Riak CS need for 5 servers?

Christopher Meiklejohn cmeiklejohn at basho.com
Fri Apr 3 14:12:59 EDT 2015

> On Apr 3, 2015, at 10:38 AM, Shankar Dhanasekaran <shankar at opendrops.com> wrote:
> hi,
> For the organization that I am working for, it seems like 25tb of storage is needed. Reading riak docs, it's mentioned that at least 5 servers should be put in place.
> what is the harm in having only 2 riak nodes? will this reduce performance? or cause loss of data? Since Riak is masterless, either one of the server will have the data at any point of time.

If you have only two nodes and you incur one failure, you will no longer be able to write to a majority quorum [1], which means that a subsequent failure will result in potential unrecoverable data loss.  This is why the official recommendation used to be 3 nodes.

We’ve changed that to 5 nodes because this is the minimum amount of nodes needed to ensure all copies of each object are stored on different physical nodes. [2]

- Chris

[1] This assumes a default configuration with n_val = 3.
[2] The partition / cluster claim algorithm attempts to ensure that all preference lists are on a disjoint set of nodes.

Christopher Meiklejohn
Senior Software Engineer
Basho Technologies, Inc.
cmeiklejohn at basho.com

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