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I need to copy data from 3 nodes to a single node. I know about snapshot command and I was able to successfully copy data from a cluster where each node had 100% data (full replication) to a single-node cluster.

But this case I have a cluster with 3 nodes where the data is divided: 50%-25%-25%.

If I create snapshot on each 'source' node and restore on a single 'target' node, will everything work correctly?

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You shouldn't do it that way.

I would change my replication factor to 1 for my keyspaces, then run nodetool decommission on the nodes I want to remove, one at a time. To check the status of the decommission, watch nodetool status and nodetool netstats.

More information about decommission can be found here : http://docs.datastax.com/en/cassandra/2.0/cassandra/tools/toolsDecommission.html

This way you let Cassandra stream the data and you are sure you won't make errors.

Also, don't forget to check if you need to change the consistency factor on your client application.

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