Documentation needed on downgrading from 10.2 to 10.1

Hi,

We need a section about upgrading/downgrading from/to MariaDB 10.2 and 10.3.

On upgrading, both 10.2 and 10.3 create a new version of the redo log. This makes it impossible to binary downgrade. The process to downgrade is:

• SET GLOBAL innodb_purge_rseg_truncate_frequency = 1;SET GLOBAL innodb_fast_shutdown=0; if downgrading from 10.3. We need to empty the undo log, because 10.3 introduces new undo log record types. Beware of MDEV-11802 and MDEV-13603; this will not always empty the undo logs!
• Shut down the server cleanly (no crash)
• Remove the redo logs: ib_logfile* (not needed when downgrading to 10.2 after MDEV-14909)
• Downgrade

Downgrading from 10.1 to 10.0 should work unless you use new 10.1 features (encryption, custom tablespaces).

When downgrading from 10.2, modifying the tables, and upgrading back to 10.2 or later, the MDEV-6076 persistent AUTO_INCREMENT counters may cause surprises, because they would not be updated by the old version.

Downgrading from 10.3 to 10.2 should work. What happens when trying to access SEQUENCE objects should be tested and documented (or bug filed if it would crash). If MDEV-11369 instant ADD COLUMN has been used, those tables will be unaccessible by earlier versions of MariaDB, but should not cause a crash in 10.2, hopefully also not in earlier versions.