wsrep_sst_mariabackup purges the joiners data directory before copying back backup data.

A certain set of files is preserved, controlled by a set of regular expression terms in the "[sst] cpat" paramter.

Datadir subdirectories not under mysqld control (like "lost+found" on mount points), and listed as such with ignore-db-dir=... are still going to be removed though.

The directory names listed in ignore-db-dir options should be added to the sst:cpat ignore list.