An undo log is a collection of undo log records associated with a single read-write transaction. An undo log record contains information about how to undo the latest change by a transaction to a clustered index record. If another transaction needs to see the original data as part of a consistent read operation, the unmodified data is retrieved from undo log records. Undo logs exist within undo log segments, which are contained within rollback segments. By default, rollback segments are physically part of the system tablespace, but they can also reside in undo tablespaces. For more information, see Section 14.6.3.3, “Undo Tablespaces”.

InnoDB supports 128 rollback segments. The innodb_rollback_segments variable defines the number of rollback segments used by InnoDB.

The number of transactions that a rollback segment supports depends on the number of undo slots in the rollback segment and the number of undo logs required by each transaction.

The number of undo slots in a rollback segment differs according to InnoDB page size.

A transaction is assigned up to two undo logs, one for each of the following operation types:

Undo logs are assigned as needed. For example, a transaction that performs INSERT, UPDATE, and DELETE operations is assigned two undo logs. A transaction that performs only INSERT operations is assigned a single undo log. Undo logs are assigned to a transaction from a rollback segment that is also assigned to the transaction.

An undo log assigned to a transaction remains tied to the transaction for its duration. For example, an undo log assigned to a transaction for an INSERT operation is used for all INSERT operations performed by that transaction.

Given the factors described above, the following formulas can be used to estimate the number of concurrent read-write transactions that InnoDB is capable of supporting.