advise the page cache about expected I/O patterns on the current file
Modifies kernel page cache behaviour when operating on the current file.
The range arguments are required by some advise commands ([*] below).
With no arguments, the POSIX_FADV_NORMAL advice is implied.
-d -- don't need these pages (POSIX_FADV_DONTNEED) [*]
-n -- data will be accessed once (POSIX_FADV_NOREUSE) [*]
-r -- expect random page references (POSIX_FADV_RANDOM)
-s -- expect sequential page references (POSIX_FADV_SEQUENTIAL)
-w -- will need these pages (POSIX_FADV_WILLNEED) [*]
Notes: these interfaces are not supported in Linux kernels before 2.6.
NORMAL sets the default readahead setting on the file.
RANDOM sets the readahead setting on the file to zero.
SEQUENTIAL sets double the default readahead setting on the file.
WILLNEED and NOREUSE are equivalent, and force the maximum readahead.
