Question

I am working on an application which does sequentially write a large file (and does not read at all), and I would like to use posix_fadvise() to optimize the filesystem behavior.

The function description in the manpage suggests that the most appropriate strategy would be POSIX_FADV_SEQUENTIAL. However, the Linux implementation description doubts that:

Under Linux, POSIX_FADV_NORMAL sets the readahead window to the default size for the backing device; POSIX_FADV_SEQUENTIAL doubles this size, and POSIX_FADV_RANDOM disables file readahead entirely.

As I’m only writing data (overwriting files possibly too), I don’t expect any readahead. Should I then stick with my POSIX_FADV_SEQUENTIAL or rather use POSIX_FADV_RANDOM to disable it?

How about other options, such as POSIX_FADV_NOREUSE? Or maybe do not use posix_fadvise() for writing at all?

Answer 1

It all depends on the temporal locality of your data. If your application won’t need the data soon after it was written, then you can go with POSIX_FADV_NOREUSE to avoid writing to the buffer cache (in a similar way as the O_DIRECT flag from open()).