HP XP Continuous Access Software - Manual (Page 22)

During normal XP Continuous Access Journal operations, the secondary array does not allow XP
Continuous Access Journal secondary data volumes to be online (mounted) and, therefore, hosts
cannot read from or write to secondary data volumes. The XP Continuous Access Journal secondary
data volume's write enable option allows write access to a secondary data volume while the pair
is split (see
Secondary data volume write option ). You can only enable the secondary data volume's
write option when splitting the pair from the primary array.
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 XP Continuous
Access 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 XP Continuous Access 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, XP Continuous Access Journal provides a secondary data volume write
option (S-Vol. Write) that enables write I/Os to the secondary data volume of a split XP Continuous
Access 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 XP Continuous
Access 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 XP Continuous
Access Journal secondary data volumes.
Creating and storing journals at primary arrays
When a primary array performs an update (host-requested write I/O) on an XP Continuous Access
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 2 (page 23) ). When the base-journal
is obtained, only metadata information is created and stored in the XP Continuous Access Journal
cache or journal volume.
22 Overview of XP Continuous Access Journal