Programs can use posix_fadvise to announce an intention to access file
data in a specific pattern in the future, thus allowing the kernel to perform
The advice applies to a (not necessarily existent) region starting at
offset and extending for len bytes (or until the end of the file
if len is 0) within the file referred to by fd. The advice is
not binding; it merely constitutes an expectation on behalf of the
Permissible values for advice include:
Indicates that the application has no advice to give about
its access pattern for the specified data. If no advice is given for an
open file, this is the default assumption.
The application expects to access the specified data
sequentially (with lower offsets read before higher ones).
The specified data will be accessed in random order.
The specified data will be accessed only once.
The specified data will be accessed in the near
The specified data will not be accessed in the near
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. These changes
affect the the entire file, not just the specified region (but other open file
handles to the same file are unaffected).
POSIX_FADV_WILLNEED and POSIX_FADV_NOREUSE both initiate a
non-blocking read of the specified region into the page cache. The amount of
data read may be decreased by the kernel depending on VM load. (A few
megabytes will usually be fully satisfied, and more is rarely useful.)
POSIX_FADV_DONTNEED attempts to free cached pages associated with the
specified region. This is useful, for example, while streaming large files. A
program may periodically request the kernel to free cached data that has
already been used, so that more useful cached pages are not discarded instead.
Pages that have not yet been written out will be unaffected, so if the
application wishes to guarantee that pages will be released, it should call
fsync or fdatasync first.