Glossary

BB credits (buffer-to-buffer credits)

Buffer-to-buffer (BB) credits are the flow-control mechanism between two directly connected Fibre Channel ports. A port has a limited amount of receive buffer memory. The sender spends credits as it transmits and waits for the neighbor port to give them back, so that frames are not lost when the buffers are full.

XorMon

How XorMon monitors it

✓XorMon graphs BB credits as the percentage of time a port was unable to transmit frames because its transmit BB credit was zero.
✓The zero-credit condition is sampled every 2.5 microseconds. 100% means the port could not send anything during the time frame, which points to congestion or to a device slowed down by latency.
✓Switch and total graphs add up the per-port values, so the result can go above 100% on a switch with many ports without being wrong.
✓Buffer credits are graphed with throughput, frames, errors and SFP power for SAN switches; switches polled over SNMP are sampled and collected every 5 minutes.

XorMon documentation: Buffer-to-buffer credit monitoring

BB credits (buffer-to-buffer credits)

How XorMon monitors it

Related terms

Sources