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

[Chao Yu]

On 2017/12/15 10:06, Jaegeuk Kim wrote:
> On 12/14, Hyunchul Lee wrote:
>> 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.
> 
> I think what we really can do would assign many user hints to our 3 DATA
> logs likewise rw_hint_to_seg_type(), since it's just hints for user data.
> Then, we can decide how to keep that as much as possible, since we have
> another filesystem metadata such as meta and nodes. In addition, I don't
> think we have to keep the original user-hints which makes F2FS logs be
> messed up.
> 
> With that mind, I can think of the below cases. Especially, if user wants
> to keep their io_hints, we'd better recommend to use direct_io w/o fs_iohints.



> In order to keep this policy, I think fs_iohints would be better to be a
> feature set by mkfs.f2fs and detected by sysfs entries for users.
> 
> 1) w/ fs_iohints
> 
> User                        F2FS               Block
> -------------------------------------------------------------------
>                             Meta               WRITE_LIFE_MEDIUM
>                             HOT_NODE           WRITE_LIFE_NOTSET
>                             WARM_NODE          -'
>                             COLD_NODE          WRITE_LIFE_NONE
> ioctl(cold)                 COLD_DATA          WRITE_LIFE_EXTREME
> extention list              -'                 -'
> WRITE_LIFE_EXTREME          -'                 -'
> WRITE_LIFE_SHORT            HOT_DATA           WRITE_LIFE_SHORT
> 
> -- buffered_io
> WRITE_LIFE_NOT_SET          WARM_DATA          WRITE_LIFE_LONG
> WRITE_LIFE_NONE             -'                 -'
> WRITE_LIFE_MEDIUM           -'                 -'
> WRITE_LIFE_LONG             -'                 -'
> 
> -- direct_io (Not recommendable)
> WRITE_LIFE_NOT_SET          WARM_DATA          WRITE_LIFE_NOT_SET
> WRITE_LIFE_NONE             -'                 WRITE_LIFE_NONE
> WRITE_LIFE_MEDIUM           -'                 WRITE_LIFE_MEDIUM
> WRITE_LIFE_LONG             -'                 WRITE_LIFE_LONG

Agreed with above IO hint mapping rule.

> 
> 2) w/o fs_iohints
> 
> User                        F2FS               Block
> -------------------------------------------------------------------
>                             Meta               -
>                             HOT_NODE           -
>                             WARM_NODE          -
>                             COLD_NODE          -
> ioctl(cold)                 COLD_DATA          -
> extention list              -'                 -
> 
> -- buffered_io
> WRITE_LIFE_EXTREME          COLD_DATA          -
> WRITE_LIFE_SHORT            HOT_DATA           -
> WRITE_LIFE_NOT_SET          WARM_DATA          -
> WRITE_LIFE_NONE             -'                 -
> WRITE_LIFE_MEDIUM           -'                 -
> WRITE_LIFE_LONG             -'                 -

Now we recommend direct_io if user wants to give IO hint for storage, I suspect
that user would suffer performance regression issue w/o buffered IO.

Another problem is that, now, in Android, it will be very hard to prompt
application to migrate their IO pattern from buffered IO to direct IO, one
possible way is distinguishing user data lifetime from FWK, e.g. set
WRITE_LIFE_SHORT for cache file or tmp file, set WRITE_LIFE_EXTREME for media file.

In order to support buffered_io, would it be better to change mapping as below?

-- buffered_io
WRITE_LIFE_EXTREME          COLD_DATA          WRITE_LIFE_EXTREME
WRITE_LIFE_SHORT            HOT_DATA           WRITE_LIFE_SHORT
WRITE_LIFE_NOT_SET          WARM_DATA          WRITE_LIFE_NOT_SET
WRITE_LIFE_NONE             -'                 -'
WRITE_LIFE_MEDIUM           -'                 -'
WRITE_LIFE_LONG             -'                 -'

Thanks,

> 
> -- direct_io
> WRITE_LIFE_EXTREME          COLD_DATA          WRITE_LIFE_EXTREME
> WRITE_LIFE_SHORT            HOT_DATA           WRITE_LIFE_SHORT
> WRITE_LIFE_NOT_SET          WARM_DATA          WRITE_LIFE_NOT_SET
> WRITE_LIFE_NONE             -'                 WRITE_LIFE_NONE
> WRITE_LIFE_MEDIUM           -'                 WRITE_LIFE_MEDIUM
> WRITE_LIFE_LONG             -'                 WRITE_LIFE_LONG
> 
> 
> Note that, I don't much care about how to manipulate streamid in nvme driver
> in terms of LIFE_NONE or LIFE_NOTSET, since other drivers can handle them
> in different ways. Taking a look at the definition, at least, we don't need
> to assume that those are same at all. For example, if we can expolit this in
> UFS driver, we can pass all the stream ids to the device as context ids.
> 
> Thanks,
> 
>>
>> 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@xxxxxxx>
>>>>>>>>
>>>>>>>> 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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> 
> .
>