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From "Jeff Jirsa (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-13442) Support a means of strongly consistent highly available replication with tunable storage requirements
Date Mon, 09 Oct 2017 17:16:00 GMT


Jeff Jirsa commented on CASSANDRA-13442:

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.

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
> -----------------------------------------------------------------------------------------------------
>                 Key: CASSANDRA-13442
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Compaction, Coordination, Distributed Metadata, Local Write-Read
>            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
> 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"
> 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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