HP XP Continuous Access Software - Manual (Page 21)

changed tracks involved in host data writes to the primary and secondary data volumes are noted
in difference bitmaps.
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.
:
When an RCU is shared with multiple MCUs, perform the pair resume operation after the system
option mode 593 is set if you resume 1025 or more pairs from multiple MCUs (range: group) at
the same time. Pair resume operations may fail if the system option mode 593 is not set.
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 XP Continuous Access Journal operations
When a primary array receives a read I/O for an XP Continuous Access 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 XP Continuous Access 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 XP Continuous Access Journal pairs in the journal group, depending on the
failure type. When the suspended XP Continuous Access Journal pair or journal group is resumed
(pairresync), the primary and secondary arrays negotiate pair resynchronization. See
XP Continuous
AccessJournal suspend types for more information.
Remote copy operations 21