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https://issues.apache.org/jira/browse/CASSANDRA-13442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16198063#comment-16198063
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Kurt Greaves commented on CASSANDRA-13442:
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Yeah OK I'm convinced (if it can be proven, obviously), however let's not go
around making it incredibly misleading.
bq. 10-20x on transient replicas. Not at full replicas or overall.
Saying 10-20x is really misleading. No one is actually going to see a 10 - 20x
improvement in disk usage. Even a reduction of 1/3 would be optimistic I'm sure.
bq. With vnodes data would be spread out over several nodes so the additional
utilization at each node could be substantially less.
Let's not pretend people running vnodes can actually run repairs.
bq. Some of it might end up being part of overlapping functionality. I can hope.
Not sure if there is a ticket for it but I've been meaning to create one which
would probably benefit from this change. Need a way to change RF without
downtime and without costing a fortune (DC migration). I can see ways in which
transient replicas would give this functionality, as will need some way to
change RF on the fly and not cause nodes to be responsible for data they don't
yet have.
If you could add a replica as transient at any time this would almost solve the
RF change problem, assuming you had some way to transition between transient
and real replicas.
> 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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