Re: [dm-devel] [PATCH v4 00/13] support non power of 2 zoned devices

[Pankaj Raghav <p.raghav@samsung.com>]

On 5/20/22 08:41, Damien Le Moal wrote:
>>>>>
>>>>> So what about creating a device-mapper target, that's taking npo2 drives and
>>>>> makes them po2 drives for the FS layers? It will be very similar code to
>>>>> dm-linear.
>>>>
Keith and Adam had a similar suggestion to go create a device mapper
(dm-unholy) when we tried the po2 emulation[1].
>>>> +1
>>>>
>>>> This will simplify the support for FSes, at least for the initial drop (if
>>>> accepted).
>>>>
>>>> And more importantly, this will also allow addressing any potential
>>>> problem with user space breaking because of the non power of 2 zone size.
>>>>
>>> Seconded (or maybe thirded).
>>>
>>> The changes to support npo2 in the block layer are pretty simple, and 
>>> really I don't have an issue with those.
>>> Then adding a device-mapper target transforming npo2 drives in po2 
>>> block devices should be pretty trivial.
>>>
>>> And once that is in you can start arguing with the the FS folks on 
>>> whether to implement it natively.
>>>
>>
>> So you are suggesting adding support for !PO2 in the block layer and
>> then a dm to present the device as a PO2 to the FS? This at least
>> addresses the hole issue for raw zoned block devices, so it can be a
>> first step.
> 
> Yes, and it also allows supporting these new !po2 devices without
> regressions (read lack of) in the support at FS level.
> 
>>
>> This said, it seems to me that the changes to the FS are not being a
>> real issue. In fact, we are exposing some bugs while we generalize the
>> zone size support.
> 
> Not arguing with that. But since we are still stabilizing btrfs ZNS
> support, adding more code right now is a little painful.
> 
>>
>> Could you point out what the challenges in btrfs are in the current
>> patches, that it makes sense to add an extra dm layer?
> 
> See above. No real challenge, just needs to be done if a clear agreement
> can be reached on zone size alignment constraints. As mentioned above, the
> btrfs changes timing is not ideal right now though.
> 
> Also please do not forget applications that may expect a power of 2 zone
> size. A dm-zsp2 would be a nice solution for these. So regardless of the
> FS work, that new DM target will be *very* nice to have.
> 
>>
>> Note that for F2FS there is no blocker. Jaegeuk picked the initial
>> patches, and he agreed to add native support.
> 
> And until that is done, f2fs will not work with these new !po2 devices...
> Having the new dm will avoid that support fragmentation which I personally
> really dislike. With the new dm, we can keep support for *all* zoned block
> devices, albeit needing a different setup depending on the device. That is
> not nice at all but at least there is a way to make things work continuously.
> 

I see that many people in the community feel it is better to target the
dm layer for the initial support of npo2 devices. I can give it a shot
and maintain a native out-of-tree support for FSs for npo2 devices and
merge it upstream as we see fit later.

[1]
https://lore.kernel.org/all/20220311223032.GA2439@dhcp-10-100-145-180.wdc.com/