Chapter 7 - 14. Congestion Management in Ethernet Storage Networks以太网存储网络的拥塞管理

PFC Watchdog

PFC watchdog works similarly to Pause timeout, but it only drops the traffic in the queue that is unable to transmit continuously for a timeout duration because of receiving PFC Pause frames. PFC 进程看门狗的工作原理与暂停超时类似，但它只会丢弃队列中因收到 PFC 暂停帧而无法在超时时间内连续传输的流量。

Depending upon the implementation, the PFC watchdog can trigger the following actions: 根据实施情况，PFC 看门狗可触发以下操作：

1. Port flap or shutdown: When the PFC watchdog is detected on a queue, the port is flapped or shut down, which affects all the traffic classes on this port. This is the same action as the pause timeout feature. Some people call it a disruptive watchdog. 端口翻转或关闭： 当检测到队列上存在 PFC 进程看门狗时，端口将被翻转或关闭，从而影响该端口上的所有流量类别。这与暂停超时功能的作用相同。有人称其为破坏性看门狗。

2. Alert only: When the PFC watchdog is detected, it generates an alert, such as Syslog, but does not take any further action of dropping the traffic. Some people call it a log-only watchdog. 仅警报： 当检测到 PFC 看门狗时，它会生成警报（如 Syslog），但不会采取任何进一步措施来丢弃流量。有人称其为日志看门狗。

3. Shutdown the queue: This is the most common implementation at the time of this writing. When the PFC watchdog is detected in a no-drop queue, the following actions are taken: 关闭队列： 这是本文撰写时最常见的实现方式。当检测到 PFC 看门狗处于无损队列中时，将采取以下操作：

a. All frames in that queue are dropped (discarded). 该队列中的所有帧都会被丢弃。

b. As long as this queue remains in an Rx Pause state, all newly arriving frames on other ports on this switch that are destined to go out of this queue are immediately dropped. 只要该队列仍处于 Rx Pause（接收暂停）状态，该交换机其他端口上所有新到达的、注定要从该队列流出的帧都会被立即丢弃。

c. All ingress traffic on this port that belongs to the same traffic class (as the egress no-drop class) is also dropped. All ingress Pause frames are dropped as well. This is a unique differentiator of the PFC watchdog. Recall that Pause timeout (available on FCoE ports on Cisco MDS, Nexus, and UCS) and no-credit-drop timeout (available on FC ports on Cisco MDS switches) do not affect ingress traffic on a port. The argument for not dropping the ingress traffic by these features is that congestion is directional, hence the reverse traffic should not be affected. In contrast, the argument for dropping the ingress traffic by PFC watchdog is that applications running on a slow-drain device would not benefit just by unidirectional traffic (egress traffic for slow-drain device which is the same as the ingress traffic for switchport where PFC watchdog is enabled). In fact, egress traffic from a slow drain device may increase the duration and severity of congestion, as explained earlier in the section on Correlating I/O Operations, Traffic Patterns, and Network Congestion. 该端口上属于相同流量类别（如出口无损类别）的所有入口流量也会被丢弃。所有入口暂停帧也会被丢弃。这是 PFC 看门狗的独特区别。请注意，暂停超时（在 Cisco MDS、Nexus 和 UCS 的 FCoE 端口上可用）和无损信元超时（在 Cisco MDS 交换机的 FC 端口上可用）不会影响端口上的入口流量。这些功能无损入口流量的理由是，拥塞是定向的，因此反向流量不应受到影响。与此相反，PFC 看门狗拒绝接收入口流量的理由是，在慢排空设备上运行的应用程序不会因为单向流量（慢排空设备的出口流量与启用了 PFC 看门狗的交换端口的入口流量相同）而受益。事实上，如前面 "I/O 操作、流量模式和网络拥塞的相关性 "一节所述，慢排空设备的出口流量可能会增加拥塞的持续时间和严重程度。

The exact implementation of PFC watchdog feature may differ based on the type of switch. For example: PFC 看门狗功能的具体实现可能因开关类型而异。例如

1. Cisco MDS Switches: PFC watchdog is not available on Cisco MDS switches at the time of this writing. Using the Pause timeout feature is a valid alternative because the MDS switches use only a single no-drop class for only FCoE traffic. Cisco MDS 交换机： 在撰写本文时，Cisco MDS 交换机上不提供 PFC 看门狗功能。使用暂停超时功能是一种有效的替代方法，因为 MDS 交换机只对 FCoE 流量使用单一的无损类。

2. Cisco Nexus Switches: PFC watchdog is available on Cisco Nexus 9000 and Nexus 3000 switches. The support and action (port flap, alert only, queue shutdown) depend on the model type and NX-OS version. Refer to the release notes for the latest information. Cisco Nexus 交换机： Cisco Nexus 9000 和 Nexus 3000 交换机上提供 PFC 看门狗功能。支持和操作（端口翻转、仅警报、队列关闭）取决于型号类型和 NX-OS 版本。有关最新信息，请参阅发布说明。

3. Cisco UCS: Cisco UCS servers support PFC watchdog in UCS Manager release 4.2 onwards. Pause timeout and PFC watchdog are mutually exclusive. In later releases, the PFC watchdog was enabled by default. 思科 UCS： Cisco UCS 服务器在 4.2 版以后的 UCS 管理器中支持 PFC 看门狗。暂停超时和 PFC 进程监控相互排斥。在以后的版本中，PFC 看门狗默认已启用。

The following are the configuration details of PFC watchdog on Cisco Nexus 9000 switches as of NX-OS 10.3(1). 以下是 NX-OS 10.3(1) 中<