InnoDB divides the allocation of undo logs into rollback segments. The DB_ROLL_PTR system column of clustered indexes can address up to 128 rollback segments. Originally, InnoDB only created one rollback segment. In MySQL 5.5 or in the InnoDB Plugin for MySQL 5.1, all 128 rollback segments were created.
MySQL 5.7 hard-codes the rollback segment IDs 1..32 for temporary undo logs. On upgrade, unless a slow shutdown (innodb_fast_shutdown=0) was performed on the old server instance, these rollback segments could be in use by transactions that are in XA PREPARE state or transactions that were left behind by a server kill followed by a normal shutdown immediately after restart.
Persistent tables cannot refer to temporary undo logs or vice versa. Therefore, we should keep two distinct sets of rollback segments: one for persistent tables and another for temporary tables. In this way, all 128 rollback segments will be available for persistent transactions, which could improve performance. Also, MariaDB 10.2 would remain more compatible than MySQL 5.7 with data files from earlier versions of MySQL or MariaDB.