Is the InnoDB flush method O _ DIRECT or O _ dsync?
Where InnoDB data and log files are located on a SAN, it has been found that setting innodb_flush_method to O_DIRECT can degrade performance of simple SELECT statements by a factor of three. Referring to High performance MySQL Ver 2 and 3, it stated that InnoDB developers found bugs with using innodb_flush_method=O_DSYNC.
How does MySQL InnoDB flush method affect performance?
mysql innodb:innodb_flush_method. it says:Different values of this variable can have a marked effect on InnoDB performance. For example, on some systems where InnoDB data and log files are located on a SAN, it has been found that setting innodb_flush_method to O_DIRECT can degrade performance of simple SELECT statements by a factor of three.
Is there a way to skip fsync in InnoDB?
O_DIRECT_NO_FSYNC: InnoDB uses O_DIRECT during flushing I/O, but skips the fsync () system call afterward. This setting is suitable for some types of file systems but not others. For example, it is not suitable for XFS.
Why does MySQL still use fsync ( ) to flush the data?
However, MySQL offers the option from v5.6.7 to configure MySQL (well, innodb) tailored to your own OS’ capabilities in this aspect by adding O_DIRECT_NO_FSYNC option. O_DIRECT skips OS cache but it does not ensure that data is persisted on disk. O_DIRECT writes only to drive write cache.
What’s the difference between fsync and fdatasync in InnoDB?
Referring to High performance MySQL Ver 2 and 3, it stated that InnoDB developers found bugs with using innodb_flush_method=O_DSYNC. O_SYNC and O_DSYNC are similar to fsync () and fdatasync (): O_SYNC syncs both data and metadata, whereas O_DSYNC syncs data only. If that all seemed like a lot of explanation with no advice, here’s the advice:
What does the variable InnoDB _ flush _ method do?
innodb_flush_method variable specifies how InnoDB opens and flushes log and data files. In Innodb optimization, setting the variable innodb_flush_method tweaks the performance most of the times but there are cases of otherwise though.
How to reduce the iowait of InnoDB flush?
In my configuration innodb_flush_method=O_DSYNC from O-DIRECT reduces about 75% the iowait, and accordingly this the load. Should I set another variables besides innodb_flush_method to reduce more the iowait?
How to apply direct I / O to InnoDB files?
To apply direct I/O only to InnoDB file operations rather than the whole file system, set innodb_flush_method = O_DIRECT. With this setting, InnoDB calls directio () instead of fcntl () for I/O to data files (not for I/O to log files).