You cannot back up an Elasticsearch cluster by simply copying the data directories of all of its nodes. Elasticsearch may be making changes to the contents of its data directories while it is running; copying its data directories cannot be expected to capture a consistent picture of their contents. If you try to restore a cluster from such a backup, it may fail and report corruption and/or missing files. Alternatively, it may appear to have succeeded though it silently lost some of its data. The only reliable way to back up a cluster is by using the snapshot and restore functionality.
To have a complete backup for your cluster:
To restore your cluster from a backup: