Chapter 11. Advanced Copy Services 543
/SM590000-gmmaxhostdelay max_host_delay
This parameter specifies the maximum time delay, in milliseconds, at which the Global 
Mirror link tolerance timer starts counting down. This threshold value determines the 
additional effect that Global Mirror operations can add to the response times of the Global 
Mirror source volumes. You can use this parameter to increase the threshold from the 
default value of 5 milliseconds.
/SM590000-maxreplicationdelay max_replication_delay
This parameter sets a maximum replication delay in seconds. The value must be a 
number 0 - 360 (0 being the default value, no delay). This feature sets the maximum 
number of seconds to be tolerated to complete a single I/O. If I/O cannot complete within 
the max_replication_delay, the 1920 event is reported. This is the system-wide setting, 
and applies to MM/GM relationships.
Use the chsystem command to adjust these values, as shown in the following example:
chsystem -gmlinktolerance 300
You can view all of these parameter values by using the lssystem <system_name> command.
Focus on the gmlinktolerance parameter in particular. If poor response extends past the 
specified tolerance, a 1920 event is logged and one or more GM relationships automatically 
stop to protect the application hosts at the primary site. During normal operations, application 
hosts experience a minimal effect from the response times because the GM feature uses 
asynchronous replication.
However, if GM operations experience degraded response times from the secondary system 
for an extended period, I/O operations begin to queue at the primary system. This queue 
results in an extended response time to application hosts. In this situation, the 
gmlinktolerance feature stops GM relationships, and the application host’s response time 
returns to normal. 
After a 1920 event occurs, the GM auxiliary volumes are no longer in the 
consistent_synchronized state. Fix the cause of the event and restart your GM relationships. 
For this reason, ensure that you monitor the system to track when these 1920 events occur.
You can disable the gmlinktolerance feature by setting the gmlinktolerance value to 0 
(zero). However, the gmlinktolerance feature cannot protect applications from extended 
response times if it is disabled. It might be appropriate to disable the gmlinktolerance feature 
under the following circumstances:
/SM590000During SAN maintenance windows in which degraded performance is expected from SAN 
components, and application hosts can stand extended response times from GM volumes.
/SM590000During periods when application hosts can tolerate extended response times and it is 
expected that the gmlinktolerance feature might stop the GM relationships. For example, 
if you test by using an I/O generator that is configured to stress the back-end storage, the 
gmlinktolerance feature might detect the high latency and stop the GM relationships. 
Disabling the gmlinktolerance feature prevents this result at the risk of exposing the test 
host to extended response times.
A 1920 event indicates that one or more of the SAN components cannot provide the 
performance that is required by the application hosts. This situation can be temporary (for 
example, a result of a maintenance activity) or permanent (for example, a result of a hardware 
failure or an unexpected host I/O workload).