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Direct I/O is a feature of the file system whereby file reads and writes go directly from the applications to the storage device, bypassing the operating system read and write caches (page cache). Direct I/O is useful by applications (such as databases) that manage their own caches, as Ignite is.

Direct I/O mode for file open enforces application to do block read/write operation, that means data having size less than block can't be written or loaded from disk.

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Enabled direct input-output mode allows Ignite to bypass the system cache pages Linux is fully conveys the management of pages to Ignite.

WAL and Native IO

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Write Ahead log does not have blocks(chunks/pages) as Durable Memory Page store has. So Direct IO currently can't be enabled for WAL logging. WAL always goes through the conventional system I/O calls. 

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This advice gives only recommendation to Linux system, "do not store the file data in the page cache, as data are not required". This results WAL data is still going to page cache first, but according this advice Linux will flush and remove these data during next page cache scan.

Direct I/O & Performance

Direct I/O can bring possible negative effects to performance of reading pages. In Direct I/O mode all pages are read bypassing Linux cache directly from the disk.

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