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

From: "Javier González" <javier@javigon.com>
To: Damien Le Moal <Damien.LeMoal@wdc.com>
Cc: "Jens Axboe" <axboe@kernel.dk>,
	"Niklas Cassel" <Niklas.Cassel@wdc.com>,
	"Ajay Joshi" <Ajay.Joshi@wdc.com>,
	"Sagi Grimberg" <sagi@grimberg.me>,
	"Keith Busch" <Keith.Busch@wdc.com>,
	"Dmitry Fomichev" <Dmitry.Fomichev@wdc.com>,
	"Aravind Ramesh" <Aravind.Ramesh@wdc.com>,
	"linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"Hans Holmberg" <Hans.Holmberg@wdc.com>,
	"Keith Busch" <kbusch@kernel.org>,
	"Matias Bjørling" <mb@lightnvm.io>,
	"Christoph Hellwig" <hch@lst.de>,
	"Matias Bjorling" <Matias.Bjorling@wdc.com>
Subject: Re: [PATCH 5/5] nvme: support for zoned namespaces
Date: Wed, 17 Jun 2020 08:18:14 +0200	[thread overview]
Message-ID: <20200617061814.7syifpwn5sqg5a4w@mpHalley.localdomain> (raw)
In-Reply-To: <CY4PR04MB37518F1A34F92049EE8FAF94E79A0@CY4PR04MB3751.namprd04.prod.outlook.com>

On 17.06.2020 00:38, Damien Le Moal wrote:
>On 2020/06/17 1:13, Javier González wrote:
>> On 16.06.2020 09:07, Keith Busch wrote:
>>> On Tue, Jun 16, 2020 at 05:55:26PM +0200, Javier González wrote:
>>>> On 16.06.2020 08:48, Keith Busch wrote:
>>>>> On Tue, Jun 16, 2020 at 05:02:17PM +0200, Javier González wrote:
>>>>>> This depends very much on how the FS / application is managing
>>>>>> stripping. At the moment our main use case is enabling user-space
>>>>>> applications submitting I/Os to raw ZNS devices through the kernel.
>>>>>>
>>>>>> Can we enable this use case to start with?
>>>>>
>>>>> I think this already provides that. You can set the nsid value to
>>>>> whatever you want in the passthrough interface, so a namespace block
>>>>> device is not required to issue I/O to a ZNS namespace from user space.
>>>>
>>>> Mmmmm. Problem now is that the check on the nvme driver prevents the ZNS
>>>> namespace from being initialized. Am I missing something?
>>>
>>> Hm, okay, it may not work for you. We need the driver to create at least
>>> one namespace so that we have tags and request_queue. If you have that,
>>> you can issue IO to any other attached namespace through the passthrough
>>> interface, but we can't assume there is an available namespace.
>>
>> That makes sense for now.
>>
>> The next step for us is to enable a passthrough on uring, making sure
>> that I/Os do not split.
>
>Passthrough as in "application issues directly NVMe commands" like for SG_IO
>with SCSI ? Or do you mean raw block device file accesses by the application,
>meaning that the IO goes through the block IO stack as opposed to directly going
>to the driver ?
>
>For the latter case, I do not think it is possible to guarantee that an IO will
>not get split unless we are talking about single page IOs (e.g. 4K on X86). See
>a somewhat similar request here and comments about it.
>
>https://www.spinics.net/lists/linux-block/msg55079.html

At the moment we are doing the former, but it looks like a hack to me to
go directly to the NVMe driver.

I was thinking that we could enable the second path by making use of
chunk_sectors and limit the I/O size just as the append_max_io_size
does. Is this the complete wrong way of looking at it?

Thanks,
Javier

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