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. 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. 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. Remote Copy Consistency Groups 10.7.2 Importance of write ordering 534  Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.2.1