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From aaron morton <aa...@thelastpickle.com>
Subject Re: Using a node in separate cluster without decommissioning.
Date Thu, 12 Jul 2012 04:31:56 GMT
> Since replication factor is 2 in first cluster, I
> won't lose any data.
Assuming you have been running repair or working at CL QUORUM (which is the same as CL ALL
for RF 2)

> Is it advisable and safe to go ahead?
um, so the plan is to turn off 2 nodes in the first cluster, restask them into the new cluster
and then reverse the process ?

If you simply turn two nodes off in the first cluster you will have reduce the availability
for a portion of the ring. 25% of the keys will now have at best 1 node they can be stored
on. If a node is having any sort of problems, and it's is a replica for one of the down nodes,
the cluster will appear down for 12.5% of the keyspace.

If you work at QUORUM you will not have enough nodes available to write / read 25% of the
keys. 

If you decomission the nodes, you will still have 2 replicas available for each key range.
This is the path I would recommend.

If you _really_ need to do it what you suggest will probably work. Some tips:

* do safe shutdowns - nodetool disablegossip, disablethrift, drain
* don't forget to copy the yaml file. 
* in the first cluster the other nodes will collect hints for the first hour the nodes are
down. You are not going to want these so disable HH. 
* get the nodes back into the first cluster before gc_grace_seconds expires. 
* bring them back and repair them.
* when you bring them back, reading at CL ONE will give inconsistent results. Reading at QUOURM
may result in a lot of repair activity.

Hope that helps. 
 
-----------------
Aaron Morton
Freelance Developer
@aaronmorton
http://www.thelastpickle.com

On 11/07/2012, at 6:35 AM, rohit bhatia wrote:

> Hi
> 
> I want to take out 2 nodes from a 8 node cluster and use in another
> cluster, but can't afford the overhead of streaming the data and
> rebalance cluster. Since replication factor is 2 in first cluster, I
> won't lose any data.
> 
> I'm planning to save my commit_log and data directories and
> bootstrapping the node in the second cluster. Afterwards I'll just
> replace both the directories and join the node back to the original
> cluster.  This should work since cassandra saves all the cluster and
> schema info in the system keyspace.
> 
> Is it advisable and safe to go ahead?
> 
> Thanks
> Rohit


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