MariaDB 10.5 and the InnoDB that is no longer XtraDB

10.5 in June replaced XtraDB with upstream InnoDB, which is a change to the storage engine that most upgrade notes describe as an implementation detail.

# 10.4 and earlier
MariaDB [(none)]> SHOW ENGINES;
| XtraDB | DEFAULT |

# 10.5
| InnoDB | DEFAULT |

# and the variables that went with it:
#   innodb_buffer_pool_instances    removed
#   innodb_log_files_in_group       removed
#   several XtraDB-specific tunables — gone, silently ignored

Removed variables in my.cnf are ignored rather than rejected, so a configuration carefully tuned over years quietly stops applying half of itself. Checking SHOW VARIABLES against the file after the upgrade is the step that catches it. The performance difference is small in either direction and the real reason to know about this is that copied tuning advice from before 10.5 may be configuring something that no longer exists.