We had a 6.8.3 single node with a lot of data (yes, I know that offers no fault tolerance but its a known limitation) and I tried to upgrade it to 7.4.2.
We followed the step by step in https://www.elastic.co/guide/en/elasticsearch/reference/current//rolling-upgrades.html but when we started the node again it resulted in the node being elected as master since I set discovery.type: single-node but then it fails to apply the cluster state.
Since the I tried every option I could think of including downgrading to 7.3.0 and 7.2.0 but the log messages for the failure are always the same and the cluster does not recover the indices.
The log messages since one start of the node are in the following link:
The config is the following (taking out commented lines):
The node has 16G in -Xxmx and -Xxms.
I really am trying not to have to loose data.
Thank you for any help in advance.