The SVC chcluster command has a (new) option:
Specifies the maximum time delay, in milliseconds, above which the Global Mirror link tolerance timer starts counting down. This threshold value determines the additional impact that Global Mirror operations can add to the response times of the Global Mirror source VDisks (volumes). You can use this parameter to increase the threshold from the default value of 5 milliseconds.
The Global Mirror code is designed to detect if it potentially causing performance impacts to the SVC cluster.
If it detects certain conditions, it will start to log 1920 errors and suspend relationships to prevent performance impacts.
While this may be preferable to causing host delays, dealing with 1920 errors and suspended pairs is also not desirable.
This tuneable was added in 220.127.116.11 code to effectively allow you to make GM more willing to wait a little longer before starting to log the 1920 errors. You would only change it if you were getting 1920s and analysis of the TPC stats (esp internode latency) indicates having a slightly higher value (say 20ms) would allow the SVC to ride these small blips out.