528 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
The following numbers correspond to the numbers that are shown in Figure 11-91 on 
page 527:
/SM590000(1) The first write is performed from the host to LBA X.
/SM590000(2) The host is provided acknowledgment that the write completed even though the 
mirrored write to the auxiliary volume is not yet complete.
/SM590000(1’) and (2’) occur asynchronously with the first write.
/SM590000(3) The second write is performed from the host also to LBA X. If this write occurs before 
(2’), the write is written to the journal file.
/SM590000(4) The host is provided acknowledgment that the second write is complete.
Delay simulation
Global Mirror provides a feature that enables a delay simulation to be applied on writes that 
are sent to the auxiliary volumes. With this feature tests can be done to detect colliding writes, 
also it provides the capability to test an application before the full deployment. The feature can 
be enabled separately for each of the intracluster or intercluster Global Mirrors. 
By using the chsystem command, the delay setting can be setup and with the lssystem 
command the delay can be checked. The gm_intra_cluster_delay_simulation field 
expresses the amount of time that intracluster auxiliary I/Os are delayed. The 
gm_inter_cluster_delay_simulation field expresses the amount of time that intercluster 
auxiliary I/Os are delayed. A value of zero disables the feature.
11.6.13  Using Change Volumes with Global Mirror
Global Mirror is designed to achieve an RPO as low as possible so that data is as up-to-date 
as possible. This design places several strict requirements on your infrastructure. In certain 
situations with low network link quality, congested hosts, or overloaded hosts, you might be 
affected by multiple 1920 congestion errors.
Congestion errors happen in the following primary situations:
/SM590000At the source site through the host or network
/SM590000In the network link or network path
/SM590000At the target site through the host or network
Global Mirror has functionality that is designed to address the following conditions, which 
might negatively affect certain Global Mirror implementations:
/SM590000The estimation of the bandwidth requirements tends to be complex.
/SM590000Ensuring that the latency and bandwidth requirements can be met is often difficult.
/SM590000Congested hosts on the source or target site can cause disruption.
/SM590000Congested network links can cause disruption with only intermittent peaks.
To address these issues, Change Volumes were added as an option for Global Mirror 
relationships. Change volumes use the FlashCopy functionality, but they cannot be 
manipulated as FlashCopy volumes because they are for a special purpose only. Change 
volumes replicate point-in-time images on a cycling period. The default is 300 seconds. 
Tip: If you are experiencing repeated problems with the delay on your link, make sure that 
the delay simulator was properly disabled.