Minimal number of nodes for production

Eric Redmond eredmond at basho.com
Wed Apr 10 17:56:47 EDT 2013



On Apr 10, 2013, at 2:26 PM, Tom Zeng <tom at intridea.com> wrote:

> Hi list,
> 
> We have a production installation with only 3 nodes and running on 1.2.1.  I'd appreciate to get some  facts to convince IT to increase the number of nodes to 7 and upgrade to 1.3.  I heard people from Basho mentioned ideally 7 nodes for production a couple of time, can someone explain why 7, is 4, or 5 nodes good enough?

I'm not sure where you heard the number 7 as a minimum, unless if was for a specific use-case. In general the minimum recommended number is 5 nodes.

Running with only 3 nodes isn't a great idea. Since a core purpose of Riak is to remain available in the face of outages, 3 will not support any outage. Less than 3 is lower than the default replication value (N=3). This is so important, in fact, that we recommend 5 solely to act as a buffer in the case where 1 of the 5 is down, the remaining 4 is dangerously close to the inflexible 3 node number. Even if you do not upgrade to 1.3, you really need to have at least 5 nodes.

There are many benefits to upgrading to 1.3, but one of the most compelling from an operations point of view is active anti-entropy (AAE). Rather than waiting on read-repair to fix inconsistent values (which is passive), AAE routinely attempts to keep all node values in sync. This can be a godsend if a node goes down, since you don't need to fore read-repair when you bring the node back up by reading every key... you just let your cluster actively self-heal.


> Also on the 3 three nodes, the file size for the bitcask directory very quite a bit: 21GB, 14GB, and 20GB. Could the node with only 14GB missing something or it's expected to have such big difference?

There are several reasons sizes could be different. Values are not yet/ever replicated (based on your N and W values). Files may have not been compacted. Some keys have been deleted but not yet reaped...

> Thanks,
> Tom
> 
> -- 
> Tom Zeng
> Director of Engineering
> Intridea, Inc. | www.intridea.com
> tom at intridea.com
> (o) 888.968.4332 x519
> (c) 240-643-8728
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