Hi Robert, Firstly, thank you very much for you help. I have some comments inline below.
On 10/09/2015 01:26, "Robert Coli" <rc...@eventbrite.com<mailto:rc...@eventbrite.com>> wrote: On Wed, Sep 9, 2015 at 7:52 AM, Richard Dawe <rich.d...@messagesystems.com<mailto:rich.d...@messagesystems.com>> wrote: I am investigating various topology changes, and their effect on replica placement. As far as I can tell, replica placement is not changing after I’ve changed the topology and run nodetool repair + cleanup. I followed the procedure described at http://docs.datastax.com/en/cassandra/2.0/cassandra/operations/ops_switch_snitch.html That's probably a good thing. I'm going to be modifying the warning in the cassandra.yaml to advise users that in practice the only change of snitch or replication strategy one can safely do is one in which replica placement does not change. It currently says that you need to repair, but there are plenty of scenarios where you lose all existing replicas for a given datum, and are therefore unable to repair. The key is that you need at least one replica to stay the same or repair is worthless. And if you only have one replica staying the same, you lose any consistency consistency contract you might have been operating under. One ALMOST NEVER ACTUALLY WANTS TO DO ANYTHING BUT A NO-OP HERE. So if you have a topology that would change if you switched from SimpleStrategy to NetworkTopologyStrategy plus multiple racks, it sounds like a different migration strategy would be needed? I am imagining: 1. Switch to a different snitch, and the keyspace from SimpleStrategy to NTS but keep it all in one rack. So effectively the same topology, but with a different snitch. 2. Set up a new data centre with the desired topology. 3. Change the keyspace to have replicas in the new DC. 4. Rebuild all the nodes in the new DC. 5. Flip all your clients over to the new DC. 6. Decommission your original DC. Or something like that. Here is my test scenario : <snip> 1. To determine the token range ownership, I used “nodetool ring <keyspace>” and “nodetool info –T <keyspace>”. I saved the output of those commands with the original topology, after changing the topology, after repairing, after changing the replication strategy, and then again after repairing. In no cases did the tokens change. It looks like nodetool ring and nodetool info –T show the owner but not the replicas for a particular range. The tokens and ranges shouldn't be changing, the replica placement should be. AFAIK neither of those commands show you replica placement, they show you primary range ownership. Use getendpoints to determine replica placement before and after. Thanks, I will play with that when I have a chance next week. I was expecting the replica placement to change. Because the racks were assigned in groups (rather than alternating), I was expecting the original replica placement with SimpleStrategy to be non-optimal after switching to NetworkTopologyStrategy. E.g.: if some data was replicated to nodes 1, 2 and 3, then after the topology change there would be 2 replicas in RAC1, 1 in RAC2 and none in RAC3. And hence when the repair ran, it would remove one replica from RAC1 and make sure that there was a replica in RAC3. I would expect this to be the case. However, when I did a query using cqlsh at consistency QUORUM, I saw that it was hitting two replicas in the same rack, and a replica in a different rack. This suggests that the replica placement did not change after the topology change. Perhaps you are seeing the quirks of the current rack-aware implementation, explicated here? https://issues.apache.org/jira/browse/CASSANDRA-3810 Thanks. I need to re-read that a few times to understand it. Is there some way I can see which nodes have a replica for a given token range? Not for a range, but for a given key with nodetool getendpoints. I wonder if there would be value to the range... in the pre-vnode past I have merely generated a key for each range. With the number of ranges increased so dramatically by vnodes, it might be easier to have an endpoint that works on ranges... Thank you again. Best regards, Rich =Rob
