hesela.devBuffer-to-buffer credit
A buffer-to-buffer credit permits a Fibre Channel sender to transmit a frame into advertised receive-buffer space on the adjacent link endpoint.
Credits are direction-specific and count frames rather than bytes. Transmission spends a credit; in basic R_RDY flow control, a receiver returns credit when a buffer becomes available. Zero credits pauses transmission. The cited implementation documents credit loss after corrupted frames or credit-return primitives. Its BB_SC_N recovery feature on E ports supports R_RDY mode, not ER_RDY mode; verify the actual mode before selecting a recovery procedure. [1]
This is link flow control, not command completion. FCP carries SCSI status separately. Treating a returned credit as proof of a successful or durable write confuses these layers. [2]
Relations
- related
- Fibre Channel, Fibre Channel frame, Fibre Channel exchange, Credit starvation, Buffer-cycle deadlock
Citations
- Cisco MDS 9000 Interfaces Configuration Guide, Release 9.x: Buffer-to-Buffer Credits and Credit Recovery
- Linux v6.12: fc_fcp.h, fcp_resp and SCSI status
Sources & caveats
- Source-backed protocol definition; no throughput, failure-rate or durability measurement is implied.