HP StorageWorks Fabric OS 6.x administrator guide (5697-7344, March 2008)

Fabric OS 6.x administrator guide 463
4. The PI continues to stage data received from the initiator, respond locally to Transfer Ready, and send
the data to the target device until the target device sends an FCP_RSP.
Figure 44 How FC fastwrite works
FC fastwrite can improve Write performance. Read performance is unaffected. The gains seen from
enabling FC fastwrite depend on several factors, including the following:
• The size of I/O vs. Transfer Ready. In general, the more times a target device sends a Transfer Ready,
the greater the performance gain.
• The number of outstanding I/Os (both Write and Read), link speed, and link congestion. FC fastwrite
may not result in significant improvement if these factors suggest that the write data is delayed because
it is sharing bandwidth.
• Target response latency - If the target is slow in responding to the write command, the data must be
held by the remote switch.
FC fastwrite flow configuration requirements
FC fastwrite is enabled on both initiator and target ports. There is a possibility where either initiator or
target ports may have flows with other devices that do not support FC fastwrite. An FC fastwrite specific
zone configuration is used to filter FC fastwrite flows.
Hardware considerations for FC fastwrite
FC fastwrite is implemented in a hardware configuration consisting of two 400 Multi-protocol Router or two
4/256 SAN Director or DC Directors with FR4-18i blades connected by Fibre Channel ISLs. Consider the
following hardware characteristics and requirements when planning to implement FC fastwrite.
• FC ports on both the 400 Multi-protocol Router and the FR4-18i blade are organized into two groups.
Ports 0-7 form one group, and ports 8-15 form the other. A maximum of four ports in each group may
be configured as FC fastwrite.
• The maximum bandwidth available for FC fastwrite is 4 Gbps per group. This bandwidth is shared by
all write flows.
• Host initiators and target devices must be directly connected to the 400 Multi-protocol Router or FR4-18i
blade on their respective ends of the ISL. FC fastwrite must be configured and enabled for the ports on
both the ends of the flow. Mismatch of the configuration results in I/O failure.