Shared Disk database systems offer a high flexibility for parallel transaction and
query processing. This is because each node can process any transaction, query
or subquery because it has access to the entire database. Compared to Shared
Nothing, this is particularly advantageous for scan queries for which the degree of
intra-query parallelism as well as the scan processors themselves can dynamically
be chosen. On the other hand, there is the danger of disk contention between subqueries,
in particular for index scans. We present a detailed simulation study to analyze
the effectiveness of parallel scan processing in Shared Disk database
systems. In particular, we investigate the relationship between the degree of declustering
and the degree of scan parallelism for relation scans, clustered index scans,
and non-clustered index scans. Furthermore, we study the usefulness of disk caches
and prefetching for limiting disk contention. Finally, we show the importance of
dynamically choosing the degree of scan parallelism to control disk contention in
multi-user mode.

A revised version of this paper ("Analysis of Parallel Scan Processing in Shared Disk Database Systems") appeared at the Europar 1995 conference.