Configuring Fabric Congestion Control and QoS
S e n d d o c u m e n t a t i o n c o m m e n t s t o m d s f e e d b a c k - d o c @ c i s c o . c o m
About Data Traffic
Online transaction processing (OLTP), which is a low volume, latency sensitive application, requires
quick access to requested information. Backup processing application require high bandwidth but are
not sensitive to latency. In a network that does not support service differentiation, all traffic is treated
identically—they experience similar latency and are allocated similar bandwidths. The QoS feature in
the Cisco MDS 9000 Family switches provides these guarantees.
Data traffic can be prioritized in distinct levels of service differentiation: low, medium, or high priority.
You can apply QoS to ensure that Fibre Channel data traffic for your latency-sensitive applications
receive higher priority over throughput-intensive applications such as data warehousing (see
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.
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.
OL-16184-01, Cisco MDS SAN-OS Release 3.x
Prioritizing Data Traffic
56-2, the OLTP traffic arriving at Switch 1 is marked with a high priority level of throughput
Cisco MDS 9000 Family CLI Configuration Guide