What happens when mysql is restarted?

I tried to restart MySQL through WHM and now its taking way too long to restart, the progress bar is still showing.

What can I do?

I can still access the server via SSH but I cannot connect to MySQL. This is urgent as my online store is now down.

Will restarting the HTTP Server (Apache) help?

BenMorel

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asked Jan 10, 2017 at 21:50

2

Without seeing any additional information, here is my best guess why MySQL is trying to shutdown for a long time: I suspect mysqld no longer has a socket file.

About 1.5 years ago, I answered mysql restart issue after move database. I learned that mysqld depends on the presence of a socket file. If there is no socket, mysqld just draws lots of dots on the screen and then gives up.

In that old post, I wrote three instructions

• kill mysqld_safe
• kill mysqld
• start mysqld

Please go do this now ...

I have discussed MySQL behavior with the socket file (or lack of it) 4 years ago (See my older post mysql restart won't kill child processes on CentOS)

answered Jan 10, 2017 at 22:08

RolandoMySQLDBARolandoMySQLDBA

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3

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.

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