The InnoDB rollback segment could be a bottleneck on high-capacity systems, because it can handle a maximum of 1023 concurrent transactions that change any data. (Read-only transactions do not count against that maximum.) Starting in InnoDB 1.1 with MySQL 5.5, the limit on concurrent transactions is greatly expanded. The single rollback segment is divided into 128 segments, each of which can support up to 1023 transactions that perform writes, for a total of approximately 128K concurrent transactions. 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 to InnoDB 1.1, or some time afterward. InnoDB makes the required changes inside the system tablespace automatically, the first time you restart after performing a slow shutdown.

For more information about performance of InnoDB under high transactional load, see Section 7.5.2, “Optimizing InnoDB Transaction Management”.