600 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
The link quality and bandwidth play a large role. Your network provider likely ensures a 
latency maximum on your network link. Therefore, be sure to stay as far beneath the Global 
Mirror round-trip-time (RTT) limit as possible. You can easily double or triple the expected 
physical latency with a lower quality or lower bandwidth network link. Then, you are within the 
range of exceeding the limit if high I/O occurs that exceeds the existing bandwidth capacity. 
When you get a 1920 event, always check the latency first. The FCIP routing layer can 
introduce latency if it is not properly configured. If your network provider reports a much lower 
latency, you might have a problem at your FCIP routing layer. Most FCIP routing devices have 
built-in tools to enable you to check the RTT. When you are checking latency, remember that 
TCP/IP routing devices (including FCIP routers) report RTT by using standard 64-byte ping 
packets.
In Figure 11-155, you can see why the effective transit time must be measured only by using 
packets that are large enough to hold an FC frame, or 2148 bytes (2112 bytes of payload and 
36 bytes of header). Allow estimated resource requirements to be a safe amount because 
various switch vendors have optional features that might increase this size. After you verify 
your latency by using the proper packet size, proceed with normal hardware troubleshooting.
Figure 11-155   Effect of packet size (in bytes) versus the link size
Before proceeding, look at the second largest component of your RTT, which is serialization 
delay. Serialization delay is the amount of time that is required to move a packet of data of a 
specific size across a network link of a certain bandwidth. The required time to move a 
specific amount of data decreases as the data transmission rate increases. 
Figure 11-155 shows the orders of magnitude of difference between the link bandwidths. It is 
easy to see how 1920 errors can arise when your bandwidth is insufficient. Never use a 
TCP/IP ping to measure RTT for FCIP traffic.