Constraints For Fc Fastwrite; Hardware Considerations; Configuring And Enabling Fc Fastwrite - HP A7533A - Brocade 4Gb SAN Switch Base Administrator's Manual

The processing outlined eliminates the latency inherent in sending Transfer Ready back to the initiator
when writing data across ISLs to geographically distant target devices.
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. 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.
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.

Constraints for FC fastwrite

Consider the following constraints when configuring FC Fastwrite:
• FC Fastwrite does not work in FICON environments.
• FC Fastwrite traffic passes over an FCIP tunnel, but it cannot share the FCIP tunnel with FCIP Fast Write.
FC Fastwrite cannot go though an FCIP tunnel with FCIP Fastwrite enabled.
• FC Fastwrite does not support loop device configurations for more than one device.

Hardware considerations

FC Fastwrite is implemented in a hardware configuration consisting of two 400 MP Routers or two 4/256
SAN Directos with FR4- 1 8i 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 MP Router and the FR4- 1 8i blade are organized into two groups. Ports 0-7
form one group, and ports 8- 1 5 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 MP Router or FR4- 1 8i 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.

Configuring and enabling FC Fastwrite

Take the following steps to configure and enable FC Fastwrite.
1. Create a zone configuration to filter FC Fastwrite flows. FC Fastwrite flows are configured by creating a
zone name with an fcacc token as a prefix. For LSAN configuration, use lsan_fcacc as a prefix, as
shown in the following example..
#zonecreate fcacc_myzone1, "initiator-wwn; target-wwn"
#zonecreate LSAN_fcacc_myzone2, "initiator-wwn; target1-wwn; target2-wwn"
#cfgcreate mycfg, "fcacc_myzone1; LSAN_fcacc_myzone2"
#cfgenable mycfg
2. Enable FC Fastwrite using the fastwritecfg command. Enabling or disabling FC Fastwrite with this
command disrupts data traffic. For the FR4- 1 8i blade, the command powers the blade off and back on.
In the case of the 400 MP Router, the switch is rebooted. The process takes up to five minutes.
#fastwritecfg —enable <slot#>
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