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.
There are times, both planned and unplanned, when it is necessary to suspend disk
mirroring and to make use of the secondary storage unit in your configuration. As a
manual process, this can be complex. However, failover and failback recovery
operations are available to simplify this process and reduce the risk of error and the
time it takes to switch sites and restart I/O operations.
Failover is the process of temporarily switching production to a backup facility
(normally your recovery site) following a scheduled maintenance period or a
disaster at your production (or local) site. A failover operation is always followed by
a failback operation, which is the process of returning production to its original
location. These operations use remote mirror and copy functions to reduce the time
that is required to synchronize volumes after switching sites during planned or
unplanned outages.
The failover and failback operations allow change recording to be enabled on the
target volumes without having to communicate between the target and source
storage units. This method eliminates the need to perform a full volume copy from
your recovery site to the production site, which can reduce the time that is required
to resume operations at your production site.
In a typical remote mirror and copy environment, processing will temporarily failover
to the storage unit at your recovery site if an outage occurs at the production site.
Through use of failover operations, the state of a storage unit in your target
configuration changes. As a result, the storage unit is recognized as the source
storage unit in the pair. Because the failover process puts the volumes into a
suspended state, changes are tracked within a bitmap. Assuming that change
Chapter 8. Optional feature codes for the DS8000 123
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