Hi... I'm doing a rolling upgrade... overall, never had an issue with this..
The cluster is 7.6.1 and I'm going to latest 7.9.0
All nodes are identical machines: Debian stable machines... so far so good.
I tried upgrading on the very first node: as expected, following documentation, No problem ...
Doing upgrade on a second node ... error: EL doesn't start.
Both machines are data nodes, basically clones ... so I see absolutely no difference beyond naming, etc...
Error at log is:
[2020-09-02T13:05:25,077][ERROR][o.e.b.Bootstrap ] [elk-data09] node validation exception
 bootstrap checks failed
: JVM is using the serial collector but should not be for the best performance; either it's the default for the VM [OpenJDK 64-Bit Server VM] or -XX:+UseSerialGC was explicitly specified
[2020-09-02T13:05:25,093][INFO ][o.e.n.Node ] [elk-data09] stopping ...
no change done at jvm.options
both machines have same jvm.options file ... I've even compared with 'ps aux' command the running processes, they look the same
root@elk-data09:~# /usr/share/elasticsearch/jdk/bin/java -version
openjdk version "14.0.1" 2020-04-14
OpenJDK Runtime Environment AdoptOpenJDK (build 14.0.1+7)
OpenJDK 64-Bit Server VM AdoptOpenJDK (build 14.0.1+7, mixed mode, sharing)
NO other Java package present ... these are EL-dedicated machines...
What could I check?