572 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
You specify the delay setting by using the chsystem command and view the delay by using the
lssystem command. 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.
10.7.12 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:
Congestion at the source site through the host or network
Congestion in the network link or network path
Congestion at 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:
The estimation of the bandwidth requirements tends to be complex.
Ensuring the latency and bandwidth requirements can be met is often difficult.
Congested hosts on the source or target site can cause disruption.
Congested 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. Depending
on the cycling mode defined, Change Volumes replicate point-in-time images on a cycling
period.
Your change rate needs to include only the condition of the data at the point-in-time that the
image was taken, rather than all the updates during the period. The use of this function can
provide significant reductions in replication volume.
Global Mirror with Change Volumes has the following characteristics:
Larger RPO
Point-in-time copies
Asynchronous
Possible system performance resource requirements because point-in-time copies are
created locally
Tip: If you are experiencing repeated problems with the delay on your link, make sure that
the delay simulator was properly disabled.
Note: The cycling mode can be either none or multi. When cycling mode is set to none,
the Global Mirror will behave identically to Global Mirror without Change Volumes.When
cycling mode is set to multi, the Global Mirror will behave as described in this section.
The cycling mode can be changed only when the relationship is stopped and in
consistent_stopped or inconsistent_stopped status.
The default cycling period is 300 seconds.
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