SRDF with SiRT
SiRT (Single RoundTrip) for Fibre Channel SRDF directors (RFs) was introduced in Enginuity 5772 for SRDF/S mode only. It is dynamically enabled for SRDF/S links > 12 Km for block sizes up to 32K in Enginuity 5773 code. SiRT is compatible with fast write/write acceleration switches and extenders, as it will measure link latency and disable automatically if connected to these devices. As a best practice, it is recommended that either the EMC SiRT feature or the third-party fast write feature should be used. Both should not be enabled simultaneously.
The Fibre Channel SiRT feature for the Fibre Channel director can be set to Off or Automatic. When set to Automatic, this feature will only accelerate write I/Os using criteria based on latency and I/O size.

Note: EMC recommends contacting your EMC Customer Service Representative to verify that the setting is enabled if required in your environment.

Figure 5 shows the normal write process without the SiRT feature.
 
Figure 5 Normal write command process

The intended purpose of this feature is to maintain SRDF/S synchronicity while improving performance by localizing the transfer-ready response to the local RF port, thereby reducing an unnecessary acknowledgement response (trip) over the dark fiber distance (step 2 in Figure 5). Immediate benefits are apparent upon activation in transparent SRDF synchronous distance extension environments.
If applicable, multiple SRDF synchronous links can maximize their I/O performance over the network (transparent WDM environment).
In the example shown in Figure 6, RF1 (R1 F_Port) and RF2 (R2 F_Port) are managing the SiRT flow control.
 
Figure 6 SRDF SiRT
Legend:

Fast write/ write acceleration
EMC Connectrix and other third-party products offer single roundtrip for Fibre Channel capabilities (fast write/write acceleration) that can also increase SRDF throughput for direct-attach or Fibre Channel switched fabric configurations over extended distances. It is transparent to SRDF FC links and is used for all SRDF modes to decrease response time (SRDF/S) or improve performance over long distance links (mostly for adaptive copy and SRDF/AR, but also for some SRDF/A configurations).
Figure 7 shows a write command with fast write features.
 
Figure 7 Write command with SiRT

For Connectrix or third-party products, refer to the EMC Support Matrix available at http://elabnavigator.EMC.com to verify which of these products are supported for SRDF configurations.
! IMPORTANT
Not all products offering this feature are supported with SRDF due to unique write commands utilized by SRDF. SiRT with distance vendor write acceleration
With this in mind SiRT usage, in combination with the distance extension device-offered write acceleration mode, must be addressed.
Essentially for environments where the distance extension device is already servicing write commands on an E_Port level, it is recommended to disable SiRT. Refer to Figure 8.
 
Figure 8 All F_Ports will benefit
Legend:

In Figure 8, by enabling the write acceleration feature on the distance extension device, potentially all F_Ports (RF ports, FA ports, tape, etc.) issuing writes traversing across the E_Port attached to the distance extension client port can also take advantage of the throughput benefits from the activated write acceleration feature.

Link initialization
For link initialization of a Fibre Channel port, Fibre Channel specifications state that the maximum tolerable response time for a response is 100 milliseconds roundtrip time. This timeframe coincides with the limited timeframe of the Receiver-Transmitter Timeout Value (R_T_TOV), which is how long an FC port listens for a link response to a link service before an error is detected.