelf life, nonerasability, and low cost of optical writeonce media make it a suitable media for archiving hydrologic data.
Bican, Frank, 1988, WORM Drives: PC Magazine, March 29,1988.
Owen, D. J., 1989, ICI Imagedata's Digital Paper: Optical Information Systems, September-October 1989, v. 9, no. 5, p 226-229.
Shier, Daniel, 1989, Optical Disk Ideas on the Rise of Exploration: Geobyte, February 1989, p6-14
Storage Dimensions, 1989, Write-Once Optical Storage for Personal Computers: A White Paper, 30 p.
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