Hi Jens,

Ping^2!

Cheers,

Michael


On 3 September 2017 at 02:45, Michael Kerrisk (man-pages)
[off-list ref] wrote:

Hi Jens,

Ping!

Cheers,

Michael


On 08/28/2017 11:16 PM, Michael Kerrisk (man-pages) wrote:

quoted

On 08/28/2017 10:15 PM, Jens Axboe wrote:

quoted

On 08/28/2017 01:19 PM, Michael Kerrisk (man-pages) wrote:

quoted

Hi Jens,

On 08/25/2017 10:55 PM, Jens Axboe wrote:

quoted

On 08/25/2017 02:51 PM, Michael Kerrisk (man-pages) wrote:

quoted

quoted

quoted

quoted

quoted

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?

You are right, I misunderstood your original question. They do follow
the file description. So the dup'ed one will return the same as the
original, even if the hints on the original fd get modified. That is the
expected behavior.

So, I am still confused. I was wondering whether the hints are
associated with the open file description (OFD), rather than the
file descriptor. You said yes, then say that the dup'ed file
descriptor will have the same hints even if the hints on the
original file descriptor are modified. To me that sounds like:
the hints are associated with the file descriptor, and not the
OFD, and during dup(2) the hints are *copied* to the the new
file descriptor, with the result that after the dup(2) the hints
can be modified independently for the two file descriptors.

Can you clarify please?

No, that's not how it behaves. If you dup(2) the file descriptor, then
the dup'ed descriptor will return the same hint as was set on the
original.  If you change/clear the hint on the original, the dup'ed
descriptor will now return the new hint.

Okay -- thanks. I'd misunderstood your earlier words. Okay, I've
hacked this text to arrive at new text below. Could you please check
it? Also, there are some details that are still missing. Could you take
a look at the questions below please.

[[[
    File read/write hints
       Write lifetime hints can be used to inform the  kernel  about  the
       relative  expected  lifetime  of  writes on a given inode or via a
       particular open file description.  (See open(2) for an explanation
       of open file desriptions.)  In this context, the term "write life‐
       time" means the expected time the data will live on media,  before
       being overwritten or erased.

       An  application  may use the different hint values specified below
       to separate writes into different write classes, so that  multiple
       users  or  applications  running  on a single storage back-end can
       aggregate their I/O patterns in  a  consistent  manner.   However,
       there are no functional semantics implied by these flags, and dif‐
       ferent I/O classes can use the write lifetime hints  in  arbitrary
       ways, so long as the hints are used consistently.

QUESTIONS:
* What are write classes?
* What are I/O classes?
* What is the purpose of using read/write hints? I assume it's a
  performance point, but the text is not explicit about that.
* You variously wrote "read/write hints" and "write hints". Let's make
  it consistent. Which is the preferred term?

       The  following  operations  can be applied to the file descriptor,
       fd:

       F_GET_RW_HINT (uint64_t *; since Linux 4.13)
              Returns the value of the read/write  hint  associated  with
              the underlying inode referred to by fd.

       F_SET_RW_HINT (uint64_t *; since Linux 4.13)
              Sets the read/write hint value associated with the underly‐
              ing inode referred to by fd.

       F_GET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
              Returns the value of the read/write  hint  associated  with
              the open file description referred to by fd.

       F_SET_FILE_RW_HINT (uint64_t *; since Linux 4.13)
              Sets  the  read/write  hint  value associated with the open
              file description referred to by fd.

       If an open file description 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 Linux 4.13:

       RWH_WRITE_LIFE_NOT_SET
              No specific hint has been set.  This is the default value.

       RWH_WRITE_LIFE_NONE
              No  specific write lifetime is associated with this file or
              inode.

       RWH_WRITE_LIFE_SHORT
              Data written to this inode or via this open  file  descrip‐
              tion is expected to have a short lifetime.

       RWH_WRITE_LIFE_MEDIUM
              Data  written  to this inode or via this open file descrip‐
              tion is expected to have a lifetime longer than data  writ‐
              ten with RWH_WRITE_LIFE_SHORT.

       RWH_WRITE_LIFE_LONG
              Data  written  to this inode or via this open file descrip‐
              tion is expected to have a lifetime longer than data  writ‐
              ten with RWH_WRITE_LIFE_MEDIUM.

       RWH_WRITE_LIFE_EXTREME
              Data  written  to this inode or via this open file descrip‐
              tion is expected to have a lifetime longer than data  writ‐
              ten with RWH_WRITE_LIFE_LONG.

       All  the  write-specific  hints are relative to each other, and no
       individual absolute meaning should be attributed to them.
]]]

Cheers,

Michael

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

-- 
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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