System partnership intermix:  All of the preceding topologies are valid for the intermix of the IBM SAN Volume Controller with the Storwize V5000 Gen2 system if the Storwize V5000 Gen2 system is set to the replication layer and running IBM Spectrum Virtualize code 6.3.0 or later. 

## 10.7.2 Importance of write ordering 

Many applications that use block storage have a requirement to survive failures, such as loss of power or a software crash, and to not lose data that existed before the failure. Because many applications must perform large numbers of update operations in parallel, maintaining write ordering is key to ensuring the correct operation of applications after a disruption. 

An application that performs a high volume of database updates is designed with the concept of dependent writes. With dependent writes, it is important to ensure that an earlier write completed before a later write is started. Reversing or performing the order of writes differently than the application intended can undermine the application’s algorithms and can lead to problems, such as detected or undetected data corruption. 

The IBM Spectrum Virtualize Metro Mirror and Global Mirror implementation operates in a manner that is designed to always keep a consistent image at the secondary site. The Global Mirror implementation uses complex algorithms that operate to identify sets of data and number those sets of data in sequence. The data is then applied at the secondary site in the defined sequence. 

Operating in this manner ensures that if the relationship is in a Consistent_Synchronized state, the Global Mirror target data is at least crash consistent, and supports quick recovery through application crash recovery facilities. 

## Remote Copy Consistency Groups 

A Remote Copy Consistency Group can contain an arbitrary number of relationships up to the maximum number of MM/GM relationships that is supported by the IBM Spectrum Virtualize system. MM/GM commands can be issued to a Remote Copy Consistency Group. 

Therefore, these commands can be issued simultaneously for all MM/GM relationships that are defined within that Consistency Group, or to a single MM/GM relationship that is not part of a Remote Copy Consistency Group. For example, when a startrcconsistgrp  command is issued to the Consistency Group, all of the MM/GM relationships in the Consistency Group are started at the same time. 

