elasticsearch upgrade is not going to work for following without a patch work:
v2.3.4 -> v5.6.0 -> v6.x
Reason:
The default datapath in v2.3.4 was '/var/lib/elasticsearch/elasticsearch/nodes'
The default datapath in 6.x is '/var/lib/elasticsearch/nodes'
When we follow upgrade sequence then after coming to 6.x the data files are still kept at '/var/lib/elasticsearch/elasticsearch/nodes' but the 6.x version is expecting to have data in '/var/lib/elasticsearch/nodes'. If I manually copy the data from '/var/lib/elasticsearch/elasticsearch/nodes' to '/var/lib/elasticsearch/nodes' before starting elasticsearch (v6.x) and the start elasticsearch then the data is available.
Please fix this issue in coming 6.x release. Fix is while instaling 6.x rpm check if any data is present in '/var/lib/elasticsearch/elasticsearch/nodes'. If yes move it to '/var/lib/elasticsearch/nodes'.