When a transaction writes data, it always inserts them in the table indexes or data (in the buffer pool or in physical files). No private copies are created. The old versions of data being modified by active XtraDB/InnoDB transactions are stored in the undo log. The original data can then be restored, or viewed by a consistent read.
Before a row is modified, it is copied into the undo log. Each normal row contains a pointer to the most recent version of the same row in the undo log. Each row in the undo log contains a pointer to previous version, if any. So, each modified row has an history chain.
Rows are never physically deleted until a transaction ends. If they were deleted, the restore would be impossible. Thus, rows are simply marked for deletion.
Each transaction uses a view of the records. The transaction level determines how this view is created. For example, READ UNCOMMITTED usually uses the current version of rows, even if they are not committed (dirty reads). Other isolation levels require that the most recent committed version of rows is searched in the undo log. READ COMMITTED uses a different view for each table, while REPEATABLE READ and SERIALIZABLE use the same view for all tables.
There is also a global history list of the data. When a transaction is committed, its history is added to this history list. The order of the list is the chronological order of the commits.
The purge thread deletes the rows in the undo log which are not needed by any existing view. The rows for which a most recent version exists are deleted, as well as the delete-marked rows.
If InnoDB needs to restore an old version, it will simply replace the newer version with the older one. When a transaction inserts a new row, there is no older version. However, in that case, the restore can be done by deleting the inserted rows.
Understanding how the undo log works helps with understanding the negative effects long transactions.
Of course read-only transactions do not write more entries in the undo log; however, they delay the purging of existing entries.
Also, long transactions can more likely result in deadlocks, but this problem is not related to the undo log.
The undo log is usually part of the physical system tablespace, but from MariaDB 10.0, the innodb_undo_directory and innodb_undo_tablespaces system variables can be used to split into different tablespaces and store in a different location (perhaps on a different storage device).
Each insert or update portion of the undo log is known as a rollback segment. The innodb_undo_logs system variable specifies the number of rollback segments to be used per transaction.
The related innodb_available_undo_logs status variable stores the total number of available InnoDB undo logs.
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