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https://issues.apache.org/jira/browse/CASSANDRA-13442?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16197326#comment-16197326
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Jeff Jirsa commented on CASSANDRA-13442:
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{quote}
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.
{quote}
Everything is meant to be cheap, but that doesn't mean it is.
In a reasonably sized cluster (for example, 250 nodes * 2 datacenters *
4tb/node = 2 million GB of disk). This ticket would reduce that to something
closer to 1,340,000 GB of disk for a cluster of that nature.
Enterprise SSDs still retail for $0.50/GB. Let's pretend you get a great deal
and you're paying $0.25/GB. The cost differential is $335k vs $500k, for a
single cluster.
If you're on AWS and using GP2 EBS, that's $0.10/GB/month. The cost
differential is $134k/month vs $200k/month, or about $1.6M/year. Per cluster.
That's JUST DISK savings, even if we pretend like everything else is free (and
it's not).
If you feel like there's more ROI to win by having denser storage, I'm sure
nobody would mind seeing patches.
> 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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