Dell EMC Unity: What is the internal BB_Credit (buffer to buffer) setting for Unity Storage Processor FC ports? (User Correctable)

Understanding the BB_Credit (B2B) setting for Dell EMC Unity on a per port basis.

BB_Credit or "buffer to buffer" credits are used as a flow control method by Fibre Channel technology and represent the number of frames a port can store. During fabric login, N_Ports and F_Ports at both ends of a link establish its Buffer to Buffer Credit (BB_Credit). Each port states the maximum BB_Credit that they can receive at.
 
In buffer-credit flow control, the source and destination set the number of unacknowledged frames (buffer credits) allowed to accumulate before the source stops sending data. A counter at the source keeps track of the number of buffer credits. Each time a frame is sent by the source, the counter increments by 1. Each time the destination receives a frame, it sends an acknowledgment (R_RDY) back to the source, which decrements the counter by 1. If the number of buffer credits reaches the maximum, the source stops transmission until it receives the next acknowledgement (R_RDY) from the destination.

The benefits of a larger data buffer help in long distance applications, when operating at higher data rates. This prevents loss of frames that may result if the source gets too far ahead of the destination. As the length of a fiber optic cable span increases for a given throughput, the number of buffer credits required to ensure optimum performance increases in direct proportion. Also, as the throughput increases for a given span of fiber optic cable, the required number of buffer credits increases in direct proportion.

Various SAN switch vendors have varied default settings and the ability to change the BB_Credit setting based upon the appropriate need. 

None

Dell EMC Unity products have pre-defined BB_Credit settings that are hard-coded (cannot be changed) to receive at a maximum of 6 BB_Credits.

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