Retrieval performance versus disc space utilization on WORM optical discs

Abstract

Steady progress in the development of optical disc technology over the past decade has brought it to the point where it is beginning to compete directly with magnetic disc technology. WORM optical discs in particular, which permanently register information on the disc surface, have significant advantages over magnetic technology for applications that are mainly archival in nature but require the ability to do frequent on-line insertions. In this paper, we propose a class of access methods that use rewritable storage for the temporary buffering of insertions to data sets stored on WORM optical discs and we examine the relationship between the retrieval performance from WORM optical discs and the utilization of disc storage space when one of these organizations is employed. We describe the performance trade off as one of fast sequential retrieval of the contents of a block versus wasted space owing to data replication. A model of a specific instance of such an organization (a buffered hash file scheme) is described that allows for the specification of retrieval performance objectives. Alternative strategies for managing data replication that allow trade offs between higher consumption rates and better average retrieval performance are also described. We then provide an expected value analysis of the amount of disc space that must be consumed on a WORM disc to meet specified performance limits. The analysis is general enough to allow easy extension to other types of buffered files systems for WORM optical discs.