518 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
11.6.3  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 ensure the correct operation of applications after a disruption.
An application that performs many 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 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.
For more information about dependent writes, see 11.1.13, “FlashCopy and image mode 
Volumes” on page 459.
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.
Figure 11-87 shows the concept of Remote Copy Consistency Groups.
Figure 11-87   Remote Copy Consistency Group
Certain uses of MM/GM require the manipulation of more than one relationship. Remote 
Copy Consistency Groups can group relationships so that they are manipulated in unison.