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

[Javier González]

On 15.03.2022 14:30, 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?  Between the lack of quoting
>from hell and overly long lines from corporate mail clients I've
>mostly stopped reading this thread because it takes too much effort
>actually extract the information.

Let me give it a try:

  - PO2 emulation in NVMe is a no-go. Drop this.

  - The arguments against supporting PO2 are:
      - It makes ZNS depart from a SMR assumption of PO2 zone sizes. This
        can create confusion for users of both SMR and ZNS

      - Existing applications assume PO2 zone sizes, and probably do
        optimizations for these. These applications, if wanting to use
        ZNS will have to change the calculations

      - There is a fear for performance regressions.

      - It adds more work to you and other maintainers

  - The arguments in favour of PO2 are:
      - Unmapped LBAs create holes that applications need to deal with.
        This affects mapping and performance due to splits. Bo explained
        this in a thread from Bytedance's perspective.  I explained in an
        answer to Matias how we are not letting zones transition to
        offline in order to simplify the host stack. Not sure if this is
        something we want to bring to NVMe.

      - As ZNS adds more features and other protocols add support for
        zoned devices we will have more use-cases for the zoned block
        device. We will have to deal with these fragmentation at some
        point.

      - This is used in production workloads in Linux hosts. I would
        advocate for this not being off-tree as it will be a headache for
        all in the future.

  - If you agree that removing PO2 is an option, we can do the following:
      - Remove the constraint in the block layer and add ZoneFS support
        in a first patch.

      - Add btrfs support in a later patch

      - Make changes to tools once merged

Hope I have collected all points of view in such a short format.