Dell Brocade G620 Brocade 8.0.1 Fabric OS Administratiors Guide - Page 127

Buffer-to-Buffer Credits and Credit Recovery ∙ Buffer credit management ...127 ∙ Buffer credit recovery ...139 ∙ Credit loss detection...141 Buffer credit management Buffer-to-buffer credit management affects performance over distances; therefore, allocating a sufficient number of buffer credits for long-distance traffic is essential to performance. To prevent a target device (either host or storage) from being overwhelmed with frames, the Fibre Channel architecture provides flow control mechanisms based on a system of credits. Each of these credits represents the ability of the device to accept additional frames. If a recipient issues no credits to the sender, no frames can be sent. Pacing the transport of subsequent frames on the basis of this credit system helps prevent the loss of frames and reduces the frequency of entire Fibre Channel sequences needing to be retransmitted across the link. Because the number of buffer credits available for use within each port group is limited, configuring buffer credits for extended links may affect the performance of the other ports in the group used for core-to-edge connections. You must balance the number of longdistance ISL connections and core-to-edge ISL connections within a switch. NOTE Configuring long-distance ISLs between core and edge switches is possible, but is not a recommended practice. All switch ports provide protection against buffer depletion through buffer limiting. A buffer-limited port reserves a minimum of eight buffer credits, allowing the port to continue to operate rather than being disabled because of a lack of buffers. Buffer-limited operations are supported for the static mode (LS) and dynamic mode (LD) extended ISL modes only. For LD, distance in kilometers is the smaller of the distance measured during port initialization versus the desired_distance value. For LS, distance in kilometers is always the desired_distance value. Buffer-to-buffer flow control Buffer-to-buffer (BB) credit flow control is implemented to limit the amount of data that a port may send, and is based on the number and size of the frames sent from that port. Buffer credits represent finite physical-port memory. Within a fabric, each port may have a different number of buffer credits. Within a connection, each side may have a different number of buffer credits. Buffer-to-buffer flow control is flow control between adjacent ports in the I/O path, for example, transmission control over individual network links. A separate, independent pool of credits is used to manage buffer-to-buffer flow control. A sending port uses its available credit supply and waits to have the credits replenished by the port on the opposite end of the link. These buffer credits are used by Class 2 and Class 3 services and rely on the Fibre Channel Receiver-Ready (R_RDY) control word to be sent by the receiving link port to the sender. The rate of frame transmission is regulated by the receiving port, and is based on the availability of buffers to hold received frames. If Virtual Channel technology is in use, the VC_RDY or EXT_VC control word is used instead of the R_RDY control word to manage buffer credits. For Virtual Channels, the buffer credits are managed for each Virtual Channel, and not for the entire physical link. The Virtual Channels used in VC_RDY flow-control mode range from VC0 through VC7. When QoS is enabled, EXT_VC_RDY flowcontrol mode allocates VC0 through VC14. VC8 through VC14 are allocated specifically for QoS VCs. Brocade Fabric OS Administration Guide, 8.0.1 53-1004111-02 127