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Date:   Tue, 22 Dec 2020 09:36:16 -0500
From:   Mike Snitzer <snitzer@redhat.com>
To:     Christoph Hellwig <hch@infradead.org>
Cc:     Palmer Dabbelt <palmer@dabbelt.com>, josef@toxicpanda.com,
        bvanassche@acm.org, corbet@lwn.net, kernel-team@android.com,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-raid@vger.kernel.org, song@kernel.org, dm-devel@redhat.com,
        linux-kselftest@vger.kernel.org, shuah@kernel.org, agk@redhat.com,
        michael.christie@oracle.com
Subject: Re: [PATCH v1 0/5] dm: dm-user: New target that proxies BIOs to
 userspace
Message-ID: <20201222143616.GB12885@redhat.com>
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 <20201222133246.GA5099@infradead.org>
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On Tue, Dec 22 2020 at  8:32am -0500,
Christoph Hellwig <hch@infradead.org> wrote:

> On Mon, Dec 14, 2020 at 07:00:57PM -0800, Palmer Dabbelt wrote:
> > I haven't gotten a whole lot of feedback, so I'm inclined to at least have some
> > reasonable performance numbers before bothering with a v2.
> 
> FYI, my other main worry beside duplicating nbd is that device mapper
> really is a stacked interface that sits on top of other block device.
> Turning this into something else that just pipes data to userspace
> seems very strange.

I agree.  Only way I'd be interested is if it somehow tackled enabling
much more efficient IO.  Earlier discussion in this thread mentioned
that zero-copy and low overhead wasn't a priority (because it is hard,
etc).  But the hard work has already been done with io_uring.  If
dm-user had a prereq of leaning heavily on io_uring and also enabled IO
polling for bio-based then there may be a win to supporting it.

But unless lower latency (or some other more significant win) is made
possible I just don't care to prop up an unnatural DM bolt-on.

Mike