Dell PowerConnect Brocade 300 Fabric OS Administrator's Guide v7.1.0 - Page 555

Buffer credit management 23 1. Connect to the switch and log in using an account assigned to the admin role. 2. Disable QoS. switch:admin> portcfgqos --disable [slot/]port If you do not disable QoS, after the second or third Link Reset (LR), ARB fill words display. 3. Disable buffer credit recovery; buffer credit recovery is not compatible with the IDLE mode. If you do not disable buffer credit recovery, it continues to perform a link reset. switch:admin> portcfgcreditrecovery --disable [slot/]port 4. Configure the port to support long-distance links. switch:admin> portcfglongdistance [slot/]port,LS,0,-distance 100 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 LS and 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. Fabric OS Administrator's Guide 555 53-1002745-02