Hi Alex,

Regarding your below point the admin need to take care of temporary uneven distribution of data util the entire process is done:

"If you can't, then I guess you can for each node (one at a time), decommission it, wipe it clean and re-bootstrap it after setting the appropriate rack."

I believe while doing so in the existing single rack cluster, the first new node joined with different rack (rac2) will get 100% loaded in terms so disk usage will be proportionally very high in comparison to other nodes in rac1. 
So until both racks have equal number of nodes and we run nodetool cleaup, the data will not be equally distributed.
 

On Wed, Mar 6, 2019 at 5:50 PM Alexander Dejanovski <alex@thelastpickle.com> wrote:
Hi Manish,

the best way, if you have the opportunity to easily add new hardware/instances, is to create a new DC with racks and switch traffic to the new DC when it's ready (then remove the old one). My co-worker Alain just wrote a very handy blog post on that technique : http://thelastpickle.com/blog/2019/02/26/data-center-switch.html

If you can't, then I guess you can for each node (one at a time), decommission it, wipe it clean and re-bootstrap it after setting the appropriate rack.
Also, take into account that your keyspaces must use the NetworkTopologyStrategy so that racks can be taken into account. Change the strategy prior to adding the new nodes if you're currently using SimpleStrategy.

You cannot (and shouldn't) try to change the rack on an existing node (the GossipingPropertyFileSnitch won't allow it).

Cheers,

On Wed, Mar 6, 2019 at 12:15 PM manish khandelwal <manishkhandelwal03@gmail.com> wrote:

We have a 6 node Cassandra cluster in which all the nodes  are in same rack in a dc. We want to take advantage of "multi rack" cluster (example: parallel upgrade on all the nodes in same rack without downtime). I would like to know what is the recommended process to change an existing cluster with single racks configuration to multi rack configuration.


I want to introduce 3 racks with 2 nodes in each rack.

 

Regards
Manish

--
-----------------
Alexander Dejanovski
France
@alexanderdeja

Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com


--

regards,
Laxmikant Upadhyay