The Cisco MDS 9000 Series 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:

• Internally generated time-critical control traffic (mostly Class F frames).
• Externally generated time-critical control traffic entering a switch in the Cisco MDS 9000 Series from a another vendor’s switch. High priority frames originating from other vendor switches are marked as high priority as they enter a switch in the Cisco MDS 9000 Series.

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.

The following example displays Current QoS Settings

switch# show qos statistics
Total number of FC frames transmitted from the Supervisor= 15767
Number of highest-priority FC frames transmitted         = 8224
Current priority of FC control frames = 0    (0 = lowest; 7 = highest)

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 Series 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.

In the above figure, 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. The following table highlights the differences between configuring QoS priorities based on VSANs versus zones.

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:

• WWN—The source WWN or the destination WWN.
• Fibre Channel ID (FC ID) —The source ID (SID) or the destination ID (DID). The possible values for mask are FFFFFF (the entire FC ID is used—this is the default), FFFF00 (only domain and area FC ID is used), or FF0000 (only domain FC ID is used).

Note	An SID or DID of 0x000000 is not allowed.

• Source interface—The ingress interface.

Tip	The order of entries to be matched within a class map is not significant.

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	For more information on implementing QoS DSCP values, see the following document: Implementing Quality of Service Policies with DSCP.

Note	Class maps are processed in the order in which they are configured in each policy map.

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.

The Cisco NX-OS software supports four scheduling queues:

• Strict priority queues are queues that are serviced in preference to other queues—it is always serviced if there is a frame queued in it regardless of the state of the other queues.
• QoS assigns all other traffic to the DWRR scheduling high, medium, and low priority traffic 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. Generation 3 and Generation 4 modules are not supported from Cisco MDS NX-OS Release 8.x and later. For more information on the modules and ports supported, refer the Cisco MDS 9000 Series Interface Configuration Guide, Release 8.x guide.

The show qos commands display the current QoS settings for data traffic (see the following examples).

switch# show qos class-map
qos class-map MyClass match-any
    match destination-wwn 20:01:00:05:30:00:28:df
    match source-wwn 23:15:00:05:30:00:2a:1f
    match input-interface fc2/1
qos class-map Class2 match-all
    match input-interface fc2/14
qos class-map Class3 match-all
    match source-wwn 20:01:00:05:30:00:2a:1f

switch# show qos class-map name MyClass
qos class-map MyClass match-any
    match destination-wwn 20:01:00:05:30:00:28:df
    match source-wwn 23:15:00:05:30:00:2a:1f
    match input-interface fc2/1

switch# show qos policy-map
qos policy-map MyPolicy
    class MyClass
    priority medium
qos policy-map Policy1
    class Class2
    priority low

switch# show qos policy-map name MyPolicy
qos policy-map MyPolicy
    class MyClass
        priority medium

switch# show qos dwrr
qos dwrr-q high weight 50
qos dwrr-q medium weight 30
qos dwrr-q low weight 20

switch# show qos service policy
qos service policy MyPolicy vsan 1
qos service policy Policy1 vsan 4

switch# show qos service policy vsan 1
qos policy-map pmap1
   class cmap1
       priority medium
   class cmap2
       priority high

switch# show qos service policy interface fc3/10
qos policy-map pmap1
   class cmap3
       priority high
   class cmap4
       priority low

switch# show qos statistics
Total number of FC frames transmitted from the Supervisor= 301431
Number of highest-priority FC frames transmitted         = 137679
Current priority of FC control frames = 7    (0 = lowest; 7 = highest)

This section describes a configuration example for the application illustrated in the following figure.

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.