WL#8843: Deprecate and remove the parameter innodb_support_xa
Affects: Server-8.0 — Status: Complete — Priority: Medium
When support for two-phase commit (XA 2PC) was introduced in InnoDB in MySQL 5.0, the session parameter innodb_support_xa=OFF was introduced to disable the functionality, due to perceived overhead in the InnoDB undo log. The default value of innodb_support_xa was ON already in May 2007 (built-in InnoDB in MySQL 5.1). This parameter is fundamentally broken. (1) When innodb_support_xa=OFF in the current session, the XA PREPARE statement will be silently ignored. That is, if the server is killed after the XA PREPARE completed, the transaction would be rolled back by InnoDB; there is no way to XA COMMIT it after restarting the server. (2) The MySQL binlog internally uses XA 2PC to make itself consistent with the InnoDB undo logs. When innodb_support_xa=OFF, this logic will be broken as well. This prevents the performance gain of group commit. This worklog will deprecate the parameter innodb_support_xa in MySQL 5.7, so that XA 2PC will always be available for InnoDB. Setting the parameter to either ON or OFF will generate a deprecation warning, and setting it to OFF will generate an additional warning that it cannot be turned OFF. Internally, InnoDB will ignore the setting in 5.7 so that it is effectively always ON, so that XA 2PC will always be available for InnoDB. In MySQL 8.0, all code related to innodb_support_xa will be removed.
In 5.7.10: FR1: A deprecation warning is issued to the error log if --skip-innodb-support-xa or --innodb-support-xa=0 is specified on startup. FR2: A deprecation warning is issued to the client connection for any SET [GLOBAL] innodb_support_xa=…; FR3: For SET [GLOBAL] innodb_support_xa=OFF, a second message is issued that warns that this setting cannot be set to OFF since it makes replication unsafe and prevents the performance gain from group commit in the binlog. In 8.0: The setting is rejected and will prevent startup if found in the config file.
Removal of low-level code in 5.7 (rb#10114) and 5.8 (rb#10115): thd_supports_xa() trx_t::support_xa In trx0undo.cc, all 3 occurrences of if (trx->support_xa) will be removed, but the "if" body will be preserved. Deprecation in 5.7 (rb#10114): innodb_support_xa_update(): A new "update callback" function for SET [GLOBAL|SESSION] innodb_support_xa. Issue a deprecation warning when either the global or the session variable is set to ON or DEFAULT. Issue an additional warning when the variable is set to OFF, saying that it cannot be set to OFF. The only possible value will be ON. innobase_init(): Issue a deprecation warning if --skip-innodb-support-xa on startup. TBD: Can we distinguish --innodb-support-xa=1 from the default? Removal in 5.8 (rb#10115): The session variable innodb_support_xa will be removed, along with the 5.7 deprecation code mentioned in the previous section.
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