> This mode is what PostgreSQL uses by default. I don't know MySQL. Assuming the InnoDB storage engine, the default recently changed to Direct I/O as of MySQL

> This mode is what PostgreSQL uses by default. I don't know MySQL. Assuming the InnoDB storage engine, the default recently changed to Direct I/O as of MySQL 8.4, see https://dev.mysql.com/doc/refman/8.4/en/innodb-parameters.html#sysvar_innodb_flush_method https://dev.mysql.com/doc/refman/8.4/en/innodb-parameters.ht... It was common to set this even before it became the default though, since InnoDB maintains its own buffer pool caching scheme, and there's no sense in duplicating data pages between the InnoDB buffer pool and the OS cache. The general idea is that you should assign the majority of the system's memory to the InnoDB buffer pool, and then use direct I/O to bypass the system cache, since InnoDB can manage memory more intelligently than the OS. This is because InnoDB has a better understanding of your workload and its own data structures, vs the OS cache being more general-purpose. Other MySQL storage engines behave differently, for example MyISAM had its own caching only for indexes but relied on the OS cache for row data. (if I'm remembering correctly, that is; been a long while since I've touched MyISAM. It's a non-ACID engine and generally should not be used.)