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https://issues.apache.org/jira/browse/CASSANDRA-13442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16196481#comment-16196481
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Kurt Greaves commented on CASSANDRA-13442:
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It seems to me that the main constraint to running dense nodes is that
streaming currently takes a long time. Considering this seems to be mostly
about reducing storage costs so write bound workloads can run "dense" nodes,
and storage is meant to be cheap, it seems to me a less complex alternative
would just be to remove the barriers to having large amounts of physical
storage per node.
While this idea does seem interesting, it seems very complex and you are still
trading off replicas for additional storage. Seems that the primary use case
would be multiple datacenters with transient replicas, which granted would be
nice, but to some degree if you're happy storing transient replicas in each DC
well you're probably able to just store less replicas in each datacenter
anyway, at least if we had more flexible consistency levels.
> Support a means of strongly consistent highly available replication with
> tunable storage requirements
> -----------------------------------------------------------------------------------------------------
>
> 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
> (where N varies with RF) 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.
> Configuration for something like this in NTS might be something similar to {
> DC1="3-1", DC2="3-2" } where the first value is the replication factor used
> for consistency and the second values is the number of transient replicas. If
> you specify { DC1=3, DC2=3 } then the number of transient replicas defaults
> to 0 and you get the same behavior you have today.
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