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 126.96.36.199, “Undo Tablespaces”.
InnoDB supports 128 rollback segments. The
defines the number of rollback segments used by
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.
|InnoDB Page Size||Number of Undo Slots in a Rollback Segment (InnoDB Page Size / 16)|
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
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 attached to the
transaction for its duration. For example, an undo log assigned to
a transaction for an
operation is used for all
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
InnoDB is capable of supporting.
It is possible to encounter a concurrent transaction limit error
before reaching the number of concurrent read-write transactions
InnoDB is capable of supporting. This
occurs when the rollback segment assigned to a transaction runs
out of undo slots. In such cases, try rerunning the transaction.
(innodb_page_size / 16) * innodb_rollback_segments
(innodb_page_size / 16 / 2) * innodb_rollback_segments