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SubjectRe: [PATCH v2 2/3] nvme-tcp: support specifying the congestion-control
On Tue, 29 Mar 2022 10:48:06 +0800 Mingbao Sun wrote:
> A server in a data-center with the following 2 NICs:
>
> - NIC_fron-end, for interacting with clients through WAN
> (high latency, ms-level)
>
> - NIC_back-end, for interacting with NVMe/TCP target through LAN
> (low latency, ECN-enabled, ideal for dctcp)
>
> This server interacts with clients (handling requests) via the fron-end
> network and accesses the NVMe/TCP storage via the back-end network.
> This is a normal use case, right?

Well, if you have clearly separated networks you can set the congestion
control algorithm per route, right? man ip-route, search congctl.

> For the client devices, we can’t determine their congestion-control.
> But normally it’s cubic by default (per the CONFIG_DEFAULT_TCP_CONG).
> So if we change the default congestion control on the server to dctcp
> on behalf of the NVMe/TCP traffic of the LAN side, it could at the
> same time change the congestion-control of the front-end sockets
> to dctcp while the congestion-control of the client-side is cubic.
> So this is an unexpected scenario.
>
> In addition, distributed storage products like the following also have
> the above problem:
>
> - The product consists of a cluster of servers.
>
> - Each server serves clients via its front-end NIC
> (WAN, high latency).
>
> - All servers interact with each other via NVMe/TCP via back-end NIC
> (LAN, low latency, ECN-enabled, ideal for dctcp).

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