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

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Date:   Fri, 10 Nov 2017 14:42:28 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Hyunchul Lee <hyc.lee@...il.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 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

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
> 
> .
>