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

[Chao Yu]

On 2017/11/14 12:20, Jaegeuk Kim wrote:
> On 11/13, Hyunchul Lee wrote:
>> 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.
> 
> How about this? We'd better to split data and node blocks as much as possible.
> 
> segment type                    hints
> ------------                    -----
> COLD_NODE & COLD_DATA		WRITE_LIFE_NONE

WRITE_LIFE_NONE means there is no hints about write life time.

Shouldn't we define COLD_NODE & COLD_DATA as WRITE_LIFE_EXTERME?

Thanks,

> WARM_DATA			WRITE_LIFE_EXTERME
> HOT_NODE & WARM_NODE		WRITE_LIFE_LONG
> HOT_DATA			WRITE_LIFE_MEDIUM
> META_DATA			WRITE_LIFE_SHORT
> 
>>
>> 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
>>>>>>>>
>>>>>>>> .
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>> .
>>>>>>
>>>>>
>>>>>
>>>>
>>>> .
>>>>
>>>
>>>
> 
> .
>