Organization of images on optical discs

©1995 by Charles A. Plesums, Austin, Texas

There are basically two ways to fill an optical disc.

Arranged by "file" or "jacket," similar to the traditional organization of paper "files." If users will continue to be encouraged to "retrieve a file" like they did with paper, all the documents in a file need to be kept together to provide efficient retrieval. When a new file is started, space must be reserved for additional items. As documents are added, they are stored near the rest of the documents that are used together - "in the same file". The down side is that, unless you can predict exactly what documents will be stored, the wrong amount of space will be reserved - either it won't all be used, or you will run out. Therefore lots of time is spent reorganizing the optical discs to recover and reallocate space, which of course requires lots of media for WORM discs. Another potential problem is the write-life of a disc - see Optical Disc Life, below.

Arranged chronologically, so that each optical disc is filled completely in the sequence the documents are received. Documents that arrive at different times are not stored together, even if they are related. If the use of the documents can be focused on retrieval of individual documents rather than entire files, this organization is much more efficient in the use of space and in later file purge.

Traditional office procedures are often based on retrieving a complete paper file. That has been a convenient and efficient process for many years, assuring that the related papers are kept together, even if only a single document is required. However, there is an alternative. When a user requests a file, they can be shown a list of the documents, not the documents themselves. The user can then select which documents actually need to be viewed.

An image system must certainly allow any or all documents to be retrieved, but no system should encourage the user to casually "retrieve all." One trick is to make the user select document 1 and 2 and 3 and 4 etc. rather than providing a single command to get all the documents. Not only will this allow the more efficient chronological organization of the optical discs, but by reducing the number of images retrieved, could substantially reduce the load on the image system and the network.

File Purge

A typical business will use an on-line computer system to keep their records in a conventional database, not on paper or in an image system. The on-line system represents the current state of the business, while the image system provides the documentation of changes to the records. For example, an application for life insurance becomes part of the policy, as well as being the source of much of the information in the data processing system. The "working" information on coverages, current address, and current premiums are in the computer system. The application for life insurance will probably be in the image file for the entire life of the policy, perhaps even 75 years.

Many documents have a short life (3 or 7 years) even if the overall file exists for a long time. Routine correspondence, such as inquiries or change of address, may need to be kept for 7 years, but not 75. When the documents are stored chronologically on the disc, all the documents that arrived at approximately the same time are on the same disc. Seven years later (or whenever) it is easy to purge the disc by copying only the "permanent" documents to new media.

Continuing with the life insurance example, when a death claim is paid, the record is updated, and the death certificates and other documentation are added to the image file. It is only 7 years later, when the statute of limitations under contract law prevents further appeal, that the image files may really be purged. Beyond the images, many companies keep at least some of their data for 15-20 years, for statutory reporting and statistical analysis. Therefore at least parts of the on-line record may be around for a long time - even longer than the optical disc images.

Back to the home page, Disc Organization, Chronological Organization, Organization by File

Optical Disc Life

Most optical discs are guaranteed readable for 10-30 years, with some as long as 100 years. However, some limit writing to only the first few years. Therefore there may be a problem on discs organized by file when adding documents 5-10 years later. Since disks organized chronologically are filled near the beginning of their life, this problem does not occur.

Although 30 or 100 years sounds like a good life, the disc drives and the associated computer hardware and software will not be supported for 100 years. Optical disc technology is constantly improving - one could argue that there are major steps (generations) of technology every couple years. Since optical discs are relatively slow and have a very large capacity, conversion takes a very long time. Therefore most users avoid conversion whenever a new technology emerges. A system may need to support the new technology for new information, while keeping the older discs for the older documents. By the third or fourth generation (7-10 years) it may be wise to discontinue maintenance on the older drives, but (with the legal statute of limitations) this is a good period for purging older documents from the discs. Following this logic, most documents are written to optical disc once in their life, and purged from the discs as the technology becomes obsolete. Only the "permanent" documents are converted to new technology storage, only every 7-10 years.

Disc Organization, Chronological Organization, Organization by File

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©1995 by Charles A. Plesums, Austin, Texas USA. ALL RIGHTS RESERVED. You may license additional copies of this document through a nominal royalty payment as specified on