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Message-ID: <20220328213353.4aca75bd@kernel.org> Date: Mon, 28 Mar 2022 21:33:53 -0700 From: Jakub Kicinski <kuba@...nel.org> To: Mingbao Sun <sunmingbao@....com> Cc: Sagi Grimberg <sagi@...mberg.me>, Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>, Christoph Hellwig <hch@....de>, Chaitanya Kulkarni <kch@...dia.com>, linux-nvme@...ts.infradead.org, linux-kernel@...r.kernel.org, Eric Dumazet <edumazet@...gle.com>, "David S . Miller" <davem@...emloft.net>, Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>, David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org, tyler.sun@...l.com, ping.gan@...l.com, yanxiu.cai@...l.com, libin.zhang@...l.com, ao.sun@...l.com Subject: Re: [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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