Beschreibung

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 database systems, 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 that disk contention in multi-user mode can be limited for
Shared Disk database systems by dynamically choosing the degree of scan parallelism.