Белая книга для Cisco Cisco MDS 9500 Series Supervisor-2 Module
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White Paper
Cisco MDS 9000 Family
Quality of Service
Quality of Service
Quality of Service (QoS) enables traffic differentiation and prioritization, allowing latency-sensitive applications such as
online transaction processing (OLTP) to share storage resources alongside throughput-intensive applications such as
data warehousing.
INTRODUCTION
This white paper serves as a guide to the advanced QoS and traffic engineering features present in Cisco
®
MDS 9000 Family switches. It describes
enhancements to QoS in second-generation Cisco line cards and supervisors and includes example storage area network (SAN) designs in which
enabling QoS will provide better overall service.
WHY IS QoS NEEDED?
The primary goal of QoS is to provide priority for traffic flows to and from specific devices. In this context, priority means providing lower latency
and higher bandwidth connections with more controlled jitter.
An underlying principle of Fibre Channel switching is that the network guarantees that no frames will be dropped. If this is the case, why do we
need QoS at all? Switches today provide high-performance, non-blocking, non-oversubscribed crossbar switch fabrics. The Cisco MDS 9513
Multilayer Director can switch more than a billion frames per second. Why would users ever need QoS when a switch fabric provides seemingly
endless amounts of frame-switching capacity?
The answer is simple: congestion.
Congestion occurs for two basic reasons:
•
Congestion will occur if multiple senders are contending with a smaller number of receivers. If the aggregate rate of traffic transmitted by senders
exceeds the size of the connection to the receivers, blocking will occur (Figure 1).
•
Any time there is a speed mismatch between senders and receivers, buffering will occur. Buffers are a finite resource on switches, typically in the
range of 16 buffers (32 KB) to 255 buffers (512 KB) per port. When these buffers are full, blocking occurs (Figure 2).
Figure 1. Congestion Caused by Senders Outnumbering Receivers
Figure 2. Congestion Caused by Speed Mismatch between Senders & Receivers