Dynamic change to innodb_flush_log_at_trx_commit

Question

This is related to this question. It does help to get better performance for InnoDB tables.

According to MySQL manual, innodb_flush_log_at_trx_commit is a global dynamic variable. Thus, I can change it using SET GLOBAL command and it seems to be working.

mysql> SET GLOBAL innodb_flush_log_at_trx_commit=2;
Query OK, 0 rows affected

mysql> SHOW VARIABLES LIKE 'innodb_flush_log_at_trx_commit';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_flush_log_at_trx_commit | 2     |
+--------------------------------+-------+
1 row in set

But, it did not make the actual MySQL setting changed. When I updated my.cnf and restarted the MySQL server, it did work. So, I cannot change the global variable at run time?

I prefer the default value innodb_flush_log_at_trx_commit=1, but I need to change it to 2 before I run a restore process for a large database to get faster. But when the process done, I want to change the value back to 1. Is it possible to do this at run time?

I don't have access to my.cnf on my shared hosting server.

Answer 1

While I agree with Rolando's recommendation to change innodb_flush_method, I wasn't 100% clear what you meant by:

it did not make the actual MySQL setting changed

I want to point out the caveat that making a change to the GLOBAL variable affects any new connections, but does not modify the current session (emphasis mine):

The global variable change does not affect the session variable for any client that is currently connected (not even that of the client that issues the SET GLOBAL statement).

So to check that:

mysql> SHOW GLOBAL VARIABLES LIKE 'innodb_flush_log%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
1 row in set (0.00 sec)


mysql> SHOW SESSION VARIABLES LIKE 'innodb_flush_log%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
1 row in set (0.00 sec)

mysql> SET GLOBAL innodb_flush_log_at_trx_commit=2;
Query OK, 0 rows affected (0.00 sec)

mysql> SHOW GLOBAL VARIABLES LIKE 'innodb_flush_log%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_flush_log_at_trx_commit | 2     |
+--------------------------------+-------+
1 row in set (0.00 sec)

mysql> SHOW SESSION VARIABLES LIKE 'innodb_flush_log%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_flush_log_at_trx_commit | 1     |
+--------------------------------+-------+
1 row in set (0.00 sec)

mysql> connect;
Connection id:    6
Current database: *** NONE ***

mysql> SHOW SESSION VARIABLES LIKE 'innodb_flush_log%';
+--------------------------------+-------+
| Variable_name                  | Value |
+--------------------------------+-------+
| innodb_flush_log_at_trx_commit | 2     |
+--------------------------------+-------+
1 row in set (0.00 sec)

Answer 2

By setting innodb_flush_log_at_trx_commit, you risk confusion with mysqld/OS interoperability. I say this because the OS is being trusted to perform the flush.

Note the caution in from the MySQL Documentation

Many operating systems and some disk hardware fool the flush-to-disk operation. They may tell mysqld that the flush has taken place, even though it has not. Then the durability of transactions is not guaranteed even with the setting 1, and in the worst case a power outage can even corrupt the InnoDB database. Using a battery-backed disk cache in the SCSI disk controller or in the disk itself speeds up file flushes, and makes the operation safer. You can also try using the Unix command hdparm to disable the caching of disk writes in hardware caches, or use some other command specific to the hardware vendor.

What this tells is the following: The OS can lie like a cheating husband. The OS says it will flush to disk and simply does not do it. Therefore, even if you set innodb_flush_log_at_trx_commit, you must divorce the OS flushing to disk from mysqld's flushing to disk.

Try setting innodb_flush_method to O_DIRECT if you haven't already done so. You may see a difference because the flush method differs greatly (See my Mar 04, 2011 post Clarification on MySQL innodb_flush_method variable ).

CAVEAT

As you mentioned, you do not have access to my.cnf. Please contact the SysAdmin at your provider and get innodb_flush_method changed.

UPDATE 2012-12-10 12:45 EDT

I am current running MySQL 5.5.12 on my PC. When I connect and run show variables like 'innodb_flush_method'; I get

mysql> show variables like 'innodb_flush_method';
+---------------------+-------+
| Variable_name       | Value |
+---------------------+-------+
| innodb_flush_method |       |
+---------------------+-------+
1 row in set (0.05 sec)
mysql>

Since it is blank, it just indicates that the default setting is used. Please read my Mar 04, 2011 post for Clarification on MySQL innodb_flush_method variable