Using Subpages for Coherency Control in Parallel Database Systems

Increasing requirements such as higher transaction rates, shorter response times etc., make it necessary to parallelize existing database management systems and to implement them on multiprocessors. Shared memory multiprocessors are not arbi- trarily scalable, and thus do not meet future requirements. Therefore, it makes sense to implement databases on large-scale distributed memory multicomputers. This raises the problem how to implement a database cache which uses shared memory concepts extensively under such a hardware architecture. In this paper we describe a solution for this problem by introducing a virtual database cache (VDBC). The VDBC is an algo- rithmic approach based on the lazy release consistency model used in virtual shared memory systems and on the transaction concept. Additionally, the VDBC uses pages subdivided into equal sized subpages to maintain concurrency control and cache coher- ency and to reduce data contention on often accessed pages. The advantage of using subpages instead of just pages or of records is to combine the simple coherency control method used by page-level locking with the reduction of data contention provided by record-level locking. Our approach is based on the assumption, that interprocessor communication is quite favorable as compared to I/O - it is thus interesting to exchange subpages with other processors instead of reading them from disk.