Ariel Weisberg created CASSANDRA-13442:
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Summary: Support a means of strongly consistent replication with
storage requirements approximating RF=2
Key: CASSANDRA-13442
URL: https://issues.apache.org/jira/browse/CASSANDRA-13442
Project: Cassandra
Issue Type: Improvement
Components: Compaction, Coordination, Distributed Metadata, Local
Write-Read Paths
Reporter: Ariel Weisberg
Replication factors like RF=2 can't provide strong consistency and availability
because if a single node is lost it's impossible to reach a quorum of replicas.
Stepping up to RF=3 will allow you to lose a node and still achieve quorum for
reads and writes, but requires committing additional storage.
The requirement of a quorum for writes/reads doesn't seem to be something that
can be relaxed without additional constraints on queries, but it seems like it
should be possible to relax the requirement that 3 full copies of the entire
data set are kept. What is actually required is a covering data set for the
range and we should be able to achieve a covering data set and high
availability without having three full copies.
After a repair we know that some subset of the data set is fully replicated. At
that point we don't have to read from a quorum of nodes for the repaired data.
It is sufficient to read from a single node for the repaired data and a quorum
of nodes for the unrepaired data.
One way to exploit this would be to have N replicas, say the last N replicas in
the preference list, delete all repaired data after a repair completes.
Subsequent quorum reads will be able to retrieve the repaired data from any of
the two full replicas and the unrepaired data from a quorum read of any replica
including the "transient" replicas.
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