linux-kernel - Re: [RFC PATCH 0/2] apply write hints to select the type of segments

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Date:   Mon, 13 Nov 2017 10:35:06 +0900
From:   Hyunchul Lee <hyc.lee@...il.com>
To:     Chao Yu <yuchao0@...wei.com>
CC:     Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Hyunchul Lee <cheol.lee@....com>
Subject: Re: [RFC PATCH 0/2] apply write hints to select the type of segments

On 11/13/2017 10:26 AM, Chao Yu wrote:
> On 2017/11/13 8:24, Hyunchul Lee wrote:
>> On 11/10/2017 03:42 PM, Chao Yu wrote:
>>> On 2017/11/10 8:23, Hyunchul Lee wrote:
>>>> Hello, Chao
>>>>
>>>> On 11/09/2017 06:12 PM, Chao Yu wrote:
>>>>> On 2017/11/9 13:51, Hyunchul Lee wrote:
>>>>>> From: Hyunchul Lee <cheol.lee@....com>
>>>>>>
>>>>>> Using write hints[1], applications can inform the life time of the data
>>>>>> written to devices. and this[2] reported that the write hints patch
>>>>>> decreased writes in NAND by 25%.
>>>>>>
>>>>>> This hints help F2FS to determine the followings.
>>>>>>   1) the segment types where the data will be written.
>>>>>>   2) the hints that will be passed down to devices with the data of segments.
>>>>>>
>>>>>> This patch set implements the first mapping from write hints to segment types
>>>>>> as shown below.
>>>>>>
>>>>>>   hints                     segment type
>>>>>>   -----                     ------------
>>>>>>   WRITE_LIFE_SHORT          CURSEG_COLD_DATA
>>>>>>   WRITE_LIFE_EXTREME        CURSEG_HOT_DATA
>>>>>>   others                    CURSEG_WARM_DATA
>>>>>>
>>>>>> The F2FS poliy for hot/cold seperation has precedence over this hints, And
>>>>>> hints are not applied in in-place update.
>>>>>
>>>>> Could we change to disable IPU if file/inode write hint is existing?
>>>>>
>>>>
>>>> I am afraid that this makes side effects. for example, this could cause
>>>> out-of-place updates even when there are not enough free segments. 
>>>> I can write the patch that handles these situations. But I wonder 
>>>> that this is required, and I am not sure which IPU polices can be disabled.
>>>
>>> Oh, As I replied in another thread, I think IPU just affects filesystem
>>> hot/cold separating, rather than this feature. So I think it will be okay
>>> to not consider it.
>>>
>>>>
>>>>>>
>>>>>> Before the second mapping is implemented, write hints are not passed down
>>>>>> to devices. Because it is better that the data of a segment have the same 
>>>>>> hint.
>>>>>>
>>>>>> [1]: c75b1d9421f80f4143e389d2d50ddfc8a28c8c35
>>>>>> [2]: https://lwn.net/Articles/726477/
>>>>>
>>>>> Could you write a patch to support passing write hint to block layer for
>>>>> buffered writes as below commit:
>>>>> 0127251c45ae ("ext4: add support for passing in write hints for buffered writes")
>>>>>
>>>>
>>>> Sure I will. I wrote it already ;)
>>>
>>> Cool, ;)
>>>
>>>> I think that datas from the same segment should be passed down with the same
>>>> hint, and the following mapping is reasonable. I wonder what is your opinion
>>>> about it.
>>>>
>>>>   segment type               hints
>>>>   ------------               -----
>>>>   CURSEG_COLD_DATA           WRITE_LIFE_EXTREME
>>>>   CURSEG_HOT_DATA            WRITE_LIFE_SHORT
>>>>   CURSEG_COLD_NODE           WRITE_LIFE_NORMAL
>>>
>>> We have WRITE_LIFE_LONG defined rather than WRITE_LIFE_NORMAL in fs.h?
>>>
>>>>   CURSEG_HOT_NODE            WRITE_LIFE_MEDIUM
>>>
>>> As I know, in scenario of cell phone, data of meta_inode is hottest, then hot
>>> data, warm node, and cold node should be coldest. So I suggested we can define
>>> as below:
>>>
>>> META_DATA			WRITE_LIFE_SHORT
>>> HOT_DATA & WARM_NODE		WRITE_LIFE_MEDIUM
>>> HOT_NODE & WARM_DATA		WRITE_LIFE_LONG
>>> COLD_NODE & COLD_DATA		WRITE_LIFE_EXTREME
>>>
>>
>> I agree, But I am not sure that assigning the same hint to a node and data
>> segment is good. Because NVMe is likely to write them in the same erase 
>> block if they have the same hint.
> 
> If we do not give the hint, they can still be written to the same erase block,
> right? it will not be worse?
> 

If the hint is not given, I think that they could be written to 
the same erase block, or not. But if we give the same hint, they are written
to the same block.
I am not sure ;)

> Thanks,
> 
>>
>> Thanks.
>>
>>> Thanks,
>>>
>>>>   others                     WRITE_LIFE_NONE
>>>>  
>>>>> Thanks,
>>>>>
>>>>>>
>>>>>> Hyunchul Lee (2):
>>>>>>   f2fs: apply write hints to select the type of segments for buffered
>>>>>>     write
>>>>>>   f2fs: apply write hints to select the type of segment for direct write
>>>>>>
>>>>>>  fs/f2fs/data.c    | 101 ++++++++++++++++++++++++++++++++----------------------
>>>>>>  fs/f2fs/f2fs.h    |   1 +
>>>>>>  fs/f2fs/segment.c |  14 +++++++-
>>>>>>  3 files changed, 74 insertions(+), 42 deletions(-)
>>>>>>
>>>>>
>>>>>
>>>>
>>>> Thanks
>>>>
>>>> .
>>>>
>>>
>>>
>>
>> .
>>
> 
>