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

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Date:   Mon, 11 Dec 2017 21:15:52 +0800
From:   Chao Yu <chao@...nel.org>
To:     Hyunchul Lee <hyc.lee@...il.com>, Chao Yu <yuchao0@...wei.com>,
        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 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.

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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