HP StorageWorks Fabric OS 6.1.1 administrator guide (5697-0235, December 2009)

Fabric OS 6.1.x administrator guide 397
Channel ISLs implemented through the FC-FC Routing Service (FRS) rather than FCIP. FC Fastwrite is
supported in Fabric OS 5.3.x and later.
Figure 54 Typical network topology for FC Fastwrite
Platforms and OS requirements for FC Fastwrite
Fabric OS supports FC Fastwrite between two 400 Multi-Protocol Routers, two 4/256 SAN Directors with
FR4-18i blades, or two DC SAN Backbone Directors with FR418i blades, connected by a Fibre Channel
network. Fabric OS 5.3.0 or later is required at both ends of the tunnel in order to enable FC Fastwrite.
FC Fastwrite and FCIP tunnels cannot be used on the same 400 Multi-Protocol Router or FC4-18i blade.
Constraints for FC Fastwrite
Consider the following constraints when configuring FC Fastwrite.
• FC Fastwrite disables the local Ethernet ports (ge0 and ge1), making it impossible to configure FC
Fastwrite and FCIP tunnels on the same 400 Multi-Protocol Router or FC4-18i blade.
• FC Fastwrite does not work in FICON environments.
• FC Fastwrite flows may be routed to another 400 Multi-Protocol Router or FC4-18i blade on the FC
network. The 400 Multi-Protocol Router or FC4-18i blade may have active FCIP tunnels over an IP
network. FC Fastwrite flows may be passed through the FCIP tunnel, but only if the FCIP Fastwrite option
is disabled on the tunnel.
• FC Fastwrite does not support loop device configurations for more than one device.
• FC Fastwrite is not supported in interop mode 2 or 3.
How FC Fastwrite works
FC Fastwrite eliminates the latency inherent in sending a Transfer Ready (X_RDY) back to the initiator when
writing data across ISLs to geographically distant target devices. FC Fastwrite provides a proxy target (PT)
local to the initiator host, and a proxy initiator (PI) local to the target storage device. Figure 55 shows how
FC Fastwrite works.
1. The initiator sends a write command (WR_CMD).
2. The PT responds with a Transfer Ready (X_RDY), enabling the host to send more data.
3. The target device processes the write command, and sends a Transfer Ready to solicit more data. The
PI intercepts the Transfer Ready, and begins sending data received from the host.