Fancy a decongestant? Cisco intros ‘slow drain’ tech for 64gig Fibre Channel

Cisco has launched its 64Gbit/s Fibre Channel offering with a slow drain technology that should prevent overwhelmed devices congesting the network.

Fibre Channel (FC) is a lossless storage networking fabric currently using generation six 32Gbit/’s technology with a transition taking place to twice-as-fast generation seven 64Gbit/s speed led by Broadcom. Cisco indicated it would support gen seven back in September 2019, so this has been a while coming.

Now Switchzilla has announced its MDS 9700 48-Port 64Gbit/s line-rate multi-speed Fibre Channel Switching Module and an NVMe-enhanced FC analytics capability in its SAN Insights product.

Thomas Scheibe, Cisco VP for Data Centre Networking, said of the move: “Cisco is staying true to its long-standing customer promise to increase value of the existing SAN infrastructure verses forcing a chassis upgrade and a replacement of optics.”

The 9700 supports 16, 32 and 64Gbit/s FC traffic and the new 64Gbit/s line card can be fitted, with a field upgrade, into an existing 9700 chassis, with the reuse of all existing Cisco FC optics across existing and new ports. That backwards compatibility helps with migration to 64gig technology but increases the speed difference between the slowest and fastest sending and receiving devices in the FC fabric.

This brings with it a risk of so-called slow drain congestion. Since FC is lossless, the frames of data need to traverse the fabric with no interruption, but interruptions can occur as too many frames hit a device or switch at a time. Ports have to have buffers to store the incoming frames and, when they are full, no more frames can be accepted. Such switch port buffer starvation can choke inter-switch links. Traffic flow then gets interrupted and data access latency across the fabric increases as the SAN performance degrades.

Slow drain technology detects and controls the draining of port frame buffers. Cisco has added Dynamic Ingress Rate Limiting (DIRL) to prevent such slow draining taking place. DIRL manages the frame rate limit at the ingress to a port, independent of HBA (Host Bus Adapter) hardware and and firmware. It’s like a traffic light-controlled on-ramp to a highway, minimising highway congestion

DIRL is an agent-less, fabric-centric approach that is compatible with hosts and targets of any FC vendor and generation, controlled and governed by the MDS 9000 devices and their intelligence.

The new 64Gbit/s line cards have a new chipset for low power consumption and enhanced on-chip Fibre Channel Analytics processing and a dedicated analytics NPU (Network Processing Unit) engine. This is used by Cisco’s SAN Insights Discovery, a free cloud-based tool that provides SAN fabric health information and insights into potential fabric issues.

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