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

[Hyunchul Lee]

On 11/13/2017 10:59 AM, Chao Yu wrote:
> On 2017/11/13 9:35, Hyunchul Lee wrote:
>> 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@xxxxxxx>
>>>>>>>>
>>>>>>>> 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,
> 
> I mean it's possible to write them 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.
> 
> IMO, Only if underlying device can support more hint type or opened channels,
> and actual temperature of data segment and node segment is quite different, we
> can separate them.
> 

Okay, If Jaegeuk Kim agrees with this, I will submit the patch that 
implements your proposed mapping.

Thank you for comments ;)

> Thanks,
> 
>> 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
>>>>>>
>>>>>> .
>>>>>>
>>>>>
>>>>>
>>>>
>>>> .
>>>>
>>>
>>>
>>
>> .
>>
> 
>