Hi Jens,

On 25 August 2017 at 22:43, Jens Axboe [off-list ref] wrote:

On 08/24/2017 04:10 PM, Michael Kerrisk (man-pages) wrote:

quoted

Hello Jens,

On 24 August 2017 at 16:39, Jens Axboe [off-list ref] wrote:

quoted

On 08/23/2017 04:41 PM, Michael Kerrisk (man-pages) wrote:

quoted

Hello Jens,

On 07/31/2017 05:24 PM, Jens Axboe wrote:

quoted

Hi Michael,

Not sure what the best way to submit this is, but I wrote an
update for the fcntl man page to cover the new set/get write
hints commands that are added in 4.13. Attaching the patch, but
you can also find it below, in a fork of your github repo:

https://github.com/axboe/man-pages/commits/fcntl-write-hints

Thanks for the patch. Some questions below.

quoted

From 3eee36f14a650e50f14b42ae1d4fec33e78e6254 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>
Date: Mon, 26 Jun 2017 16:58:29 -0600
Subject: [PATCH] man2/fcntl.2: add description of new read/write hint commands

This adds a description of the new read/write hint commands:

F_GET_RW_HINT                Get inode read/write hint
F_SET_RW_HINT                Set inode read/write hint
F_GET_FILE_RW_HINT   Get file read/write hint
F_SET_FILE_RW_HINT   Set file read/write hint

and the currently available values, as of Linux 4.13.

Signed-off-by: Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>
---
 man2/fcntl.2 | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 58 insertions(+)

diff --git a/man2/fcntl.2 b/man2/fcntl.2
index 70758d2fcd99..07eacc9564fa 100644
--- a/man2/fcntl.2
+++ b/man2/fcntl.2

@@ -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-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
 .\"     Document F_ADD_SEALS and F_GET_SEALS
+.\" 2017-06-26, Jens Axboe <axboe-tSWWG44O7X1aa/9Udqfwiw@public.gmane.org>
+.\"     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.

I read the above, and it is still not clear to me what a "write lifetime"
is. Can you add some text to explain? (I'm guessing that it means the
amount of time before the data is overwritten by another write, but I'm
not sure of that, and it needs to be more explicit in the text.)

You are right, it's exactly what it refers to. It's essentially data
lifetime hints for a write command. I'd be happy to take suggestions to
improve the description of it.

Well, a sentence something like my above is probably a start, but I'd
prefer you worded something, since it might contain more insight than
anything that I propose.

Could perhaps insert a sentence (after the first one) like the following:

"Life time refers to the expected time the data will live on media, before
being overwritten or erased."

Sounds good.

quoted

quoted

quoted

quoted

+.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.

Do you mean here "file descriptor" or "file description (i.e., the
open file handle)? Maybe you mean the former, but I want to confirm.

I do mean file descriptor.

So, what are the semantics if a file descriptor is duplicated using
dup(2) or similar? If I understand correctly, then the write lifetime
hint has no effect for the new file descriptor, right?

If it's dup(2)'ed, then the new file descriptor will refer to the same
hints as the previous. See attached test file.

But then isn't this exactly the point I asked about: are the hints
private to a file descriptor or are they associated with the open file
description (open file table entry, "struct file")? You said "I do
mean file descriptor", but actually I understand what you just said
now as "hints are associated with the open file description, which may
be referred to by multiple duplicated file descriptors". Can you
clarify?

Thanks,

Michael



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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