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posix_fadvise.2: Reorganize some text 

Details for various flags were hidden under NOTES.
Move them to DESCRIPTION, to make the details more
obvious.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2017-01-26 14:37:21 +13:00
parent dfbb4842f9
commit a6b80261c8
1 changed files with 35 additions and 35 deletions
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70
man2/posix_fadvise.2
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@ -78,12 +78,47 @@ The specified data will be accessed in random order.
.TP
.B POSIX_FADV_NOREUSE
The specified data will be accessed only once.
In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
same semantics as \fBPOSIX_FADV_WILLNEED\fP.
This was probably a bug; since kernel 2.6.18, this flag is a no-op.
.TP
.B POSIX_FADV_WILLNEED
The specified data will be accessed in the near future.
\fBPOSIX_FADV_WILLNEED\fP initiates a
nonblocking read of the specified region into the page cache.
The amount of data read may be decreased by the kernel depending
on virtual memory load.
(A few megabytes will usually be fully satisfied,
and more is rarely useful.)
.TP
.B POSIX_FADV_DONTNEED
The specified data will not be accessed in the near future.
\fBPOSIX_FADV_DONTNEED\fP 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.
Requests to discard partial pages are ignored.
It is preferable to preserve needed data than discard unneeded data.
If the application requires that data be considered for discarding, then
.I offset
and
.I len
must be page-aligned.
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
.BR fsync (2)
or
.BR fdatasync (2)
first.
.SH RETURN VALUE
On success, zero is returned.
On error, an error number is returned.
@ -134,41 +169,6 @@ default size for the backing device; \fBPOSIX_FADV_SEQUENTIAL\fP doubles
this size, and \fBPOSIX_FADV_RANDOM\fP disables file readahead entirely.
These changes affect the entire file, not just the specified region
(but other open file handles to the same file are unaffected).
\fBPOSIX_FADV_WILLNEED\fP initiates a
nonblocking read of the specified region into the page cache.
The amount of data read may be decreased by the kernel depending
on virtual memory load.
(A few megabytes will usually be fully satisfied,
and more is rarely useful.)
In kernels before 2.6.18, \fBPOSIX_FADV_NOREUSE\fP had the
same semantics as \fBPOSIX_FADV_WILLNEED\fP.
This was probably a bug; since kernel 2.6.18, this flag is a no-op.
\fBPOSIX_FADV_DONTNEED\fP 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.
Requests to discard partial pages are ignored.
It is preferable to preserve needed data than discard unneeded data.
If the application requires that data be considered for discarding, then
.I offset
and
.I len
must be page-aligned.
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
.BR fsync (2)
or
.BR fdatasync (2)
first.
.SS C library/kernel differences
The name of the wrapper function in the C library is
.BR posix_fadvise ().