Re: [PATCH 5/5] nvme: support for zoned namespaces

["Javier González" <javier@javigon.com>]

On 17.06.2020 18:55, Matias Bjorling wrote:
>> -----Original Message-----
>> From: Javier González <javier@javigon.com>
>> Sent: Wednesday, 17 June 2020 20.29
>> To: Matias Bjørling <mb@lightnvm.io>
>> Cc: Christoph Hellwig <hch@lst.de>; Keith Busch <Keith.Busch@wdc.com>;
>> linux-nvme@lists.infradead.org; linux-block@vger.kernel.org; Damien Le Moal
>> <Damien.LeMoal@wdc.com>; Matias Bjorling <Matias.Bjorling@wdc.com>;
>> Sagi Grimberg <sagi@grimberg.me>; Jens Axboe <axboe@kernel.dk>; Hans
>> Holmberg <Hans.Holmberg@wdc.com>; Dmitry Fomichev
>> <Dmitry.Fomichev@wdc.com>; Ajay Joshi <Ajay.Joshi@wdc.com>; Aravind
>> Ramesh <Aravind.Ramesh@wdc.com>; Niklas Cassel
>> <Niklas.Cassel@wdc.com>; Judy Brock <judy.brock@samsung.com>
>> Subject: Re: [PATCH 5/5] nvme: support for zoned namespaces
>>
>> On 17.06.2020 19:57, Matias Bjørling wrote:
>> >On 17/06/2020 16.42, Javier González wrote:
>> >>On 17.06.2020 09:43, Christoph Hellwig wrote:
>> >>>On Tue, Jun 16, 2020 at 12:41:42PM +0200, Javier González wrote:
>> >>>>On 16.06.2020 08:34, Keith Busch wrote:
>> >>>>>Add support for NVM Express Zoned Namespaces (ZNS) Command Set
>> >>>>>defined in NVM Express TP4053. Zoned namespaces are discovered
>> >>>>>based on their Command Set Identifier reported in the namespaces
>> >>>>>Namespace Identification Descriptor list. A successfully discovered
>> >>>>>Zoned Namespace will be registered with the block layer as a host
>> >>>>>managed zoned block device with Zone Append command support. A
>> >>>>>namespace that does not support append is not supported by the driver.
>> >>>>
>> >>>>Why are we enforcing the append command? Append is optional on the
>> >>>>current ZNS specification, so we should not make this mandatory in
>> >>>>the implementation. See specifics below.
>> >>>
>> >>>Because Append is the way to go and we've moved the Linux zoned block
>> >>>I/O stack to required it, as should have been obvious to anyone
>> >>>following linux-block in the last few months.  I also have to say I'm
>> >>>really tired of the stupid politics tha your company started in the
>> >>>NVMe working group, and will say that these do not matter for Linux
>> >>>development at all.  If you think it is worthwhile to support devices
>> >>>without Zone Append you can contribute support for them on top of
>> >>>this series by porting the SCSI Zone Append Emulation code to NVMe.
>> >>>
>> >>>And I'm not even going to read the rest of this thread as I'm on a
>> >>>vacation that I badly needed because of the Samsung TWG bullshit.
>> >>
>> >>My intention is to support some Samsung ZNS devices that will not
>> >>enable append. I do not think this is an unreasonable thing to do. How
>> >>/ why append ended up being an optional feature in the ZNS TP is
>> >>orthogonal to this conversation. Bullshit or not, it ends up on
>> >>devices that we would like to support one way or another.
>> >
>> >I do not believe any of us have said that it is unreasonable to
>> >support. We've only asked that you make the patches for it.
>> >
>> >All of us have communicated why Zone Append is a great addition to the
>> >Linux kernel. Also, as Christoph points out, this has not been a secret
>> >for the past couple of months, and as Martin pointed out, have been a
>> >wanted feature for the past decade in the Linux community.
>>
>> >
>> >I do want to politely point out, that you've got a very clear signal
>> >from the key storage maintainers. Each of them is part of the planet's
>> >best of the best and most well-respected software developers, that
>> >literally have built the storage stack that most of the world depends
>> >on. The storage stack that recently sent manned rockets into space.
>> >They each unanimously said that the Zone Append command is the right
>> >approach for the Linux kernel to reduce the overhead of I/O tracking
>> >for zoned block devices. It may be worth bringing this information to
>> >your engineering organization, and also potentially consider Zone
>> >Append support for devices that you intend to used with the Linux
>> >kernel storage stack.
>>
>> I understand and I have never said the opposite.
>>
>> Append is a great addition that
>
>One may have interpreted your SDC EMEA talk the opposite. It was not
>very neutral towards Zone Append, but that is of cause one of its least
>problems. But I am happy to hear that you've changed your opinion.

As you are well aware, there are some cases where append introduces
challenges. This is well-documented on the bibliography around nameless
writes. Part of the talk was on presenting an alternative for these
particular use cases.

This said, I am not afraid of changing my point of view when I am proven
wrong.

>
>> we also have been working on for several months (see patches additions from
>> today). We just have a couple of use cases where append is not required and I
>> would like to make sure that they are supported.
>>
>> At the end of the day, the only thing I have disagreed on is that the NVMe
>> driver rejects ZNS SSDs that do not support append, as opposed to doing this
>> instead when an in-kernel user wants to utilize the drive (e.g., formatting a FS
>> with zoned support) This would allow _today_
>> ioctl() passthru to work for normal writes.
>>
>> I still believe the above would be a more inclusive solution with the current ZNS
>> specification, but I can see that the general consensus is different.
>
>The comment from the community, including me, is that there is a
>general requirement for Zone Append command when utilizing Zoned
>storage devices. This is similar to implement an API that one wants to
>support. It is not a general consensus or opinion. It is hard facts and
>how the Linux kernel source code is implemented at this point. One must
>implement support for ZNS SSDs that do not expose the Zone Append
>command natively. Period.

Again, I am not saying the opposite. Read the 2 lines below...

>>
>> So we will go back, apply the feedback that we got and return with an
>> approach that better fits the ecosystem.
>>
>> >
>> >Another approach, is to use SPDK, and bypass the Linux kernel. This
>> >might even be an advantage, your customers does not have to wait on the
>> >Linux distribution being released with a long term release, before they
>> >can even get started and deploy in volume. I.e., they will actually get
>> >faster to market, and your company will be able to sell more drives.
>>
>> I think I will refrain from discussing our business strategy on an open mailing
>> list. Appreciate the feedback though. Very insightful.
>
>I am not asking for you to discuss your business strategy on the mailing list. My comment was to give you genuinely advise that may save a lot of work, and might even get better results.
>
>>
>> Thanks,
>> Javier