Chapter 7 - 12. Congestion Management in Ethernet Storage Networks以太网存储网络的拥塞管理

Bit Rate Differences between FC and FCoE

Differences in the bit rates must be carefully accounted for when a switch transfers traffic between FC and FCoE ports. As explained in Chapter 2, the sections on Fibre Channel Bit Rate, Difference Between Fibre Channel Speed and Bit Rate, and Table 2-4, some Fibre Channel bit rates have considerable differences from the advertised speeds. For example, 8GFC port has a bit rate of 8.5 Gbps, but can transfer data only at 6.8 Gbps after accounting for the 8b/10b encoding. This makes 10 GbE almost 50% “faster” than 8GFC. Because of this reason, congestion due to speed mismatch may be seen when FCoE traffic is switched from the 10 Gbps Ethernet ports to 8GFC ports. Another common condition of congestion happens when traffic from 16GFC ports (bit rate of 14.025 Gbps) is switched to FCoE ports operating at 10 Gbps. As this chapter explains, the higher the speed or capacity mismatch the more likely congestion will occur and spread. 交换机在 FC 和 FCoE 端口之间传输流量时，必须仔细考虑比特率的差异。如第 2 章 "光纤通道比特率"、"光纤通道速度与比特率之间的差异 "和表 2-4 所述，某些光纤通道比特率与宣传的速度有很大差异。例如，8GFC 端口的比特率为 8.5 Gbps，但考虑到 8b/10b 编码，只能以 6.8 Gbps 的速度传输数据。这使得 10 GbE 的 "速度 "几乎比 8GFC 快 50%。因此，当 FCoE 流量从 10 Gbps 以太网端口切换到 8GFC 端口时，可能会出现速度不匹配导致的拥塞。当流量从 16GFC 端口（比特率为 14.025 Gbps）切换到工作速率为 10 Gbps 的 FCoE 端口时，会出现另一种常见的拥塞情况。正如本章所述，速度或容量不匹配程度越高，拥塞就越容易发生和蔓延。

These differences in bit rates of Ethernet and Fibre Channel can’t be completely eliminated, but they must be minimized by keeping their speeds as close as possible, for example, 32GFC and 25 GbE. Besides the speed of the ports, the minimum bandwidth guarantee on the shared FCoE links should also be accounted for because the FC/FCoE traffic may not be allowed to consume the entire capacity of the link. 虽然无法完全消除以太网和光纤通道在比特率上的差异，但必须尽可能地缩小它们之间的差距，例如 32GFC 和 25 GbE。除了端口速度外，还应考虑共享 FCoE 链路的最低带宽保证，因为 FC/FCoE 流量可能不允许占用链路的全部容量。

This should be a design-level decision on the switches with FC and FCoE ports, such as Cisco UCS Fabric Interconnect (Refer to Chapter 9). After an initial design, monitor the traffic patterns continuously, and add the additional capacity as needed. 对于带有 FC 和 FCoE 端口的交换机，如 Cisco UCS Fabric Interconnect（参见第 9 章），这应该是一个设计层面的决策。初步设计完成后，应持续监控流量模式，并根据需要增加额外容量。

Multiple no-drop Classes on the Same Link

When multiple no-drop classes are enabled in lossless Ethernet networks, follow the congestion troubleshooting methodology in one class (CoS) at a time. 当无损以太网网络中启用了多个无损类时，请按照每次一个类（CoS）的拥塞故障排除方法进行操作。

The converged topology in