IBM DS8000 - ADDITIONAL INFORMATION, User Manual, Page: 145 / 250

volumes across multiple storage units that are managed together when creating
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.
With Global Mirror functions, consistency groups can be formed many times per
hour, as opposed to once or twice per day. By combining many volumes into a
session, which is managed by the Global Mirror master storage unit, multiple
volumes can be copied to the recovery site simultaneously while maintaining
point-in-time consistency across those volumes.
The following properties control how often consistency groups are formed. You can
modify these values using the Web-based interface called the IBM System Storage
DS Storage Manager. Access the Global Mirror page, and then Define Properties
from the summary of actions menu.
Consistency group interval time
Indicates how long (in seconds) that the storage unit waits between the
formation of consistency groups. If you set this value to zero (requires
sufficient bandwidth), the consistency group forms continuously, which
means that a consistency group starts forming as soon as the previous
consistency group has been formed.
Maximum coordination interval
Indicates the maximum time (in milliseconds) that the master storage unit
communicates with its subordinate storage units to form a consistent data
point. The default is 50 milliseconds.
Because host write I/O is delayed while the consistent point is being set,
performance might be affected by allowing too much time for this interval.
If the time set for the maximum coordination interval expires before
formation of the consistent data point is complete, the consistency group
fails.
Maximum time writes are inhibited to the remote site
Indicates the maximum time (in seconds) that write operations are not
allowed at the recovery site before the storage unit stops forming a current
consistency group. If the drain time is maintained for an extended period
of time, the number of write operations that are required to transfer the
data to the recovery site can become large enough to increase the time to
form a consistency group.
Note: As distance increases, there is an increasing lag in the time it takes
for the data to be written to the recovery site. This lag is referred to
as a drain time.
Loss of data can also be increased in the event of a disaster. If it is set to
zero, it defaults to either four minutes or twice the consistency group
interval value, depending on which value is larger. The first consistency
group is formed regardless of the consistency group drain time. For the
remainder of the consistency groups, if the specified time expires before
the data is drained to the recovery site, consistency group formation stops.
After the consistency group formation has stopped five consecutive times,
the timer is disabled and the next consistency group is formed regardless
of the required time.
Session management:
Chapter 8. Optional feature codes for the DS8000
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