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

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Date:   Wed, 15 Nov 2017 08:27:30 -0800
From:   Jaegeuk Kim <jaegeuk@...nel.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     Hyunchul Lee <hyc.lee@...il.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, kernel-team@....com,
        Hyunchul Lee <cheol.lee@....com>, Chao Yu <chao@...nel.org>
Subject: Re: [RFC PATCH 0/2] apply write hints to select the type of segments

On 11/14, Chao Yu wrote:
> 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@....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
> >>>>>>>
> >>>>>>
> >>>>>> 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?

The assumption would be to split different types of blocks by flash firmware,
so I think we can use WRITE_LIFE_NONE as a type as well.

Thanks,

> 
> 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
> >>>>>>>>
> >>>>>>>> .
> >>>>>>>>
> >>>>>>>
> >>>>>>>
> >>>>>>
> >>>>>> .
> >>>>>>
> >>>>>
> >>>>>
> >>>>
> >>>> .
> >>>>
> >>>
> >>>
> > 
> > .
> >