Fibre Channel devices employ a credit-based flow control mechanism to guarantee delivery between storage devices and to ensure that the rate at which the data i sent by the source does not exceed that at which it can be received at the destination.
To establish a link-be-link flow control, ports of the two connected storage devices first exchange and agree on the number of frames each can receive (server HBA and switch port, for example). This is called the buffer-to-buffer credit (BB_Credits). Whenever the source device sends out a frame, it increments the credit counter by one. The receiving device will send back an acknowledment message R_RDY primitive upon the receipt of each transmitted frame. Once the source device receives the R_RDY, it lowers the BB_Credit counter by one. If the BB_Credit counter reaches the previously agreed upon credit threshold, the source device simple stops sending frames until the BB_Credit counter is lowered below its threshold by receipt of another R_RDY from the destination.
The same behavior happens in another direction. BB_Creding max value can be different on two ends of an FC link.
Default receive BB_Credints depend on the module. For 16 port module max is 255 (Fx_Port default is 16, and E_Port default is 255). For the 32-port linecard, it is fixed at Fx_Port 12 and E_Port 12.
Transmit BB_Credits are automatically set to the advertised receive BB_Credits of the peer port. This is done during FLOGI process.
Receive Performance buffers (no transmit) are on by default. They range from 1 to 145. They are used in addition to the BB_Credits. SAN-OS uses build-in algorithm to calculate how many performance credits are needed. On E_Port it is typically at 145 and on F_Port it is typically 12.
mds9124(config)# show int fc1/3 bbcredit
fc1/3 is up
Transmit B2B Credit is 3
Receive B2B Credit is 16
16 receive B2B credit remaining
3 transmit B2B credit remaining
3 low priority transmit B2B credit remaining
To establish a link-be-link flow control, ports of the two connected storage devices first exchange and agree on the number of frames each can receive (server HBA and switch port, for example). This is called the buffer-to-buffer credit (BB_Credits). Whenever the source device sends out a frame, it increments the credit counter by one. The receiving device will send back an acknowledment message R_RDY primitive upon the receipt of each transmitted frame. Once the source device receives the R_RDY, it lowers the BB_Credit counter by one. If the BB_Credit counter reaches the previously agreed upon credit threshold, the source device simple stops sending frames until the BB_Credit counter is lowered below its threshold by receipt of another R_RDY from the destination.
The same behavior happens in another direction. BB_Creding max value can be different on two ends of an FC link.
Default receive BB_Credints depend on the module. For 16 port module max is 255 (Fx_Port default is 16, and E_Port default is 255). For the 32-port linecard, it is fixed at Fx_Port 12 and E_Port 12.
Transmit BB_Credits are automatically set to the advertised receive BB_Credits of the peer port. This is done during FLOGI process.
Receive Performance buffers (no transmit) are on by default. They range from 1 to 145. They are used in addition to the BB_Credits. SAN-OS uses build-in algorithm to calculate how many performance credits are needed. On E_Port it is typically at 145 and on F_Port it is typically 12.
mds9124(config)# show int fc1/3 bbcredit
fc1/3 is up
Transmit B2B Credit is 3
Receive B2B Credit is 16
16 receive B2B credit remaining
3 transmit B2B credit remaining
3 low priority transmit B2B credit remaining
BB_Credits can be different on two links in a Port Channel. BB_Credits affect only individual inks.
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