Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) - Manual (Page 390)

Types of bottlenecks
The bottleneck detection feature detects two types of bottlenecks:
• Latency bottleneck
• Congestion bottleneck
A latency bottleneck is a port where the offered load exceeds the rate at which the other end of the link
can continuously accept traffic, but does not exceed the physical capacity of the link. This condition
can be caused by a device attached to the fabric that is slow to process received frames and send
back credit returns. A latency bottleneck caused by such a device can spread through the fabric and
can slow down unrelated flows that share links with the slow flow.
By default, bottleneck detection detects latency bottlenecks that are severe enough that they cause 98
percent loss of throughput. This default value can be modified to a different percentage.
A congestion bottleneck is a port that is unable to transmit frames at the offered rate because the
offered rate is greater than the physical data rate of the line. For example, this condition can be
caused by trying to transfer data at 8 Gbps over a 4 Gbps ISL.
You can use the bottleneckMon command to configure separate alert thresholds for congestion and
latency bottlenecks.
Advanced settings allow you to refine the criterion for defining latency bottleneck conditions to allow
for more (or less) sensitive monitoring at the sub-second level. For example, you would use the
advanced settings to change the default value of 98 percent for loss of throughput. Refer to
Advanced
bottleneck detection settings on page 401 for specific details.
If a bottleneck is reported, you can investigate and optimize the resource allocation for the fabric.
Using the zone setup and Top Talkers, you can also determine which flows are destined to any
affected F_Ports.
How bottlenecks are reported
Bottleneck detection uses the concept of an affected second when determining whether a bottleneck
exists on a port. Each second is marked as being affected or unaffected by a latency or congestion
bottleneck, based on certain criteria.
The bottleneck detection feature maintains two histories of affected seconds for each port —one
history for latency bottlenecks and another for congestion bottlenecks. A history is maintained for a
maximum of three hours for each port. You can view the history using the bottleneckmon --show
command, as described in
Displaying bottleneck statistics on page 404.
Bottlenecks are also reported through RASLog alerts and SNMP traps. These two alerting
mechanisms cannot be turned on and off independently.
You can use the bottleneckMon command to specify the following alerting parameters:
• Whether alerts are to be sent when a bottleneck condition is detected
• The size of the time window to look at when determining whether to alert
• How many affected seconds are needed to generate the alert
• How long to stay quiet after an alert
• If an enabled alert is for congestion, for latency, or for both
NOTE
Changing alerting parameters affects RASLog alerting as well as SNMP traps.
For more detailed information on the bottleneckMon command, refer to the Fabric OS Command
Reference.
Types of bottlenecks
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