In this blog post, we are going to talk about the variable InnoDB_flush_log_at_trx_commit. We are going to discuss what each value that we can assign to this variable means, and how it can affect performance and durability.
Innodb_flush_log_at_trx_commit controls the durability in ACID compliance
A – atomicity
C – consistency
I – isolation
D – durability
The possible values for this variable is 0, 1, and 2.
The specific path of the query goes to the innodb buffer pool, then log buffer (redo logs), then OS buffer, then finally to log file.
When innodb flush log at transaction commit is set to 0, your write goes thru memory, thru the buffer pool into the log buffer. That write then flushes from the log buffer to the log file on disk for every 1 second or when the OS flushes.
If this variable is set to 1, which is maximum durability. Your write goes to the log buffer, but the commit of the file ensures that it is written all the way on disk. This value will have a bit of performance hit compared to value 0.
If the value is set to 2, the write goes to the log buffer, but the file will be committed all the way to the OS buffer. And then the OS will flush to disk roughly every 1 second.
Advantages and Disadvantages
0 – when the database crashes, the log buffer within memory will get loss, and there is a possibility of losing those transaction. This setting is for performance, but not for durability.
1 – every write will surely be written to the redo log on disk. You will not lose that write regardless of the crash.
2 – You will lose only about 25% performance as compared to 1. If the DB crashes, the file is still written to disk cache, and then written to disk later. But if the DB server itself crashes, that DB server’s disk buffer may lose its data. This can be prevented though if we have battery backup or SAN.
