How can we mount NFS mount on elastic operator based pods (Elasticsearch on Kubernetes)

Elastic Orchestration Elastic Cloud on Kubernetes (ECK)
2

I am not sure I fully understand your question. What do you mean by

There is no such restriction. You can mount anything to the pods as you have full access to the podTemplate.

But we don't recommend using NFS for the Elasticsearch data directory for performance reasons. Now you mention "backup" and I am not sure how to understand that. Two thoughts on that though:

  1. Elasticsearch has built-in redundancy if you configure your indices to have replicas that provide some form of "backup" in case a node fails.
  2. Elasticsearch comes with a snapshot mechanism that allows you together with Snapshot Lifecycle Management to create backups of your data in regular intervals. You can choose from a variety of storage solutions for the snapshot repository (AWS S3, Azure, Google Storage, HDFS, Openstack Swift)

Please also have a look this related answer from @sebgl that gives a good overview of possible approaches to storage on ECK: