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Fibre Channel (SAN)

Posts: 0

Buffer to Buffer credit

Please explain me what is BB_Credit

In my environment , i am getting switch alert like congestion bottleneck .

My tech Lead asked to check the value of Time BB_Credit Zero .

Please let me know what this value presents . How much should this normal value .

I have a little idea through my studies , but i need explanation from expertsperfor

Valued Contributor
Posts: 761
Registered: ‎06-11-2010

Re: Buffer to Buffer credit


BB credit definiton:

The congestion bottleneck alert has the following definition:

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.

Time BB credit zero:

The number of times that the port was unable to transmit frames because

the transmit BB credit was zero. The purpose of this statistic is to detect

congestion or a device affected by latency. This parameter is sampled at

intervals of 2.5 μs (microseconds), and the counter is incremented if the

condition is true. Each sample represents 2.5 μs of time with zero Tx BB

Credit. An increment of this counter means that the frames could not be sent

to the attached device for 2.5 μs, indicating degraded performance.


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