Brocade Fabric OS Administrators Guide (Supporting Fabric OS v7.3.0) - Manual (Page 117)

Buffer-to-Buffer Credits and Credit Recovery
● Buffer credit management ............................................................................................117
● Buffer credit recovery ................................................................................................... 129
● Credit loss..................................................................................................................... 131
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 long-distance 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
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