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Abstract:
Large multimedia document archives may hold a major fraction of their data in
tertiary storage libraries for cost reasons. This paper develops an integrated
approach to the vertical data migration between the tertiary, secondary, and
primary storage in that it reconciles speculative prefetching, to mask the high
latency of the tertiary storage, with the replacement policy of the document
caches at the secondary and primary storage level, and also considers the
interaction of these policies with the tertiary and secondary storage request
scheduling.
The integrated migration policy is based on a continuous-time Markov chain
model for predicting the expected number of accesses to a document within a
specified time horizon. Prefetching is initiated only if that expectation is
higher than those of the documents that need to be dropped from secondary
storage to free up the necessary space. In addition, the possible resource
contention at the tertiary and secondary storage is taken into account by
dynamically assessing the response-time benefit of prefetching a document
versus the penalty that it would incur on the response time of the pending
document requests.
The parameters of the continuous-time Markov chain model, the probabilities of
co-accessing certain documents and the interaction times between successive
accesses, are dynamically estimated and adjusted to evolving workload patterns
by keeping online statistics. The integrated policy for vertical data migration
has been implemented in a prototype system. The system makes profitable use of
the Markov chain model also for the scheduling of volume exchanges in the
tertiary storage library. Detailed simulation experiments with Web-server-like
synthetic workloads indicate significant gains in terms of client response
time. The experiments also show that the overhead of the statistical
bookkeeping and the computations for the access predictions is affordable.
