[dm-devel] Question about which types of dm need to support DAX

[dm-devel] Question about which types of dm need to support DAX

[Yang, Xiao/杨 晓]

Hi Ross, Toshi, Jeff and others

After reading the kernel patches[1][2][3] about dm, I think only three 
types(linear, stripe and log-writes) of dm can support DAX now, right?

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=98d82f48f1983ceef5c8d2f6c87bfee2918790ee
[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=beec25b4573bc310f5a12cf33207b20ecc30945c
[3] 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=84b22f8378cf493524043a0a8dd567c58c64546f

In addition, I also saw that Jeff has added a check[4] for the DAX 
support of dm in xfstests.

[4] 
https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/commit/?id=fc7b3903894a6213c765d64df91847f4460336a2

I wonder why only three types of dm need to support DAX? Is there any 
reason/history and use case?

Is it necessary to make other types (for example, thin-pool) support 
DAX? If necessary, is there any use case for other types?

Best Regards,
Xiao Yang

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Re: [dm-devel] Question about which types of dm need to support DAX

[Yang, Xiao/杨 晓]

Hi All,

Ping. Is there any feedback about the question?

Best Regards,
Xiao Yang

On 2022/9/16 14:56, Yang, Xiao/杨 晓 wrote:
> Hi Ross, Toshi, Jeff and others
> 
> After reading the kernel patches[1][2][3] about dm, I think only three 
> types(linear, stripe and log-writes) of dm can support DAX now, right?
> 
> [1] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=98d82f48f1983ceef5c8d2f6c87bfee2918790ee 
> 
> [2] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=beec25b4573bc310f5a12cf33207b20ecc30945c 
> 
> [3] 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=84b22f8378cf493524043a0a8dd567c58c64546f 
> 
> 
> In addition, I also saw that Jeff has added a check[4] for the DAX 
> support of dm in xfstests.
> 
> [4] 
> https://git.kernel.org/pub/scm/fs/xfs/xfstests-dev.git/commit/?id=fc7b3903894a6213c765d64df91847f4460336a2 
> 
> 
> I wonder why only three types of dm need to support DAX? Is there any 
> reason/history and use case?
> 
> Is it necessary to make other types (for example, thin-pool) support 
> DAX? If necessary, is there any use case for other types?
> 
> Best Regards,
> Xiao Yang
> 
> -- 
> dm-devel mailing list
> dm-devel@redhat.com
> https://listman.redhat.com/mailman/listinfo/dm-devel
> 

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Re: [dm-devel] Question about which types of dm need to support DAX

[Jeff Moyer]

"Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:

> Hi Ross, Toshi, Jeff and others
>
> After reading the kernel patches[1][2][3] about dm, I think only three
> types(linear, stripe and log-writes) of dm can support DAX now, right?

Right.

> I wonder why only three types of dm need to support DAX? Is there any
> reason/history and use case?

It was easy to add DAX support to those targets.  Note that if a target
needs to intercept I/O, then it is fundamentally incompatible with DAX.

Cheers,
Jeff
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Re: [dm-devel] Question about which types of dm need to support DAX

[Yang, Xiao/杨 晓]

Hi Jeff,

Thanks a lot for your reply.

On 2022/9/27 3:52, Jeff Moyer wrote:
> "Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:
> 
>> Hi Ross, Toshi, Jeff and others
>>
>> After reading the kernel patches[1][2][3] about dm, I think only three
>> types(linear, stripe and log-writes) of dm can support DAX now, right?
> 
> Right.
> 
>> I wonder why only three types of dm need to support DAX? Is there any
>> reason/history and use case?
> 
> It was easy to add DAX support to those targets.  Note that if a target
> needs to intercept I/O, then it is fundamentally incompatible with DAX.

How can I know which target needs to intercept I/O? Could you tell me 
why it is fundamentally incompatible with DAX?

