MySQL, optimize and replication
Your First Question


  why does a OPTIMIZE query block the replication catching up ?


The SQL thread in MySQL Replication processes SQL commands from the Relay Logs as a FIFO queue (i.e. first-come, first-server basis). Once the SQL thread see a command, it processes it to completion. Meanwhile, the Seconds_Behind_Master just keeps creeping up.

While the SQL thread on the Slave is running OPTIMIZE TABLE, once you run STOP SLAVE; it SQL thread has complete the running OPTIMIZE TABLE before the SQL thread can terminate. That's why STOP SLAVE; hangs. To verify this, open another mysql session and run SHOW PROCESSLIST;, You should see a line that has OPTIMIZE TABLE with the status Killing slave....

Your Second Question


  can I prevent those OPTIMIZE queries ? I didn't put any cron to do that, and as far as I can see, nothing in /etc/cron.{d,daily,hourly,weekly} does this.


Someone or something is running the OPTIMIZE TABLE. You have to hunt it down in all your jobs or PHP apps. Use the mysqlbinlog utility to dumps all the binary logs into a text file. Then, grep the text file for OPTIMIZE TABLE or optimize table. The timestamp of when it was submitted into the binlogs will be a few lines about the command.

When you do find that OPTIMIZE TABLE command in your jobs or PHP code, simply run this:

SET SQL_LOG_BIN=0; OPTIMIZE TABLE tblname; SET SQL_LOG_BIN=1;


This will not register the OPTIMIZE TABLE in the binlogs, thus preventing a Slave from replicating it.

As was just pointed out in the comments below, You can also replace

OPTIMIZE TABLE tblname;


with one of the following:

OPTIMIZE NO_WRITE_TO_BINLOG TABLE tblname;
OPTIMIZE LOCAL TABLE tblname;


to restrict the OPTIMIZE TABLE command from landing in the binlogs and thus prevent replication of it.