HP XP Continuous Access Software - Manual (Page 31)

NOTE:
To reduce the overhead associated with these remote copy activities and maximize data transfers,
the local array uses a special write command that is allowed only for Continuous Access XP Journal
initial and update copy operations. This command transfers control parameters and FBA-format data
for consecutive updated records in a track using a single write operation. The special Continuous
Access XP Journal write command eliminates the overhead required for performing FBA-to-CKD and
CKD-to-FBA conversions.
Secondary data volume write option
For additional flexibility, Continuous Access XP Journal provides a secondary data volume write option
(S-Vol. Write) that enables write I/Os to the secondary data volume of a split Continuous Access XP
Journal pair. You can select the secondary data volume's write option during the pairsplit-r operation,
and it applies only to the selected pairs. You can access the secondary data volume's write option only
when connected to the primary array. When you resync a split Continuous Access XP Journal pair that
has the secondary data volume's write option enabled, the secondary array sends the secondary data
volume track bitmap to the primary array, and the primary array merges the primary and secondary data
volume bitmaps to determine which tracks are out-of sync. This ensures proper pair resynchronization.
Difference management
Changed track data (updated by write I/Os during pair split or suspension) between the primary and
secondary data volumes is represented in each track bitmap. When a split/suspended pair is resumed
(pvol-to-svol pairresync), the primary array merges the primary and secondary data volume bitmaps, and
uses that information to copy changed data to the secondary data volume.
NOTE:
The number of bitmap areas affects the maximum number of pairs you can create in the array. For
more information, see Maximum number of pairs .
Journal processing
Journal data contains primary data volume updates and metadata information (associated control
information), which enables the secondary array to maintain update consistency of the Continuous
Access XP Journal secondary data volumes.
Creating and storing journals at primary arrays
When a primary array performs an update (host-requested write I/O) on a Continuous Access XP Journal
primary data volume, the primary array creates journal data that is transferred to the secondary array.
Journal data is stored in cache first, and then in the journal volume.
Metadata information is attached to journal data (see
Table 3 ). When the base-journal is obtained, only
metadata information is created and stored in the Continuous Access XP Journal cache or journal volume.
Continuous Access XP Journal user guide 31