Re: [RFC PATCH -next v3 01/10] block: introduce BLK_FEAT_WRITE_ZEROES_UNMAP to queue limits features

Zhang Yi
2025-04-10T09:35:50Z

On 2025/4/10 16:20, Keith Busch wrote:
> On Thu, Apr 10, 2025 at 09:15:59AM +0200, Christoph Hellwig wrote:
>> On Thu, Apr 10, 2025 at 11:52:17AM +0800, Zhang Yi wrote:
>>>
>>> Thank you for your review and comments. However, I'm not sure I fully
>>> understand your points. Could you please provide more details?
>>>
>>> AFAIK, the NVMe protocol has the following description in the latest
>>> NVM Command Set Specification Figure 82 and Figure 114:
>>>
>>> ===
>>> Deallocate (DEAC): If this bit is set to `1´, then the host is
>>> requesting that the controller deallocate the specified logical blocks.
>>> If this bit is cleared to `0´, then the host is not requesting that
>>> the controller deallocate the specified logical blocks...
>>>
>>> DLFEAT:
>>> Write Zeroes Deallocation Support (WZDS): If this bit is set to `1´,
>>> then the controller supports the Deallocate bit in the Write Zeroes
>>> command for this namespace...
>>
>> Yes. The host is requesting, not the controller shall. It's not
>> guaranteed behavior and the controller might as well actually write
>> zeroes to the media. That is rather stupid, but still.
>
> I guess some controllers _really_ want specific alignments to
> successfully do a proper discard. While still not guaranteed in spec, I
> think it is safe to assume a proper deallocation will occur if you align
> to NPDA and NPDG. Otherwise, the controller may do a read-modify-write
> to ensure zeroes are returned for the requested LBA range on anything
> that straddles an implementation specific boundary.
>

I understand. A proper deallocation has certain constraints, but I
guess it should be useful for most scenarios. Thank you for
the explanation.

Thanks,
Yi.