Re: [PATCH 0/6] power_of_2 emulation support for NVMe ZNS devices

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 3/16/22 09:23, Luis Chamberlain wrote:
> On Wed, Mar 16, 2022 at 09:07:18AM +0900, Damien Le Moal wrote:
>> On 3/16/22 02:00, Luis Chamberlain wrote:
>>> On Tue, Mar 15, 2022 at 02:30:52PM +0100, Christoph Hellwig wrote:
>>>> On Tue, Mar 15, 2022 at 02:26:11PM +0100, Javier González wrote:
>>>>> but we do not see a usage for ZNS in F2FS, as it is a mobile
>>>>> file-system. As other interfaces arrive, this work will become natural.
>>>>>
>>>>> ZoneFS and butrfs are good targets for ZNS and these we can do. I would
>>>>> still do the work in phases to make sure we have enough early feedback
>>>>> from the community.
>>>>>
>>>>> Since this thread has been very active, I will wait some time for
>>>>> Christoph and others to catch up before we start sending code.
>>>>
>>>> Can someone summarize where we stand?
>>>
>>> RFCs should be posted to help review and evaluate direct NPO2 support
>>> (not emulation) given we have no vendor willing to take a position that
>>> NPO2 will *never* be supported on ZNS, and its not clear yet how many
>>> vendors other than Samsung actually require NPO2 support. The other
>>> reason is existing NPO2 customers currently cake in hacks to Linux to
>>> supoport NPO2 support, and so a fragmentation already exists. To help
>>> address this it's best to evaluate what the world of NPO2 support would
>>> look like and put the effort to do the work for that and review that.
>>
>> And again no mentions of all the applications supporting zones assuming
>> a power of 2 zone size that will break.
> 
> What applications? ZNS does not incur a PO2 requirement. So I really
> want to know what applications make this assumption and would break
> because all of a sudden say NPO2 is supported.

Exactly. What applications ? For ZNS, I cannot say as devices have not
been available for long. But neither can you.

> Why would that break those ZNS applications?

Please keep in mind that there are power of 2 zone sized ZNS devices out
there. Applications designed for these devices and optimized to do bit
shift arithmetic using the power of 2 size property will break. What the
plan for that case ? How will you address these users complaints ?

>> Allowing non power of 2 zone size may prevent applications running today
>> to run properly on these non power of 2 zone size devices. *not* nice.
> 
> Applications which want to support ZNS have to take into consideration
> that NPO2 is posisble and there existing users of that world today.

Which is really an ugly approach. The kernel zone user interface is
common to all zoned devices: SMR, ZNS, null_blk, DM (dm-crypt,
dm-linear). They all have one point in common: zone size is a power of
2. Zone capacity may differ, but hey, we also unified that by reporting
a zone capacity for *ALL* of them.

Applications correctly designed for SMR can thus also run on ZNS too.
With this in mind, the spectrum of applications that would break on non
power of 2 ZNS devices is suddenly much larger.

This has always been my concern from the start: allowing non power of 2
zone size fragments userspace support and has the potential to
complicate things for application developers.

> 
> You cannot negate their existance.
> 
>> I have yet to see any convincing argument proving that this is not an issue.
> 
> You are just saying things can break but not clarifying exactly what.
> And you have not taken a position to say WD will not ever support NPO2
> on ZNS. And so, you can't negate the prospect of that implied path for
> support as a possibility, even if it means work towards the ecosystem
> today.

Please do not bring in corporate strategy aspects in this discussion.
This is a technical discussion and I am not talking as a representative
of my employer nor should we ever dicsuss business plans on a public
mailing list. I am a kernel developer and maintainer. Keep it technical
please.


-- 
Damien Le Moal
Western Digital Research