content_image/971429.jpg
Figure 6-25 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. 

<img src='content_image/971429.jpg'>

In Figure 6-25, the amount of time in microseconds that is required to transmit a packet across network links of varying bandwidth capacity is compared. The following packet sizes are used: 

GLYPH<SM590000> 64 bytes: The size of the common ping packet 

GLYPH<SM590000> 1500 bytes: The size of the standard TCP/IP packet 

GLYPH<SM590000> 2148 bytes: The size of an FC frame 

Finally, your path maximum transmission unit (MTU) affects the delay that is incurred to get a packet from one location to another location. An MTU might cause fragmentation, or be too large and cause too many retransmits when a packet is lost. 

## Recovery after 1920 errors 

After a 1920 error occurs, the Global Mirror auxiliary volumes are no longer in a Consistent Synchronized  state. You must establish the cause of the problem and fix it before you restart the relationship. 

When the relationship is restarted, you must resynchronize it. During this period, the data on the Metro Mirror or Global Mirror auxiliary volumes on the secondary system is inconsistent, and your applications cannot use the volumes as backup disks. To address this data consistency exposure on the secondary system, a FlashCopy of the auxiliary volumes can be created to maintain a consistent image until the Global Mirror (or the Metro Mirror) relationships are synchronized again and back in a consistent state. 

IBM Spectrum Virtualize provides the Remote Copy Consistency Protection  feature that automates this process. When Consistency Protection is configured, the relationship between the primary and secondary volumes does not go in to the Inconsistent copying  status once restarted. Instead, the system uses a secondary change volume  to automatically copy the previous consistent state of the secondary volume. 

