HP XP Continuous Access Software - Manual (Page 29)

When using Continuous Access XP Journal as a data migration tool, host failover is recommended but not
required. When using Continuous Access XP Journal as a disaster recover tool, host failover is required to
ensure effective disaster recovery operations. When a Continuous Access XP Journal pair is suspended
due to an error condition, the primary array generates sense information that should be transferred to the
remote site using the host failover software for effective disaster detection and recovery.
Remote copy operations
Figure 5 illustrates the types of Continuous Access XP Journal remote copy operations: initial copy
and update copy.
Figure 5 Remote copy operations
Initial copy operations
Initial copy operations synchronize data in the primary data volume with data in the secondary data
volume and are performed independently from host I/Os. Initial copy operations are performed when
creating a data volume pair or when resynchronizing a suspended pair. The initial copy operation copies
base-journal data obtained from the primary data volume at the primary array to the secondary array,
and then restores the base-journal to the secondary data volume.
The journal-obtain operation uses all of the primary data volume's data as base-journal data, in sequence.
The base-journal contains a replica of the entire data volume and a replica of data volume updates. The
base-journal is copied from the primary array to the secondary array after the secondary array issues a
read-journal command. After the base-journal is copied to the secondary array, the base-journal is stored
in a restore journal volume in a restore journal group to which the secondary data volume belongs. After
that, data in the restore journal volume is restored (destaged) to the secondary data volume, so data in
the secondary data volume is representative of data in the primary data volume.
Base-journal data reflects the entire data volume plus updates. The difference area within the base-journal
is used during differential resynchronization operations. Journal data for the entire data volume is created
when the data volume pair is created. Difference journal data is used when the data volume's pair status
changes from Suspended to Pairresync. Merging difference bitmaps recorded in regards to the primary
and secondary data volumes allows you to obtain out-of-order data for only what changed while the pair
was suspended. When a data volume pair is suspended, changed tracks involved in host data writes to
the primary and secondary data volumes are noted in difference bitmaps.
Continuous Access XP Journal user guide 29