MySQL Bugs: #78790: Document the difference between native aio and background threads for InnoDB

Bug #78790	Document the difference between native aio and background threads for InnoDB
Submitted:	9 Oct 2015 17:48	Modified:	8 Mar 2017 22:34
Reporter:	Mark Callaghan	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: Documentation	Severity:	S3 (Non-critical)
Version:	5.6.26	OS:	Any
Assigned to:		CPU Architecture:	Any

Description:
Before InnoDB had the option to use native IO, the number of background threads were the limit to concurrent background IO requests. innodb_write_io_threads and innodb_read_io_threads limited the number of concurrent writes & reads done in the background. I don't think that is the case when native aio is used, well there is a limit but it is much larger. One of the much larger limits might be the per-thread array size. But if that is 256 and you set innodb_write_io_threads to 1 then you can still get 256 concurrent background writes and that is a lot.

It might not be good for disk read response time to intermittently submit 256 or 500 writes down to the IO scheduler.

Regardless, the docs don't have much today to explain this.

How to repeat:
read the docs, read fil0fil.cc, look at iostat avgqu-sz during write-intensive workloads

Suggested fix:
Update the docs to explain how to limit concurrent background IO requests when native aio is used.

http://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_write_io_threa...

http://dev.mysql.com/doc/refman/5.7/en/innodb-performance-multiple_io_threads.html

http://dev.mysql.com/doc/refman/5.7/en/innodb-parameters.html#sysvar_innodb_use_native_aio

Thank you for the bug report.

Posted by developer:
 
This blog post was published when the native AIO feature was introduced:

https://blogs.oracle.com/mysqlinnodb/entry/innodb_now_supports_native_aio

Posted by developer:
 
The following documentation was added:
https://dev.mysql.com/doc/refman/5.7/en/innodb-linux-native-aio.html

Thank you for the bug report.