[budabudimir]

It is skipping the buffer cache that is true, but, if I understand things correctly, it allows kernel to use user provided buffers directly, thus skipping the copying of the data from kernel to user land. That is why buffers used in O_DIRECT context have to be aligned properly.

It would be fun to run the experiment non the less.

From open(2) man pages on O_DIRECT: Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user- space buffers