I found this calculator very convenient: http://www.ecyrd.com/cassandracalculator/
Regardless of your other DCs you need RF=3 if you write at LOCAL_QUORUM, RF=2 if you write/read at ONE. Obviously using ONE as CL makes your cluster very fragile. -- Jacques-Henri Berthemet -----Original Message----- From: Rakesh Kumar [mailto:rakeshkumar46...@gmail.com] Sent: vendredi 25 mars 2016 18:14 To: user@cassandra.apache.org Subject: Re: How many nodes do we require On Fri, Mar 25, 2016 at 11:45 AM, Jack Krupansky <jack.krupan...@gmail.com> wrote: > It depends on how much data you have. A single node can store a lot of data, > but the more data you have the longer a repair or node replacement will > take. How long can you tolerate for a full repair or node replacement? At this time, for a foreseeable future, size of data will not be significant. So we can safely disregard the above as a decision factor. > > Generally, RF=3 is both sufficient and recommended. Are you telling a SimpleReplication topology with RF=3 or NetworkTopology with RF=3. taken from: https://docs.datastax.com/en/cassandra/2.0/cassandra/architecture/architectureDataDistributeReplication_c.html " Three replicas in each data center: This configuration tolerates either the failure of a one node per replication group at a strong consistency level of LOCAL_QUORUM or multiple node failures per data center using consistency level ONE." In our case, with only 3 nodes in each DC, wouldn't a RF=3 effectively mean ALL. I will state our requirement clearly: If we are going with six nodes (3 in each DC), we should be able to write even with a loss of one DC and loss of one node of the surviving DC. I am open to hearing what compromise we have to do with the reads during the time a DC is down. For us write is critical, more than reads. May be this is not possible with 6 nodes, and requires more. Pls advise.
