Bug #118051 Semi-Sync Replication Relay Log Flushing Discrepancy

• Submitted: 24 Apr 10:51
• Modified: 5 May 11:35
• Reporter: Yunus UYANIK
• Status: Not a Bug
• Category: MySQL Server
• Severity: S3 (Non-critical)
• Version: 8.0
• OS: Any
• Assigned to: MySQL Verification Team
• CPU Architecture: Any
• Tags: semi-sync

[24 Apr 10:51] Yunus UYANIK

Description:
The official documentation states: https://dev.mysql.com/doc/refman/8.0/en/replication-semisync.html#:~:text=The%20replica%20....

"The replica acknowledges receipt of a transaction's events only after the events have been written to its relay log and flushed to disk."

However, examination of the source code reveals that:

The semi-sync plugin (semisync_replica.cc) sends acknowledgment after writing to the relay log but before an explicit fsync. There is no direct fsync operation in the semi-sync acknowledgment path.

Code Path: https://github.com/mysql/mysql-server/blob/ff05628a530696bc6851ba6540ac250c7a059aa7/plugin...

The acknowledgment is sent in ReplSemiSyncSlave::slaveReply() (semisync_replica.cc) after flush() but without explicit fsync

Documentation implies fsync-before-ack, but implementation shows ack-before-fsync

How to repeat:
Codes and docs provided.

Suggested fix:
Fix needed to be done depending on which part is broken docs or codes.

[24 Apr 14:25] Jervin Real

There is nothing implied in the documentation IMHO, it only flushes, no fsync. Doc and code aligns.

[5 May 11:35] MySQL Verification Team

This is not a bug.

Code does flush as described in the documentation.