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Re: [RFC PATCH 0/5] hw/block/nvme: support multi-path for ctrl/ns

From: Minwoo Im
Subject: Re: [RFC PATCH 0/5] hw/block/nvme: support multi-path for ctrl/ns
Date: Sat, 16 Jan 2021 03:38:18 +0900
User-agent: Mutt/1.11.4 (2019-03-13)

On 21-01-15 18:47:20, Klaus Jensen wrote:
> On Jan 15 09:35, Keith Busch wrote:
> > On Fri, Jan 15, 2021 at 02:57:45PM +0100, Klaus Jensen wrote:
> > > 
> > > As you already mentioned, the problem I see with this approach is that
> > > we have separate namespaces attached to separate controllers. So we are
> > > faking it to the max and if I/O starts going through the other
> > > controller we end up on a namespace that is unrelated (different data).
> > > Havoc ensues.
> > > 
> > > My approach looks a lot like yours, but I hacked around this by adding
> > > extra 'ctrl-0', 'ctrl-1', ..., link-parameters to the namespace device,
> > > replacing the bus. This works well because the namespace then just
> > > registers with multiple controllers. But adding more parameters like
> > > that just isnt nice, so I've been trying to figure out how to allow a
> > > parameter to be specified multiple times, so we could just do more
> > > 'ctrl'-parameters.
> > > 
> > > Alas, since I started thinking about namespace sharing I have been
> > > regretting that I didn't reverse the bus-mechanic for namespace
> > > attachment. It would align better with the theory of operation if it was
> > > the controllers that attached to namespaces, and not the other way
> > > around. So what I would actually really prefer, is that we had multiple
> > > 'ns' link parameters on the controller device.
> > 
> > Would this work better if we introduce a new device in the nvme hierarchy:
> > the nvme-subsystem? You could attach multi-path namespaces and
> > controllers to that, and namespaces you don't want shared can attach
> > directly to controllers like they do today. You could also auto-assign
> > cntlid, and you wouldn't need to duplicate serial numbers in your
> > parameters.
> I kinda POC'ed that, but I think I tried to make it work with a bus and
> walking it and all kinds of fancy stuff.
> I think it can just be a 'link' parameter, so something like:
>   -device nvme-subsys,id=subsys0

Do we have any plan for default subsys hierarchy?  Or is it going to be
a mandatory root node of nvme controllers and namespaces?

>   -device nvme,id=nvme0,subsys=subsys0
>   -device nvme,id=nvme1,subsys=subsys0
>   -device nvme-ns,id=shared-ns1,nsid=1,subsys=subsys0

In this case, what is the default set-up for shared-ns1?  Is this
namespace going to be ready right after the two nvme controllers being
realized?  If so, do we iterate all the namespace devices in the NVM
subsystem and attach them to this controller in the initial time?

If so, I agree with this approach.

>   -device nvme-ns,id=private-ns2,nsid=2,bus=nvme0

This must be the case what Keith mentioned of directly attaching to a
controller.  It looks nice.  But, one concerning point here is that, in
!shared namespace, if we don't specify 'subsys' property here to attach
it to directly to a controller, it means it implicitly will belong to
the subsys0 where the nvme0 belongs to.  It means that user should give
nsid different than 1 which is already shared.

So, how do we make subsys property as a mandatory for namespace device
and provide optional choice for bus.  If bus is given to a controller,
then it can mean a private namespace, otherwise it can be shared among
controllers in a subsystem.

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