Pradeep, it sounds like what you're proposing counts as a topology change because you are changing the datacenter name and rack name.

Please refer to the documentation here about what to do in that situation:
https://docs.datastax.com/en/cassandra/3.0/cassandra/operations/opsSwitchSnitch.html

In particular:

Simply altering the snitch and replication to move some nodes to a new datacenter will result in data being replicated incorrectly.

Topology changes may occur when the replicas are placed in different places by the new snitch. Specifically, the replication strategy places the replicas based on the information provided by the new snitch.

If the topology of the network has changed, but no datacenters are added:
a. Shut down all the nodes, then restart them.
b. Run a sequential repair and nodetool cleanup on each node.

On Sun, Aug 26, 2018 at 11:14 AM, Pradeep Chhetri <pradeep@stashaway.com> wrote:
Hello everyone,

Since i didn't hear from anyone, just want to describe my question again:

Am i correct in understanding that i need to do following steps to migrate data from SimpleSnitch to GPFS changing datacenter name and rack name to AWS region and Availability zone respectively

1) Update the rack and datacenter fields in cassandra-rackdc.properties file and rolling restart cassandra with this flag "-Dcassandra.ignore_dc=true -Dcassandra.ignore_rack=true"

2) Run nodetool repair --sequential and nodetool cleanup.

3) Rolling restart cassandra removing the flag  "-Dcassandra.ignore_dc=true -Dcassandra.ignore_rack=true"

Regards,
Pradeep

On Thu, Aug 23, 2018 at 10:53 PM, Pradeep Chhetri <pradeep@stashaway.com> wrote:
Hello,

I am currently running a 3.11.2 cluster in SimpleSnitch hence the datacenter is datacenter1 and rack is rack1 for all nodes on AWS. I want to switch to GPFS by changing the rack name to the availability-zone name and datacenter name to region name.

When I try to restart individual nodes by changing those values, it failed to start throwing the error about dc and rack name mismatch but gives me an option to set ignore_dc and ignore_rack to true to bypass it.

I am not sure if it is safe to set those two flags to true and if there is any drawback now or in future when i add a new datacenter to the cluster. I went through the documentation on Switching Snitches but didn't get much explanation.

Regards,
Pradeep











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Joshua Galbraith | Lead Software Engineer | New Relic