IBM TotalStorage DS6000 Introduction And Planning Manual page 29

I/O to all duplex remote mirror and copy volumes on LSS pairs will be queued
either until a consistency group created operation is run or the consistency group
timeout time expires, allowing external automation to use the consistency group
created operation to create a dependent write consistent set of target volumes over
any number of LSS and disk storage units.
External automation or command-line interface commands can issue a "freeze" to
all LSS pairs that relate to the application, affecting the set of source volumes in the
consistency group. The freeze operation causes the source volumes to suspend
and their associated paths to end. Therefore, all associated updates at the recovery
site are halted, resulting in a consistent data checkpoint. The automation program is
not a part of the software provided with the storage unit; it must be supplied by the
user. However, IBM has offerings to assist with this automation. For more
information, contact your IBM storage representative.
A consistency grouping timeout function gives automation software the time to issue
the automated requests to ensure consistency. The consistency grouping timeout is
a value that specifies the length of time that the volume remains in long-busy
(unavailable) following an error. You can change this value by accessing Copy
Services functions from the Web interface (DS Storage Manager ) and navigating to
the Paths page and then to the LSS options page. The default value for this
parameter is set to two minutes.
Note: The consistency group option is not required for Global Mirror or for Global
Copy. Global Copy always keeps a inconsistent copy of the volumes at the
recovery site. The volume pairs become consistent only when all updates
are transmitted and copied to the recovery site while the application write
operations are quiesced. Global Mirror uses a function called a session to
control the formation of consistency groups. (A session is a collection of
volumes across multiple storage units that are managed together to create
consistent copies of data.) The formation of these consistency groups is
coordinated by the "master" storage unit, which sends commands over
remote mirror and copy links to its "subordinate" storage units.
The following describes the behavior of these volume pairs when an error occurs
that affects a volume in a consistency group:
v Without the consistency group option enabled, the storage unit causes the
volume where the error is detected to enter a suspended state, but updates to
that volume are still allowed.
v With the consistency group option enabled, the volume becomes suspended and
additionally enters a long-busy state. Updates are not possible to volumes from
the source LSS to target LSS that are sharing the same path.
A critical mode determines the behavior of the remote mirror and copy pairs or
consistency groups after a failure in communication between the source and target
storage units when all the paths between a pair are lost. You can enable this mode
also by accessing Copy Services functions from the DS Storage Manager and by
navigating to the Paths page and then to the LSS options page.
Failover and failback operations
The failover operation is the process of switching production to a backup facility
(normally your recovery site). A failback operation is the process of returning
production to its original location after a disaster or a scheduled maintenance
period.
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Chapter 1. Introduction