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From "Stu Hood" <stu.h...@rackspace.com>
Subject RE: Best practice for adding new nodes to ring
Date Tue, 26 Oct 2010 17:45:00 GMT
While the "adding virtual tokens/nodes to Cassandra" discussion is a good one, there are a
few factors that might delay (or remove?) the necessity of adding that complexity:

* In Cassandra 0.7, removing load from a node is fairly cheap: a bounded number of reads are
used to determine which portions of the large sorted data files (sstables) to stream, followed
by "sendfile" calls to deliver the data to the destination
* For a replication factor RF, RF nodes can send data to a new node: this means that to have
all existing N nodes in your cluster participate in adding K nodes, you only need to add N
/ RF = K nodes per expansion: this is a much easier factor to achieve than a power of 2.

While the added nodes will not be immediately balanced, there are some possible improvements
to our existing load-balancing facilities to better handle unbalanced cases: see https://issues.apache.org/jira/browse/CASSANDRA-1418

Finally, virtual nodes are not a panacea: reviewing the papers on https://issues.apache.org/jira/browse/CASSANDRA-192
suggests that they are significantly more difficult to implement than our current solution.

We haven't ruled virtual nodes out, but I think many of us are leaning toward exploring improvements
to our current architecture.

Thanks,
Stu

-----Original Message-----
From: "Greg Kim" <gkim@netflix.com>
Sent: Tuesday, October 26, 2010 12:21pm
To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: Best practice for adding new nodes to ring

Hi,

I have a question regarding the best practices for adding new nodes to an existing cluster.
 From reading the following wiki: http://wiki.apache.org/cassandra/Operations  -- I understand
that when creating a brand new cluster -- we can use the following to calculate the initial
token for each node to achieve balance in the ring:  
  def tokens(nodes):
     for i in range(1, nodes + 1):
         print (i * (2 ** 127 - 1) / nodes)


My question is on the best practice for adding new nodes to an existing cluster.  There is
a recommendation in the wiki which is to basically to compute new tokens for every node and
assign them manually using the nodetool command.  We're planning on running either 16GB or
32GB heaps on each of our nodes, so token re-assignment for each node in the cluster sounds
like a very expensive operation especially in situations where we're adding new nodes to handle
scaling issues w/ the existing cluster. 

I'm bit of a noob to cassandra, so wanted to see how others are currently coping w/ this.
 One option can be to grow the cluster in the power of 2 and use bootstraping w/ automatic
token generation.  Is this an option that people are using? (but this gets exponentially expensive
when you already have a large # of nodes)

Does anyone know why cassandra doesn't use virtual tokens (e.g. one node token - creating
256 virtual node tokens in the ring)?  This way adding new nodes to an existing cluster will
significantly mitigate the unbalance issue in the ring.  


Thanks
gkim


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