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> In database land, most databases open(2)Linux their WAL and data files with O_DIRECT

Sounds surprising. Which ones? PG and Sqlite don't. MySQL maybe does. Redis and MongoDB apparently don't from a quick skim.

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I'll rephrase the line sometime, but the intention is to communicate that it's the default choice to go with if you're making this decision in 2024. Most of the projects which aren't using some form of direct IO are doing it because they predate O_DIRECT (e.g. postgres), didn't architect for using async IO (e.g. mongo), or have excessive portability concerns which means they only offer it as an option (e.g. innodb).

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