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

[Johannes Thumshirn]

On 15/03/2022 14:52, Javier González wrote:
> 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

(+ linux-btrfs )

Please also make sure to support btrfs and not only throw some patches 
over the fence. Zoned device support in btrfs is complex enough and has 
quite some special casing vs regular btrfs, which we're working on getting
rid of. So having non-power-of-2 zone size, would also mean having NPO2
block-groups (and thus block-groups not aligned to the stripe size).

Just thinking of this and knowing I need to support it gives me a 
headache.

Also please consult the rest of the btrfs developers for thoughts on this.
After all btrfs has full zoned support (including ZNS, not saying it's 
perfect) and is also the default FS for at least two Linux distributions.

Thanks a lot,
	Johannes