Preventive Service Planning
Abstract
This document details changes to supported configurations, and best practice, for the Global Mirror with Change Volumes feature.
Content
Changed support for Global Mirror with Change Volumes
The maximum number of Global Mirror with Change Volumes relationships per system changed in IBM Spectrum Virtualize version 7.7.0 or later.
| Release | Platform | Configured cycle period | Maximum number of GMCV relationships per system | Notes |
| V7.7.0 or later |
|
60-300 seconds | 256 | Unchanged. Not recommended – test carefully before deployment. |
|
300-86400 seconds | 256 | Unchanged. | |
|
300-86400 seconds | 1500, maximum of 256 relationships per consistency group. | Previously unsupported. | |
|
300-86400 seconds | 2500, maximum of 256 relationships per consistency group. | Previously unsupported. | |
| V8.2.0 or later |
|
300-86400 seconds | 2500, maximum of 256 relationships per consistency group. | Previously unsupported. |
When mixing multiple SVC node types within a system, or when mixing different cycle periods, use the lowest value for the maximum number of relationships. Only use more than 256 Global Mirror with Change Volume relationships if all have a cycle period of 300 seconds or greater, and all nodes are SVC 2145-DH8, 2145-SV1, 2147-SV1, 9848-AC2, 9848-AC3 or FS9100 family.
Note that the CLI and GUI will not prevent you from using unsupported configurations.
Some customers may already be running configurations that are not listed as supported in the above table under a specific agreement with IBM. This change in support does not affect those configurations.
Best practices when using Global Mirror with Change Volumes replication
The Global Mirror with Change Volumes (GMCV) replication periodically uses FlashCopy to establish a new set of data to replicate. Establishing that set of data takes a finite length of time, during which new IO to the volumes is delayed.
This delay is reported through TPC as the “peak IO response time”, and is the aspect of replicated volumes’ performance most affected by the configuration of GMCV relationships in the system. The average IO response time is largely unaffected. Non-replicated volumes are not typically affected by this process.
Decreasing the number of GMCV relationships in a consistency group will lower the delay in establishing the consistent image to copy each cycle. The relative peak IO response time for a given number of relationships configured as a varying number of relationships per consistency group is given in the following table:
| Relationships per consistency group |
standalone relationships
|
25
|
50
|
150
|
256
|
| Relative peak IO response time (approximately) |
1.0x
|
1.2x
|
2.0x
|
3.0x
|
5.0x
|
Decreasing the total number of relationships in a consistency group (where possible) will decrease the peak IO response time.
Relationships should be in the same consistency group if data from the same point in time is required to recover an application. While minimising the number of relationships in each consistency group, take care not to exclude volumes that are vital for disaster recovery.
In some cases, it may be possible to use fewer larger volumes for an application, which can result in smaller consistency groups and a smaller total number of relationships.
Consistency groups spanning multiple I/O groups will have an increased peak IO response time due to the increased inter-node messaging required.
Increasing the cycle period will not typically decrease the peak IO response time.
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Document Information
Modified date:
28 March 2023
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ssg1S1010102