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Date:   Thu, 24 Mar 2022 07:20:53 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Sagi Grimberg <sagi@grimberg.me>
Cc:     Jens Axboe <axboe@kernel.dk>, Kanchan Joshi <joshiiitr@gmail.com>,
        Kanchan Joshi <joshi.k@samsung.com>,
        Christoph Hellwig <hch@lst.de>,
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        io-uring@vger.kernel.org, linux-nvme@lists.infradead.org,
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        Javier =?iso-8859-1?Q?Gonz=E1lez?= <javier@javigon.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Adam Manzanares <a.manzanares@samsung.com>,
        Anuj Gupta <anuj20.g@samsung.com>
Subject: Re: [PATCH 05/17] nvme: wire-up support for async-passthru on
 char-device.
Message-ID: <20220324062053.GA12519@lst.de>
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On Wed, Mar 16, 2022 at 04:50:53PM +0200, Sagi Grimberg wrote:
>>> I know, and that was my original question, no one cares that this
>>> interface completely lacks this capability? Maybe it is fine, but
>>> it is not a trivial assumption given that this is designed to be more
>>> than an interface to send admin/vs commands to the controller...
>>
>> Most people don't really care about or use multipath, so it's not a
>> primary goal.
>
> This statement is generally correct. However what application would be 
> interested in speaking raw nvme to a device and gaining performance that
> is even higher than the block layer (which is great to begin with)?

If passthrough is faster than the block I/O path we're doing someting
wrong.  At best it should be the same performance.

That being said multipathing is an integral part of the nvme driver
architecture, and the /dev/ngX devices.  If we want to support uring
async commands on /dev/ngX it will have to support multipath.