Re: [Qemu-devel] [Qemu-block] [RFC] nvme: how to support multiple namespaces

[Kevin Wolf <kwolf@redhat.com>]

Am 24.06.2019 um 22:46 hat Laszlo Ersek geschrieben:
> On 06/24/19 12:18, Kevin Wolf wrote:
> > Am 24.06.2019 um 10:01 hat Klaus Birkelund geschrieben:
> >> On Thu, Jun 20, 2019 at 05:37:24PM +0200, Laszlo Ersek wrote:
> >>> On 06/17/19 10:12, Klaus Birkelund wrote:
> >>>> Hi all,
> >>>>
> >>>> I'm thinking about how to support multiple namespaces in the NVMe
> >>>> device. My first idea was to add a "namespaces" property array to the
> >>>> device that references blockdevs, but as Laszlo writes below, this might
> >>>> not be the best idea. It also makes it troublesome to add per-namespace
> >>>> parameters (which is something I will be required to do for other
> >>>> reasons). Some of you might remember my first attempt at this that
> >>>> included adding a new block driver (derived from raw) that could be
> >>>> given certain parameters that would then be stored in the image. But I
> >>>> understand that this is a no-go, and I can see why.
> >>>>
> >>>> I guess the optimal way would be such that the parameters was something
> >>>> like:
> >>>>
> >>>>    -blockdev raw,node-name=blk_ns1,file.driver=file,file.filename=blk_ns1.img
> >>>>    -blockdev raw,node-name=blk_ns2,file.driver=file,file.filename=blk_ns2.img
> >>>>    -device nvme-ns,drive=blk_ns1,ns-specific-options (nsfeat,mc,dlfeat)...
> >>>>    -device nvme-ns,drive=blk_ns2,...
> >>>>    -device nvme,...
> >>>>
> >>>> My question is how to state the parent/child relationship between the
> >>>> nvme and nvme-ns devices. I've been looking at how ide and virtio does
> >>>> this, and maybe a "bus" is the right way to go?
> >>>
> >>> I've added Markus to the address list, because of this question. No
> >>> other (new) comments from me on the thread starter at this time, just
> >>> keeping the full context.
> >>>
> >>
> >> Hi all,
> >>
> >> I've succesfully implemented this by introducing a new 'nvme-ns' device
> >> model. The nvme device creates a bus named from the device id ('id'
> >> parameter) and the nvme-ns devices are then registered on this.
> >>
> >> This results in an nvme device being creates like this (two namespaces
> >> example):
> >>
> >>   -drive file=nvme0n1.img,if=none,id=disk1
> >>   -drive file=nvme0n2.img,if=none,id=disk2
> >>   -device nvme,serial=deadbeef,id=nvme0
> >>   -device nvme-ns,drive=disk1,bus=nvme0,nsid=1
> >>   -device nvme-ns,drive=disk2,bus=nvme0,nsid=2
> >>
> >> How does that look as a way forward?
> > 
> > This looks very similar to what other devices do (one bus controller
> > that has multiple devices on its but), so I like it.
> 
> +1
> 
> Also, I believe it's more modern nowadays to express the same example
> with "blockdev" syntax, rather than "drive". (Not that I could suggest
> the exact spelling for that :)) I don't expect the modern syntax to
> behave differently, I just guess it's better to stick with the new in
> examples / commit messages etc.

As this example uses only raw files, it's actually pretty simple:

-blockdev driver=file,filename=nvme0n1.img,node-name=disk1
-blockdev driver=file,filename=nvme0n2.img,node-name=disk2

The -device options stay the same, their drive=... value just refers to
the node-name now. (-drive IDs and node-names have a shared namespace,
so this is unambiguous.)

For the sake of completeness, if nvme0n1.img were actually a qcow2
image, you would add a second -blockdev for the format layer:

-blockdev driver=file,filename=nvme0n1.img,node-name=disk1-file
-blockdev driver=qcow2,file=disk1-file,node-name=disk1

Kevin