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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 for applications (such as databases) that manage their own caches, as Ignite isdoes.

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.

Plugin works on Linux with the version of the kernel over higher than 2.4.2. This plugin switches the input of the output for durable (page) memory to use the mode Direct IO for files. If incompatible OS or FS is used, plugin has no effect and fallbacks to regular I/O implementation. Durable memory page size should be not less than physical device block and Linux system page size. Durable memory page size should be divisible by underlying OS and FS blocks sizes. Usually both sizes are 4Kbytes, so using default page size is usually sufficient to enable plugin.

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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 is 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.

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