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Date:   Mon, 3 Oct 2022 21:12:46 -0700
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Subject: Re: [PATCH] xfs: fail dax mount if reflink is enabled on a partition
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To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     =?UTF-8?B?WWFuZywgWGlhby/mnagg5pmT?= <yangx.jy@fujitsu.com>,
        Brian Foster <bfoster@redhat.com>,
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        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "david@fromorbit.com" <david@fromorbit.com>, zwisler@kernel.org,
        Jeff Moyer <jmoyer@redhat.com>, dm-devel@redhat.com,
        toshi.kani@hpe.com
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On 2022/10/03 17:12, Darrick J. Wong wrote:
> On Fri, Sep 30, 2022 at 09:56:41AM +0900, Gotou, Yasunori/五島 康文 wrote:
>> Hello everyone,
>>
>> On 2022/09/20 11:38, Yang, Xiao/杨 晓 wrote:
>>> Hi Darrick, Brian and Christoph
>>>
>>> Ping. I hope to get your feedback.
>>>
>>> 1) I have confirmed that the following patch set did not change the test
>>> result of generic/470 with thin-volume. Besides, I didn't see any
>>> failure when running generic/470 based on normal PMEM device instaed of
>>> thin-volume.
>>> https://lore.kernel.org/linux-xfs/20211129102203.2243509-1-hch@lst.de/
>>>
>>> 2) I can reproduce the failure of generic/482 without thin-volume.
>>>
>>> 3) Is it necessary to make thin-volume support DAX. Is there any use
>>> case for the requirement?
>>
>>
>> Though I asked other place(*), I really want to know the usecase of
>> dm-thin-volume with DAX and reflink.
>>
>>
>> In my understanding, dm-thin-volume seems to provide similar feature like
>> reflink of xfs. Both feature provide COW update to reduce usage of
>> its region, and snapshot feature, right?
>>
>> I found that docker seems to select one of them (or other feature which
>> supports COW). Then user don't need to use thin-volume and reflink at same
>> time.
>>
>> Database which uses FS-DAX may want to use snapshot for its data of FS-DAX,
>> its user seems to be satisfied with reflink or thin-volume.
>>
>> So I could not find on what use-case user would like to use dm-thin-volume
>> and reflink at same time.
>>
>> The only possibility is that the user has mistakenly configured dm-thinpool
>> and reflink to be used at the same time, but if that is the case, it seems
>> to be better for the user to disable one or the other.
>>
>> I really wander why dm-thin-volume must be used with reflik and FS-DAX.
> 
> There isn't a hard requirement between fsdax and dm-thinp.  The /test/
> needs dm-logwrites to check that write page faults on a MAP_SYNC
> mmapping are persisted directly to disk.  dm-logwrites requires a fast
> way to zero an entire device for correct operation of the replay step,
> and thinp is the only way to guarantee that.

Thank you for your answer. But I still feel something is strange.
Though dm-thinp may be good way to execute the test correctly,
I suppose it seems to be likely a kind of workaround to pass the test,
it may not be really required for actual users.

Could you tell me why passing test by workaround is so necessary?

Thanks,


> 
> --D
> 
>> If my understanding is something wrong, please correct me.
>>
>> (*)https://lore.kernel.org/all/TYWPR01MB1008258F474CA2295B4CD3D9B90549@TYWPR01MB10082.jpnprd01.prod.outlook.com/