MEMS storage devices are new non-volatile secondary storages that have outstanding advantages over magnetic disks. MEMS storage devices, however, are much different from magnetic disks in the structure and access characteristics in the following ways. They have thousands of heads called probe tips and provide the following two major access facilities: (1) flexibility : freely selecting a set of probe tips for accessing data, (2) parallelism: simultaneously reading and writing data with the set of probe tips selected. Due to these characteristics, it is nontrivial to find data placements that fully utilize the capability of MEMS storage devices. In this paper, we propose a simple logical model called the Region-Sector (RS) model that abstracts major characteristics affecting data retrieval performance, such as flexibility and parallelism, from the physical MEMS storage model. We also suggest heuristic data placement strategies based on the RS model. To show the usability of the RS model, we derive new data placements for relational data and two-dimensional spatial data by using these strategies. Experimental results show that the proposed data placements improve the data retrieval performance by up to 4.7 times for relational data and by up to 18.7 times for two-dimensional spatial data of approximately 320 Mbytes compared with those of existing data placements. Further, these improvements are expected to be more marked as the database size grows.

Many object-oriented database systems have used the notion of implicit authorization to avoid the overhead caused by explicitly storing all authorizations for each object. In implicit authorization, it is very important to detect efficiently conflicts between existing authorizations and new authorizations to be added. In this article we propose a conflict detection mechanism in the OODBMSs using implicit authorization with the notion of intention type authorization. When we grant an authorization on a node n in the database granularity hierarchy, the existing method is inefficient in determining the conflicts since it needs to examine all authorizations on the descendants of the node n. In contrast, our mechanism has the advantage of detecting the conflicts at the node n where an explicit authorization is to be granted without examining any authorizations below the node n. Thus, the proposed mechanism can detect a conflict with the average time complexity of O(d), which is smaller than O(md) of existing methods, where m is the number of children nodes at an arbitrary level and d is the difference of levels between the node with an existing explicit authorization and the higher node where an explicit authorization is to be granted. We also show that the additional storage overhead of storing all authorizations is negligible when compared with the total number of all explicit authorizations.