Monday, 19 November 2012

Beware !!! Danger ahead "sql_log_bin"...

Some days ago, one of our clients reported that data for present day then, was missing from the slaves, upon querying the table data, we were taken by surprise to find that none of the data was replicated to the slaves, none of the slaves. The large slave lag cleared in seconds, which was not the case earlier with the client generating more than a Gig of data daily. 

To start the investigation we started checking if the relay log is writing the event data properly, checking the relay we saw that the latest data was absent from that. Strange. It was strange because, none of the problems occurred on the slave or master which would have caused the issue, the "exec_master_log_pos" wasn't changing anymore.

The only thing happened the prior weekend was a table conversion. We had a adequately large, almost 40G table to convert to InnoDB, the client needed timelines for that, hence we started the process on one of the slaves. We got the timeline, client was OK with it, then we proceeded with the master's table. Just as obvious we didn't want this statement to replicate to the slaves, hence asked the upcoming team to turn off "sql_log_bin" before starting the alter. 

Boom !!! We had done this before, using sql_log_bin was a cake walk on 5.0 and 5.1, but we were on a Percona 5.5, also we had executed "SET GLOBAL sql_log_bin=OFF", had it been executed on 5.1 or 5.0, this would have returned an error, since its a "SESSION" variable. However in mysql 5.5 this became a "GLOBAL" and "SESSION" variable, all the threads executing DMLs on the servers were not logged in binary logs. The replication had no issues, however it became a pain when we discovered it, and that was almost after a day. 

We had no choice but to refresh the slaves, but the data was huge 1.2T, after 3 days of boredom, we finally had the slaves in sync.

If there was a "Remove feature request" in MySQL or Percona, I would strongly recommend removing the global scope of this configuration.

As an advice, make a habit of using "SESSION" while turning ON or OFF such params or even while viewing the server STATUS.

Happy Replicating...

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