Michał Górny Michał Górny - 1 month ago 11
C Question

What posix_fadvise() args for sequential file write?

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

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()).

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