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I turned innodb_flush_log_at_trx_commit = 2 and get a very fast write speed. But is it safe be used in production web site?

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3 Answers 3

up vote 15 down vote accepted

You can lose up to one second's worth of transactions. The default value is 1, which helps keep InnoDB ACID Compliant.

According to the MySQL Documentation on innodb_flush_log_at_trx_commit

If the value of innodb_flush_log_at_trx_commit is 0, the log buffer is written out to the log file once per second and the flush to disk operation is performed on the log file, but nothing is done at a transaction commit. When the value is 1 (the default), the log buffer is written out to the log file at each transaction commit and the flush to disk operation is performed on the log file. When the value is 2, the log buffer is written out to the file at each commit, but the flush to disk operation is not performed on it. However, the flushing on the log file takes place once per second also when the value is 2. Note that the once-per-second flushing is not 100% guaranteed to happen every second, due to process scheduling issues.

The default value of 1 is required for full ACID compliance. You can achieve better performance by setting the value different from 1, but then you can lose up to one second worth of transactions in a crash. With a value of 0, any mysqld process crash can erase the last second of transactions. With a value of 2, only an operating system crash or a power outage can erase the last second of transactions. InnoDB's crash recovery works regardless of the value.

For the greatest possible durability and consistency in a replication setup using InnoDB with transactions, use innodb_flush_log_at_trx_commit=1 and sync_binlog=1 in your master server my.cnf file.

Caution

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.

Based on this, values other than 1 put InnoDB at risk of losing 1 second's worth of transactions, or a transaction commit's worth of data.

The documentation also says use sync_binlog=1.

According to the MySQL Documentation on sync_binlog

A value of 1 is the safest choice because in the event of a crash you lose at most one statement or transaction from the binary log. However, it is also the slowest choice (unless the disk has a battery-backed cache, which makes synchronization very fast).

Your safest choice is

[mysqld]
innodb_flush_log_at_trx_commit=1
sync_binlog=1

If you do not mind possible data loss (up to 1 second's worth) then you can use either 0 or 2 at your own risk if the rewards (faster write speed) are worth it.

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Rolando :+1 for last few lines sync_binlog=1... –  Abdul Manaf Feb 10 '12 at 5:58

My opinion differs from other. innodb_flush_log_at_trx_commit = 0 if: it is my development computer or home mini database where is no sensitive data.

innodb_flush_log_at_trx_commit = 2 if: it is blog/stats/e-commerce (with ~100x shop in day), etc.

innodb_flush_log_at_trx_commit = 1 if: you have a lot of customers or you need to work with money transaction like bank. so this time you should split your dataflow between several servers to have speed & safety.

I prefer 2, because it has ~75x faster write speed and it fails ONLY if hardware fails.

Anyway you should know what you need more much more writing speed or up to 1 second information?

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The innodb_flush_log_at_trx_commit is used with the purpose as ..

If the value of innodb_flush_log_at_trx_commit is 0, the log buffer is written out to the log file once per second and the flush to disk operation is performed on the log file, but nothing is done at a transaction commit.

When the value is 1 (the default), the log buffer is written out to the log file at each transaction commit and the flush to disk operation is performed on the log file.

When the value is 2, the log buffer is written out to the file at each commit, but the flush to disk operation is not performed on it. However, the flushing on the log file takes place once per second also when the value is 2. Note that the once-per-second flushing is not 100% guaranteed to happen every second, due to process scheduling issues.

The default value of 1 is required for full ACID compliance. You can achieve better performance by setting the value different from 1, but then you can lose up to one second worth of transactions in a crash. With a value of 0, any mysqld process crash can erase the last second of transactions. With a value of 2, only an operating system crash or a power outage can erase the last second of transactions. InnoDB's crash recovery works regardless of the value.

In My opinion using innodb_flush_log_at_trx_commit to 2 should not be an issue.But to use 1 is the safest.

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I just noticed that you answered only 18 seconds after me with essentially the same answer. +1 !!! –  RolandoMySQLDBA Apr 24 '12 at 18:16

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