Chapter 3-3. Detecting Congestion in Fibre Channel Fabrics

Congestion Direction — Ingress or Egress

Congestion is directional. That is, each link is comprised of two unidirectional links bundled together. Congestion usually occurs in one direction, while the reverse direction may be uncongested. Consequently, understanding the direction of the congestion is important. Attempting to follow the congestion in the wrong direction does not lead to finding the source and the cause of it. 拥塞是定向的。也就是说，每个链路都由捆绑在一起的两个单向链路组成。拥塞通常发生在一个方向上，而反方向可能不拥塞。因此，了解拥塞的方向非常重要。如果试图沿着错误的方向跟踪拥塞情况，则无法找到拥塞的源头和原因。

For example, Figure 3-1 shows that traffic flows from hosts to targets and from targets to hosts (bi-directional), but only the traffic from targets to hosts is affected by congestion if Host-1 is a slow-drain device. Based on the port location, the congestion can be in the ingress direction or egress direction. But the overall congestion direction is against the traffic flow—from the destination to the source of traffic. 例如，图 3-1 显示流量从主机流向目标和从目标流向主机（双向），但如果 Host-1 是慢排空设备，则只有从目标流向主机的流量会受到拥塞影响。根据端口位置，拥塞可能发生在入口方向，也可能发生在出口方向。但总的拥塞方向与流量流向相反--从目的地到流量源。

Egress Congestion

Egress congestion indicates congestion toward the neighbor on that port. For example, if the neighbor is a host (initiator), then this initiator is throttling the switchport, thus slowing down the frame transmission from the switch to the initiator. 出口拥塞表示该端口上的邻居出现拥塞。例如，如果邻居是主机（启动程序），那么这个启动程序就会对交换端口进行节流，从而减慢从交换机到启动程序的帧传输速度。

As Figure 3-1 shows, congestion on an egress switchport results in congestion on the ingress port, which causes egress congestion on the upstream ports along the traffic path towards the source. In other words, egress congestion causes ingress congestion. Ingress congestion does not cause egress congestion. Because of this reason, congestion detection workflow and metrics in the egress direction are more important for identifying the source of congestion. 如图 3-1 所示，出口交换端口的拥塞会导致入口端口的拥塞，入口端口的拥塞又会导致流向源的上游端口的出口拥塞。换句话说，出口拥塞会导致入口拥塞。入口拥塞不会导致出口拥塞。因此，出口方向的拥塞检测工作流程和指标对于识别拥塞源更为重要。

原文地址：https://blog.csdn.net/mounter625/article/details/144780831

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