Best Regards,
Xiao Yang
> 
> Cheers,
> Jeff
> 

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Re: [dm-devel] Question about which types of dm need to support DAX

[Jeff Moyer]

"Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:

> Hi Jeff,
>
> Thanks a lot for your reply.
>
> On 2022/9/27 3:52, Jeff Moyer wrote:
>> "Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:
>>
>>> Hi Ross, Toshi, Jeff and others
>>>
>>> After reading the kernel patches[1][2][3] about dm, I think only three
>>> types(linear, stripe and log-writes) of dm can support DAX now, right?
>>
>> Right.
>>
>>> I wonder why only three types of dm need to support DAX? Is there any
>>> reason/history and use case?
>>
>> It was easy to add DAX support to those targets.  Note that if a target
>> needs to intercept I/O, then it is fundamentally incompatible with DAX.
>
> How can I know which target needs to intercept I/O?

There's no better way than reading and understanding what each target
does.

> Could you tell me why it is fundamentally incompatible with DAX?

DAX stands for direct access, and it is a mechanism by which a
user-space process can perform I/O without going through the kernel.

-Jeff
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Re: [dm-devel] Question about which types of dm need to support DAX

[y-goto]

Hello Jeff-san,

> "Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:
> 
> > Hi Jeff,
> >
> > Thanks a lot for your reply.
> >
> > On 2022/9/27 3:52, Jeff Moyer wrote:
> >> "Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:
> >>
> >>> Hi Ross, Toshi, Jeff and others
> >>>
> >>> After reading the kernel patches[1][2][3] about dm, I think only
> >>> three types(linear, stripe and log-writes) of dm can support DAX now,
> right?
> >>
> >> Right.
> >>
> >>> I wonder why only three types of dm need to support DAX? Is there
> >>> any reason/history and use case?
> >>
> >> It was easy to add DAX support to those targets.  Note that if a
> >> target needs to intercept I/O, then it is fundamentally incompatible with
> DAX.

I would like to know its use case.
Though my team has been made effort for a long time to remove experimental of FS-DAX,
we still need one more time to solve thin-pool (or other DM feature) v.s. DAX problem even if it is easy.

To be honest, I'm under criticism for taking such a long time and still not be able to remove experimental.
In addition, there are a few persons who insist Fujitsu should stop development of FS-DAX due to
too much time for development

I hope that my team can continue to make effort for FS-DAX.
If there is a use-case of thin-pool, then I'll be able to persuade such persons by it. 
So I would appreciate it if you could tell me about the use case.

Thanks,
---
Yasunori Goto


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Re: [dm-devel] Question about which types of dm need to support DAX

[Yang, Xiao/杨 晓]

On 2022/9/27 22:38, Jeff Moyer wrote:
> "Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:
> 
>> Hi Jeff,
>>
>> Thanks a lot for your reply.
>>
>> On 2022/9/27 3:52, Jeff Moyer wrote:
>>> "Yang, Xiao/杨 晓" <yangx.jy@fujitsu.com> writes:
>>>
>>>> Hi Ross, Toshi, Jeff and others
>>>>
>>>> After reading the kernel patches[1][2][3] about dm, I think only three
>>>> types(linear, stripe and log-writes) of dm can support DAX now, right?
>>>
>>> Right.
>>>
>>>> I wonder why only three types of dm need to support DAX? Is there any
>>>> reason/history and use case?
>>>
>>> It was easy to add DAX support to those targets.  Note that if a target
>>> needs to intercept I/O, then it is fundamentally incompatible with DAX.
>>
>> How can I know which target needs to intercept I/O?
> 
> There's no better way than reading and understanding what each target
> does.

Hi Jeff,

After reading the code about thin provisioning target, thin provisioning 
target depends on DM_BUFIO kconfig which is designed to cache I/O. Is it 
what you said about "intercept I/O"? Perhaps, we have to drop DM_BUFIO 
for thin provisioning target if we want to make it support DAX.

Best Regards,
Xiao Yang

> 
>> Could you tell me why it is fundamentally incompatible with DAX?
> 
> DAX stands for direct access, and it is a mechanism by which a
> user-space process can perform I/O without going through the kernel.
> 
> -Jeff
> 

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