HP XP Continuous Access Software - Manual (Page 30)

The primary array's base-journal data is sent to the secondary array journal volume and then the
secondary data volume (in response to a read command) issued from the secondary array. The initial
copy operation finishes when all base-journals have been destaged to the remote secondary volume.
NOTE:
If you manipulate volumes (not journal groups) to create or resynchronize two or more data volume pairs
within the same journal group, the base journal for one pair is stored in the restore journal volume, and
then the base journal for another pair is stored in the restore journal volume. Therefore, the operation
for restoring the latter base journal is delayed.
NOTE:
You can specify None as the copy mode for initial copy operations. If None is selected, initial copy
operations are not performed. You are responsible for using the None copy mode only when you are sure
that data in the primary data volume is already exactly the same as data in the secondary data volumes.
Update copy operations
When a host performs a write I/O operation to a primary data volume of a data volume pair, an update
remote copy operation is performed. During an update copy operation, update data written to the
primary data volume is obtained as an update journal entry. The update journal entry is copied to the
secondary array, and then restored to the secondary data volume.
Update journal entries are stored in journal volumes in the journal group to which the primary data
volume belongs. When the secondary array issues read journal commands, update journal entries are
copied from the primary array to the secondary array asynchronously with regards to completion of write
I/Os by the host. Update journal entries copied to the secondary array are stored in journal volumes in
the journal group to which the secondary data volume belongs. The secondary array restores update
journals to the secondary data volumes in the order in which write I/Os were made, so the secondary
data volumes are updated in the same order as the primary data volumes.
Read and write I/O operations during Continuous Access XP Journal operations
When a primary array receives a read I/O for a Continuous Access XP Journal primary data volume, the
primary array performs the read from the primary data volume. If the read fails, the redundancy provided
by RAID-1 or RAID-5 technology recovers the failure. The primary array does not read the Continuous
Access XP Journal secondary data volume for recovery.
When a primary array receives a write I/O for a primary data volume with PAIR status, the primary array
performs the update copy operation and writes to the primary volume.
The primary array completes the primary data volume write operations independently of the update copy
operations at the secondary data volume. The secondary array updates data in the secondary data
volume according to the write sequence numbers of journal data. This maintains data consistency
between the primary and secondary data volumes. If the primary data volume write operation fails, the
primary array reports a unit check and does not create a journal data entry for the operation. If the update
copy operation fails, the secondary array suspends either the affected pair or all Continuous Access
XP Journal pairs in the journal group, depending on the failure type. When the suspended Continuous
Access XP Journal pair or journal group is resumed (pairresync), the primary and secondary arrays
negotiate pair resynchronization. See Continuous Access XP Journal suspend types for more information.
During normal Continuous Access XP Journal operations, the secondary array does not allow Continuous
Access XP Journal secondary data volumes to be online (mounted) and, therefore, hosts cannot read from
or write to secondary data volumes. The Continuous Access XP 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.
30 Overview of Continuous Access XP Journal