unbuffered I/O in Linux

Question

I\'m writing lots and lots of data that will not be read again for weeks - as my program runs the amount of free memory on the machine (displayed with \'free\' or \'top\') drops

Answer 1

The closest equivalent to the Windows flags you mention I can think of is to open your file with the open(2) flags O_DIRECT | O_SYNC:

   O_DIRECT (Since Linux 2.4.10)
          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.  The O_DIRECT flag on its
          own makes at an effort to transfer data synchronously, but does not
          give the guarantees of the O_SYNC that data and necessary metadata are
          transferred.  To guarantee synchronous I/O the O_SYNC must be used in
          addition to O_DIRECT.  See NOTES below for further discussion.

          A semantically similar (but deprecated) interface for block devices is
          described in raw(8).

Granted, trying to do research on this flag to confirm it's what you want I found this interesting piece telling you that unbuffered I/O is a bad idea, Linus describing it as "brain damaged". According to that you should be using madvise() instead to tell the kernel how to cache pages. YMMV.