HP StorageWorks XP Remote Web Console Software - Manual (Page 25)

Continuous Access XP Journal user guide 25
NOTE: To reduce the overhead associated with remote copy activities and maximize data transfer, the
local disk 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 write option during the pairsplit-r operation, and it
applies only to the selected pairs. The secondary data volume write option can be accessed only when
you are connected to the primary disk array. When you resync a split Continuous Access XP Journal pair
that has the secondary data volume write option enabled, the secondary disk array sends the secondary
data volume track bitmap to the primary disk array, and the primary disk array merges the primary and
secondary data volume bitmaps to determine which tracks are out-of sync. This ensures proper pair
resynchronization.
Difference management
Differential data (updated by write I/Os during split or suspension) between the primary and secondary
data volumes is stored in each track bitmap. When a split/suspended pair is resumed (pairresync), the
primary disk array merges the primary and secondary data volume bitmaps, and differential data is
copied to the secondary data volume.
NOTE: The number of bitmap areas affects the maximum number of pairs that can be created in the disk
array. For more information, see ” Maximum number of pairs ” on page 38.
Journal processing
Continuous Access XP Journal journal data contains primary data volume updates and metadata
information (associated control information), which enables the secondary disk array to maintain update
consistency of the Continuous Access XP Journal secondary data volumes.
Creating and storing journals at primary disk arrays
When a primary disk array performs an update (host-requested write I/O) on a Continuous Access XP
Journal primary data volume, the primary disk array creates journal data that is transferred to that
secondary disk 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 Continuous Access XP Journal cache or the journal volume.
Table 3 Metadata information
Type Description
Journal type Journal type (for example, base-journal or update journal)
LDEV No. (data) Number of the primary data volume that stores the original data
Original data storing
position
Primary data volume’s slot number and start and end of sub-block number (data
length)
LDEV No. (journal) Volume number of the master journal volume that stores the journal data
Journal data storing position Master journal volume’s slot number of master journal volume, and start sub-block
number