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fcntl.2: Describe the set/get write hints commands that are added in Linux 4.13 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
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Jens Axboe 2017-08-26 02:55:07 +02:00 committed by Michael Kerrisk
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@ -5,6 +5,7 @@
.\" and Copyright (C) 2002-2010, 2014 Michael Kerrisk;
.\" and Copyright (C) 2014 Jeff Layton
.\" and Copyright (C) 2014 David Herrmann
.\" and Copyright (C) 2017 Jens Axboe
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
@ -61,6 +62,8 @@
.\" Document F_SETPIPE_SZ and F_GETPIPE_SZ.
.\" 2014-07-08, David Herrmann <dh.herrmann@gmail.com>
.\" Document F_ADD_SEALS and F_GET_SEALS
.\" 2017-06-26, Jens Axboe <axboe@kernel.dk>
.\" Document F_{GET,SET}_RW_HINT and F_{GET,SET}_FILE_RW_HINT
.\"
.TH FCNTL 2 2017-05-03 "Linux" "Linux Programmer's Manual"
.SH NAME
@ -1490,6 +1493,61 @@ Furthermore, if there are any asynchronous I/O operations
.RB ( io_submit (2))
pending on the file,
all outstanding writes will be discarded.
.SS File read/write hints
Write life time hints can be used to inform the kernel about the relative
expected life time of writes on a given inode or file. An application may
use this interface for separating writes into different write classes,
so that multiple users or applications running on a single storage back-end
can aggregate their IO patterns in a consistent manner. However, there are
no functional semantics implied by these flags, and different IO classes
can use the write life time hints in arbitrary ways so long as they are used
consistently.
.TP
.BR F_GET_RW_HINT " (\fIuint64_t\fP; since Linux 4.13)"
Returns the value of the read/write hint associated with the underlying inode
for the file descriptor.
.TP
.BR F_SET_RW_HINT " (\fIuint64_t\fP; since Linux 4.13)"
Sets the read/write hint value associated with the underlying inode for the
file descriptor.
.TP
.BR F_GET_FILE_RW_HINT " (\fIuint64_t\fP; since Linux 4.13)"
Returns the value of the read/write hint associated with the file descriptor.
.TP
.BR F_SET_FILE_RW_HINT " (\fIuint64_t\fP; since Linux 4.13)"
Sets the read/write hint value associated with the file descriptor.
.P
If a given file descriptor has not been assigned a read/write hint, then it
shall use the value assigned to the inode, if any. The following read/write
hints are valid since kernel 4.13:
.TP
.BR RWH_WRITE_LIFE_NOT_SET
No specific hint has been set. This is the default value.
.TP
.BR RWH_WRITE_LIFE_NONE
No specific write life time is associated with this file or inode.
.TP
.BR RWH_WRITE_LIFE_SHORT
Data written to this file or inode is expected to have a short life time.
.TP
.BR RWH_WRITE_LIFE_MEDIUM
Data written to this file or inode is expected to have a life time longer than
data written with
.BR RWH_WRITE_LIFE_SHORT .
.TP
.BR RWH_WRITE_LIFE_LONG
Data written to this file or inode is expected to have a life time longer than
data written with
.BR RWH_WRITE_LIFE_MEDIUM .
.TP
.BR RWH_WRITE_LIFE_EXTREME
Data written to this file or inode is expected to have a life time longer than
data written with
.BR RWH_WRITE_LIFE_LONG .
.P
All the write specific hints are relative to each other, and no individual
absolute meaning should be attributed to them.
.P
.SH RETURN VALUE
For a successful call, the return value depends on the operation:
.TP 0.9i