Add data nodes in peak time

Hi,

I have a self-managed Elasticsearch cluster with 3 dedicated master nodes, in peak time I would simply add data nodes to increase cluster throughput. And I don't think this action would bring any side affects to the existing cluster.

However, when I gonna to scale up another cluster hosted by cloud service. The service advisor told me that adding data nodes in peak time would make the cluster unstable. Can anyone tell me what kinds of side effect would happen in that action?

Forwarding any answer

When you add nodes the existing shard allocation will change. That may have an impact depending on how many, and how large the shards are.