What is Congestion in SAN fabric?

Congestion

When over-subscription occurs, it leads to a condition called congestion.

When a node is unable to use as much bandwidth as it wants, because of contention with another node, then there is congestion. A port, link, or fabric can be congested.

Congestion effects application performance.

Congestion can be difficult to detect because it can also be directly related to buffer-to-buffer credit starvation in the switch port. Therefore, when you look at the data throughput from the switch, it seems like normal or less traffic is flowing through the ports. However, the server I/O is unable to perform because the data cannot be transported because of a lack of buffer-to-buffer credits.

Congestion versus over-subscription

Congestion occurs when a channel is bottle-necked and fully utilized. This kind of bottleneck is a congestion bottleneck. You should be aware that “over-subscription” does not have the same meaning as “congestion”. Over-subscription refers only to the potential for congestion; an over-subscribed link may go through a lifetime of normal operation and never be congested. The term over-subscription is not to be used in place of congestion, which is the actual contention for bandwidth by devices through an ISL.

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