Re: man page update (fcntl(2) new set/get write hints)

Michael Kerrisk (man-pages)
2017-08-28T21:16:06Z

On 08/28/2017 10:15 PM, Jens Axboe wrote:
> On 08/28/2017 01:19 PM, Michael Kerrisk (man-pages) wrote:
>> Hi Jens,
>>
>> On 08/25/2017 10:55 PM, Jens Axboe wrote:
>>> On 08/25/2017 02:51 PM, Michael Kerrisk (man-pages) wrote:
>>>>>>>> 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,