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

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Date:   Fri, 01 Dec 2017 17:28:23 +0900
From:   Hyunchul Lee <hyc.lee@...il.com>
To:     Chao Yu <yuchao0@...wei.com>, Jaegeuk Kim <jaegeuk@...nel.org>
CC:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        kernel-team@....com, Jens Axboe <axboe@...nel.dk>,
        Hyunchul Lee <cheol.lee@....com>
Subject: Re: [PATCH 1/2] f2fs: pass down write hints to block layer for bufferd
 write

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.

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,
> 
>