Take the 2-minute tour ×
Database Administrators Stack Exchange is a question and answer site for database professionals who wish to improve their database skills and learn from others in the community. It's 100% free, no registration required.

I would like to restart mysql gracefully just like httpd where threads are served before restart. I would dislike queries breaking.

share|improve this question
    
Short answer, no. –  Roy Feb 25 '13 at 19:50

1 Answer 1

up vote 6 down vote accepted

Any "requested" shutdown sequence in MySQL (short of kill -9) is somewhat graceful, since transactions in progress (on transactional tables) are rolled back, but here are a couple of ways to make a restart as clean as possible.

Note: if you are shutting down the server for an upgrade, then don't use this process; instead, follow the process detailed in this answer.

Otherwise, if you're just restarting an otherwise-healthy server so that you can change a read-only global variable or something similar, here is a graceful path:

First, enable innodb_fast_shutdown if it isn't already. This isn't directly related to the gracefulness of the shutdown, but it should bring your server back faster.

mysql> SHOW VARIABLES LIKE 'innodb_fast_shutdown';
+----------------------+-------+
| Variable_name        | Value |
+----------------------+-------+
| innodb_fast_shutdown | 0     |
+----------------------+-------+
1 row in set (0.00 sec)

mysql> SET GLOBAL innodb_fast_shutdown = 1;
Query OK, 0 rows affected (0.01 sec)

Next, instruct the server to close all open tables as soon as no currently-running queries are referencing them. This step also has nothing to do with the graceful shutdown, but it will make the subsequent step go faster:

mysql> FLUSH LOCAL TABLES;
Query OK, 0 rows affected (41.12 sec)

The FLUSH TABLES statement (with the optional LOCAL keyword, which avoids an unnecessary but otherwise harmless flush of any slaves) will block and your prompt won't return until all of the tables can be closed. Once each table has been "flushed" (closed), if a query subsequently references the table, it will be automatically reopened, but that's okay. What we're accomplishing with this step is making less work for the final step:

mysql> FLUSH TABLES WITH READ LOCK;
Query OK, 0 rows affected (13.74 sec)

mysql>

This statement flushes all tables (hence the advantage of getting some of that out of the way less disruptively with the prior step) and acquires a global (server-wide) read-only lock on them.

You can't have a global read lock until every currently running "write" query (i.e., pretty much everything but SELECT) is done. Issuing the lock request will allow existing queries to finish but won't allow new ones to start.

Your prompt doesn't return until you hold this global lock, so every query that is in progress when you request the lock is able to finish, and you know they're finished, because you get the prompt back. Any subsequent queries that try to write anything to any table will just stall, changing no data, waiting indefinitely for the lock, until...

  • you change your mind about the restart and release the lock manually (UNLOCK TABLES;)
  • you restart the server, or
  • you accidentally or intentionally disconnect the command line client from this thread (so don't do that). Keep this window connected and sitting at the mysql prompt:

Resist the temptation to close this.

mysql>

This idle console prompt is what's holding the global lock for you. Lose this, lose the lock.

From another console window, restart MySQL the way you normally would, either with initscripts (e.g., your local variant of service mysql.server restart) or with mysqladmin shutdown followed by a manual restart.

share|improve this answer
    
Fantastic reply, thanks! –  giorgio79 Feb 26 '13 at 5:28

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.