is it possible to splunk 6 version to version 9 rolling upgrade?
ii is not possible. Rolling update option comes to splunk on version 7.x not in 6.
You must check from installation/update guide to which versions you must go first before you could update to 9. If I recall right you need to use at least two middle versions. Firstly you must do offline upgrades and start splunk with those middle versions later you could do rolling updates.
is it possible rolling upgrade 6 -> 7
you mean version 6 can't upgrade to 9?
You must do offline upgrade first to 7.1.0 or newer version as rolling update has presented in 7.1.0. https://docs.splunk.com/Documentation/Splunk/9.0.3/Indexer/Searchablerollingupgrade
After that you can perform rolling updates to the newer version as described on manual. BUT you cannot do it directly from 7 to 9! Instead of it you must do those intermediate updates first.