Chapter 10. Copy services 561
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.
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