Better Scalability with Multiple Rollback Segments
Starting in InnoDB 1.1 with MariaDB 5.5, the limit on concurrent transactions is greatly expanded, removing a bottleneck with the InnoDB rollback segment that affected high-capacity systems. The limit applies to concurrent transactions that change any data; read-only transactions do not count against that maximum.
The single rollback segment is now divided into 128 segments, each of which can support up to 1023 transactions that perform writes, for a total of approximately 128K concurrent transactions. The original transaction limit was 1023.
Each transaction is assigned to one of the rollback segments, and remains tied to that rollback segment for the duration. This enhancement improves both scalability (higher number of concurrent transactions) and performance (less contention when different transactions access the rollback segments).
To take advantage of this feature, you do not need to create any new database or tables, or reconfigure anything. You must do a slow shutdown before upgrading from MariaDB 5.1 or earlier, or some time afterward. InnoDB makes the required changes inside the system tablespace automatically, the first time you restart after performing a slow shutdown.
If your workload was not constrained by the original limit of 1023 concurrent transactions, you can reduce the number of rollback segments used within a MariaDB instance or within a session by setting the configuration option innodb_rollback_segments.
For more information about performance of InnoDB under high transactional load, see , "Optimizing InnoDB Transaction Management".