However, all nodes that remain active
are needed to process a slice from the replica. In order to allow up to R nodes to
become unavailable, there must be R nonoverlapping replica slice sets.
If we desire y nodes to be able to be off-line simultaneously when a single replica
Figure 14. Schema in node X with replicated schema from node Y
224 Furtado
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is used, then the y nodes must not contain replica slices of each other. Partitioned
replicas (PRs).guarantee this by creating groups and placing replica slices from one
group in a different group. This way we can take a whole group off-line simultaneously
for maintenance or other functionality, because the complete set of replica
slices are elsewhere. This strategy is a hybrid between FPR and FR.
If replicas are partitioned into x slices, we denote it by PR(x). If x = N, we have a
fully partitioned replica. A very simple algorithm to generate less than N slices is:
Number nodes linearly;
The data for node i is partitioned into X slices starting at 1;
For slice set j = 0 to R
For (slice x from 1 to X) Place slice x in node (i+j+ x) MOD N
Figure 15 compares the response time
(line) for query TPC-H Q9 and the
minimum number of replicas needed (bars) when 5 out of 20 nodes are off-line using
full replicas (FRs), fully partitioned replicas (FPRs), and partitioned replicas (PRs).
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