Chapter 10. Copy services 589
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.
We 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 until you 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:
During SAN maintenance windows in which degraded performance is expected from SAN
components, and application hosts can withstand extended response times from GM
volumes.
During 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).
If 1920 events are occurring, it can be necessary to use a performance monitoring and
analysis tool, such as the IBM Virtual Storage Center, to help identify and resolve the
problem.
10.9.4 System partnership
To create an IBM SAN Volume Controller or an IBM Storwize system partnership, use the
mkfcpartnership command for traditional Fibre Channel (FC or FCoE) connections or
mkippartnership for IP-based connections.
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