linux-kernel - Re: [f2fs-dev] [PATCH 1/2] f2fs: pass down write hints to block layer for bufferd write

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Date:   Thu, 14 Dec 2017 10:33:59 +0900
From:   Hyunchul Lee <hyc.lee@...il.com>
To:     Chao Yu <yuchao0@...wei.com>, Chao Yu <chao@...nel.org>,
        Jaegeuk Kim <jaegeuk@...nel.org>
CC:     Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, kernel-team@....com,
        linux-fsdevel@...r.kernel.org, Hyunchul Lee <cheol.lee@....com>
Subject: Re: [f2fs-dev] [PATCH 1/2] f2fs: pass down write hints to block layer
 for bufferd write

Hi Jaegeuk,

I need your comment about the fs_iohint mount option.

a) w/o fs_iohint, propagate user hints to low layer.
b) w/ fs_iohint, ignore user hints, and use hints which is generated
with F2FS.

Chao suggests this option. because user hints are more accurate than
file system.

This is resonable, But I have some concerns about this option. 
The first thing is that blocks of a segments have different hints. This
could make GC less effective. 
The second is that the separation between LIFE_MEDIUM and LIFE_LONG is 
really needed. I think that difference between them is a little ambigous 
for users, and LIFE_SHORT and LIFE_EXTREME is converted to different 
hints by F2FS.

Thanks.

On 12/12/2017 11:45 AM, Chao Yu wrote:
> Hi Hyunchul,
> 
> On 2017/12/12 10:15, Hyunchul Lee wrote:
>> Hi Chao,
>>
>> On 12/11/2017 10:15 PM, Chao Yu wrote:
>>> Hi Hyunchul,
>>>
>>> On 2017/12/1 16:28, Hyunchul Lee wrote:
>>>> Hi Chao,
>>>>
>>>> On 11/30/2017 04:06 PM, Chao Yu wrote:
>>>>> Hi Hyunchul,
>>>>>
>>>>> On 2017/11/28 8:23, Hyunchul Lee wrote:
>>>>>> From: Hyunchul Lee <cheol.lee@....com>
>>>>>>
>>>>>> This implements which hint is passed down to block layer
>>>>>> for datas from the specific segment type.
>>>>>>
>>>>>> segment type                     hints
>>>>>> ------------                     -----
>>>>>> COLD_NODE & COLD_DATA            WRITE_LIFE_EXTREME
>>>>>> WARM_DATA                        WRITE_LIFE_NONE
>>>>>> HOT_NODE & WARM_NODE             WRITE_LIFE_LONG
>>>>>> HOT_DATA                         WRITE_LIFE_MEDIUM
>>>>>> META_DATA                        WRITE_LIFE_SHORT
>>>>>
>>>>> Just noticed, if our user do not give the hint via ioctl, f2fs can
>>>>> provider hint to lower layer according to hot/cold separation ability,
>>>>> it will be okay. But once user give his hint which may be more accurate
>>>>> than filesystem, hint converted by f2fs may be wrong.
>>>>>
>>>>> So what do you think of adding an option to control whether filesystem
>>>>> can convert hint user given?
>>>>>
>>>>
>>>> I think it is okay for LIFE_SHORT and LIFE_EXTREME. because they are 
>>>> converted to different hints.
>>>
>>> What I mean is introducing a mount option, e.g. fs_iohint,
>>> a) w/o fs_iohint, propagate file/inode io_hint to low layer.
>>> b) w/ fs_iohint, ignore file/inode io_hint, use io_hint which is generated
>>> with filesystem's private rule.
>>>
>>
>> Okay, I will implement this option and send this patch again.
> 
> Let's wait for Jaegeuk's comments first?
> 
>>
>> Without fs_iohint, Even if data blocks are moved due to GC, 
>> we should keep user hints. And if user hints are not given, 
>> any hints are not passed down to block layer, right?
> 
> Hmm.. that will be a problem, IMO, we can store last user's io_hint into inode
> layout, so later when we trigger GC, we can use the last io_hint in inode rather
> than giving no hint or fs' hint.
> 
> I think it needs to discuss with original author of IO hint, what is the IO hint
> policy when filesystem move block by itself after inode has been released in system.
> 
> Thanks,
> 
>>
>> Thank you for comments.
>>
>>> Thanks,
>>>
>>>>
>>>> file hint      segment type        io hint
>>>> ---------      ------------        -------
>>>> LIFE_SHORT     HOT_DATA            LIFE_MEDIUM
>>>> LIFE_MEDIUM    WARM_DATA           LIFE_NONE
>>>> LIFE_LONG      WARM_DATA           LIFE_NONE
>>>> LIFE_EXTREME   COLD_DATA           LIFE_EXTREME
>>>>
>>>> the problem is that LIFE_MEDIUM and LIFE_LONG are converted to 
>>>> the same hint, LIFE_NONE. I am not sure that the seperation between 
>>>> LIFE_MEDIUM and LIFE_LONG is really needed. Because I guess that the 
>>>> difference between them is a little ambigous for users, and if WARM_DATA 
>>>> segment has two different hints, it can makes GC non-efficient.
>>>>
>>>> I wonder your thought about this.
>>>>
>>>> Thanks.
>>>>
>>>>> Thanks,
>>>>>
>>>>>
>>>>
>>>> ------------------------------------------------------------------------------
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>>>> _______________________________________________
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>>>>
>>>
>>
>> .
>>
> 
> 
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