HP Cisco MDS 9020 Cisco MDS 9000 Family Fabric Manager Configuration Guide, Re - Page 1237

Chapter 66 Configuring Fabric Congestion Control and QoS QoS Send documentation comments to [email protected] Figure 66-2 Prioritizing Data Traffic OLTP server Backup server VOQ(s) Absolute Congestion VOQ(s) Absolute Disk FC High High Medium Medium Low Low 105228 Switch 1 Switch 2 In Figure 66-2, the OLTP traffic arriving at Switch 1 is marked with a high priority level of throughput classification (class map) and marking (policy map). Similarly, the backup traffic is marked with a low priority level. The traffic is sent to the corresponding priority queue within a virtual output queue (VOQ). A deficit weighted round robin (DWRR) scheduler configured in the first switch ensures that high priority traffic is treated better than low priority traffic. For example, DWRR weights of 70:20:10 implies that the high priority queue is serviced at 7 times the rate of the low priority queue. This guarantees lower delays and higher bandwidths to high priority traffic if congestion sets in. A similar configuration in the second switch ensures the same traffic treatment in the other direction. If the ISL is congested when the OLTP server sends a request, the request is queued in the high priority queue and is serviced almost immediately since the high priority queue is not congested. The scheduler assigns its priority over the backup traffic in the low priority queue. Note When the high priority queue does not have traffic flowing through, the low priority queue uses all the bandwidth and is not restricted to the configured value. A similar occurrence in Switch 2 sends a response to the transaction request. The round trip delay experienced by the OLTP server is independent of the volume of low priority traffic or the ISL congestion. The backup traffic uses the available ISL bandwidth when it is not used by the OLTP traffic. Tip To achieve this traffic differentiation, be sure to enable FCC (see the "Enabling FCC" section on page 66-2). VSAN Versus Zone-Based QoS While you can configure both zone-based QoS and VSAN-based QoS configurations in the same switch, both configurations have significant differences. Table 66-1 highlights the differences between configuring QoS priorities based on VSANs versus zones. OL-16184-01, Cisco MDS SAN-OS Release 3.x Cisco MDS 9000 Family CLI Configuration Guide 66-5