Upgrade from squeeze?
 

Hi,
I have few legacy production servers with Debian 6.0.
I wondering if it is actually possible to upgrade to a more recent Debian version, or should I consider complete reinstall and migration.

Thanks

Complete reinstall. You cannot upgrade directly from 6.0 to 11. The upgrade sequence would be 6.0 → 7 → 8 → 9 → 10 → 11. The time and effort this will require is definitely more than complete reinstall.

And that's not talking about what time all the necessary checks and clean-ups after each upgrade step will take. As well as reading release notes and taking all the necessary precautions before each upgrade step.

Re install - there were some very awkward changes with the Buster release, if I remember right, which made the usual upgrade sequence break some of it, anyway Debian suggested a complete re installation.