linux-kernel - Re: [PATCH v6 00/16] nvmet: add target passthru commands support

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Date:   Sat, 27 Jul 2019 00:50:47 +0000
From:   "Stephen  Bates" <sbates@...thlin.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Hannes Reinecke <hare@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
CC:     Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>,
        Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Max Gurtovoy <maxg@...lanox.com>
Subject: Re: [PATCH v6 00/16] nvmet: add target passthru commands support

> This is different from multipath on say a multi-controller PCI device
> and trying to expose both those controllers through passthru. this is
> where the problems we are discussing come up.

I *think* there is some confusion. I *think* Sagi is talking about network multi-path (i.e. the ability for the host to connect to a controller on the target via two different network paths that fail-over as needed). I *think* Logan is talking about multi-port PCIe NVMe devices that allow namespaces to be accessed via more than one controller over PCIe (dual-port NVMe SSDs being the most obvious example of this today).

> But it's the multipath through different ports that
>  seems important for fabrics. ie. If I have a host with a path through
>  RDMA and a path through TCP they should both work and allow fail over.

Yes, or even two paths that are both RDMA or both TCP but which take a different path through the network from host to target.

> Our real-world use case is to support our PCI device which has a bunch
> of vendor unique commands and isn't likely to support multiple
> controllers in the foreseeable future.

I think solving passthru for single-port PCIe controllers would be a good start.

Stephen