IBM DS8000 User Manual page 151

The process of forming consistency groups using Global Mirror involves
the following steps:
1. The consistency group is created.
2. Data in the consistency group is sent to the remote site.
3. Data in the consistency group is saved as a group.
In a Metro/Global Mirror configuration, Metro Mirror and Global Copy volume
pairs, combined with consistency groups using Global Mirror and FlashCopy, help
maintain consistent and current (usually within seconds) data at the remote site to
provide a true backup at the remote site. Metro/Global Mirror introduces Global
Mirror and FlashCopy change recording and out-of-sync bitmap features that allow
faster subsequent resynchronization of the volumes at the local site.
During a disaster while the intermediate site is in production mode, the change
recording feature continues to be used to copy changed data from the B volumes at
the intermediate site to the C volumes at the remote site as consecutive sets of
consistency groups. When the original local site is ready to resume production
mode, the direction of the Metro Mirror configuration must be reversed. The
out-of-sync bitmaps are used to resynchronize the volumes at the local site.
During normal operations, consistency groups are continually being created from
the volumes at the intermediate site. While consistency groups are being formed,
host write I/Os are momentarily delayed while a consistent data point is attained
(because Metro Mirror from the local to intermediate site is synchronous). Bitmaps
that track updates for the consistency group are created and data is transferred, or
drained, to the volumes at the remote site using Global Copy. Application writes for
the next consistency group are recorded in the change recording bitmap.
The master storage unit coordinates the consistency group formation for all
volumes that belong to a Global Mirror session over FCP links that are based on
user-specified parameters. It also coordinates I/O with optional subordinates to all
primary volumes to form consistency groups. The frequency that consistency
groups are formed is based on the number of seconds that is specified for the
Consistency Group Interval option and the amount of time that it takes to form
consistency groups. Other Global Mirror options are described in "Consistency
group tunable values" on page 124.
Note: Be aware that an insufficient bandwidth (caused by a link failure or
insufficient link configuration) can also increase the amount of time that it
takes to drain a consistency group.
In a Metro/Global Mirror configuration, if a Metro Mirror volume pair between
the local and intermediate sites is either duplex pending or becomes suspended,
the Global Mirror session will fail consistency group formation. This is to ensure
that the consistency group at the remote site is, in fact, truly consistent with the
local site.
In addition, there might be cases where the secondary volume in a Metro Mirror
relationship at the intermediate site is not aware that its status as a secondary is
suspended and the consistency group formation would continue. The Global
Mirror session fails the consistency group formation as a way to ensure that the
consistency group that is formed at the remote site is consistent with the local site.
You can query information about the intermediate volume's status in the Global
Mirror session using the showgmir -metrics command.
Chapter 8. Optional feature codes for the DS8000
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