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This chapter provides details on the QoS features provided in all switches.
This section includes the following topics:
The Cisco MDS 9000 Family supports QoS for internally and externally generated control traffic. Within a switch, control traffic is sourced to the supervisor module and is treated as a high priority frame. A high priority status provides absolute priority over all other traffic and is assigned in the following cases:
By default, the QoS feature for certain critical control traffic is enabled. These critical control frames are assigned the highest (absolute) priority.
Tip We do not recommend disabling this feature as all critical control traffic is automatically assigned the lowest priority once you issue this command.
To disable the high priority assignment for control traffic, follow these steps:
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Use the show qos statistics command to view the current state of the QoS configuration for critical control traffic. This command displays the current QoS settings along with the number of frames marked high priority. The count is only for debugging purposes and cannot be configured (see Example 2-1).
Example 2-1 Displays Current QoS Settings
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 Figure 2-1).
Figure 2-1 Prioritizing Data Traffic
In Figure 2-1, 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.
While you can configure both zone-based QoS and VSAN-based QoS configurations in the same switch, both configurations have significant differences. Table 2-1 highlights the differences between configuring QoS priorities based on VSANs versus zones.
To configure QoS, follow these steps:
Step 1 Enable the QoS feature.
Step 2 Create and define class maps.
Step 3 Define service policies.
Step 4 Apply the configuration.
By default, the QoS data traffic feature is disabled for data traffic. To configure QoS for data traffic, you must first enable the data traffic feature in the switch.
Tip QoS is supported in interoperability mode. For more information, refer to the Cisco MDS 9000 Family Switch-to-Switch Interoperability Configuration Guide.
To enable the QoS data traffic feature, follow these steps:
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Removes the currently applied QoS configuration and disables QoS. You can no longer configure data traffic parameters. |
Use the class map feature to create and define a traffic class with match criteria to identify traffic belonging to that class. The class map name is restricted to 63 alphanumeric characters and defaults to the match-all option. Flow-based traffic uses one of the following values:
Note An SID or DID of 0x000000 is not allowed.
Tip The order of entries to be matched within a class map is not significant.
Use the class-map command to create and define a traffic class with match criteria to identify traffic belonging to that class. Define each match criterion with one match statement from the class map configuration (switch(config-cmap)) mode.
Note The enhanced mode for the source-device-alias or destination-device-alias option is not supported.
Note The QoS attribute with IVR zone set and VSAN is not supported.
To create a class map, follow these steps:
Service policies are specified using policy maps. Policy maps provide an ordered mapping of class maps to service levels. You can specify multiple class maps within a policy map, and map a class map to a high, medium, or low service level. The default priority is low. The policy map name is restricted to 63 alphanumeric characters.
As an alternative, you can map a class map to a differentiated services code point (DSCP).The DSCP is an indicator of the service level for a specified frame. The DSCP value ranges from 0 to 63, and the default is 0. A DSCP value of 46 is disallowed.
The order of the class maps within a policy map is important to determine the order in which the frame is compared to class maps. The first matching class map has the corresponding priority marked in the frame.
Note Refer to http://www.cisco.com/en/US/tech/tk543/tk757/technologies_tech_note09186a00800949f2.shtml for further information on implementing QoS DSCP values.
Note Class maps are processed in the order in which they are configured in each policy map.
To specify a service policy, follow these steps:
When you have configured a QoS data traffic policy, you must enforce the data traffic configuration by applying that policy to the required VSAN(s). If you do not apply the policy to a VSAN, the data traffic configuration is not enforced. You can only apply one policy map to a VSAN.
Note You can apply the same policy to a range of VSANs.
To apply a service policy, follow these steps:
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The Cisco NX-OS software supports four scheduling queues:
The DWRR scheduler services the queues in the ratio of the configured weights. Higher weights translate to proportionally higher bandwidth and lower latency. The default weights are 50 for the high queue, 30 for the medium queue, and 20 for the low queue. Decreasing order of queue weights is mandated to ensure the higher priority queues have a higher service level, though the ratio of the configured weights can vary (for example, one can configure 70:30:5 or 60:50:10 but not 50:70:10).
Note Generation 1 and Generation 2 modules are not supported from Cisco MDS NX-OS Release 6.x and later.
To associate a weight with a DWRR queue, follow these steps:
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Associates a relative weight (10) to a specified queue (default queue). |
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The show qos commands display the current QoS settings for data traffic (see Examples 2-2 to 2-10 ).
Example 2-2 Displays the Contents of all Class Maps
Example 2-3 Displays the Contents of a Specified Class Map
Example 2-4 Displays All Configured Policy Maps
Example 2-5 Displays a Specified Policy Map
Example 2-6 Displays Scheduled DWRR Configurations
Example 2-7 Displays All Applied Policy Maps
Example 2-8 Displays the Policy Map Associated with a Specified VSAN
Example 2-9 Displays the Class Map Associated with a Specified Interface
Example 2-10 Displays QoS Statistics
This section describes a configuration example for the application illustrated in Figure 2-2.
Figure 2-2 Example Application for Traffic Prioritization
Both the OLTP server and the backup server are accessing the disk. The backup server is writing large amounts of data to the disk. This data does not require specific service guarantees. The volumes of data generated by the OLTP server to the disk are comparatively much lower but this traffic requires faster response because transaction processing is a low latency application.
The point of congestion is the link between Switch 2 and the disk, for traffic from the switch to the disk. The return path is largely uncongested as there is little backup traffic on this path.
Service differentiation is needed at Switch 2 to prioritize the OLTP-server-to-disk traffic higher than the backup-server-to-disk traffic.
To configure traffic prioritization for the example application, follow these steps:
Step 3 Assign the service policy.
Step 4 Assign the weights for the DWRR queues.
Step 5 Repeat Create the class maps. through Assign the weights for the DWRR queues. on Switch 1 to address forward path congestion at both switches.
Congestion could occur anywhere in the example configuration. To address congestion of the return path at both switches, you need to create two more class maps and include them in the policy map as follows:
Step 1 Create two more class maps.
Step 2 Assign the class maps to the policy map.
Step 3 Repeat Create two more class maps. through Assign the class maps to the policy map. on Switch 1 to address return path congestion at both switches.
A port rate limiting feature helps control the bandwidth for individual Fibre Channel ports. Port rate limiting is also referred to as ingress rate limiting because it controls ingress traffic into a Fibre Channel port. The feature controls traffic flow by limiting the number of frames that are transmitted out of the exit point on the MAC. Port rate limiting works on all Fibre Channel ports. The rate limit ranges from 1 to 100% and the default is 100%.
Note Port rate limiting can be configured on all Cisco MDS switches and linecards only if the QoS feature is enabled.
To configure the port rate limiting value, follow these steps: