HP StorageWorks XP Remote Web Console Software - Manual (Page 24)

24 Continuous Access XP Journal
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 of one pair is stored in the restore journal
volume, and then the base journal of 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 the None mode is selected,
initial copy operations are not performed. You are responsible for using the None mode only when you are
sure that data in the primary data volume is exactly the same as data in the secondary data volumes.
Update copy operation
The primary disk array obtains update data that the host writes to the primary data volume as update
journals. Update journals are stored in journal volumes in the journal group to which the primary data
volume belongs. When the secondary disk array issues read journal commands, update journals are
copied from the primary disk array to the secondary disk array asynchronously with completion of write
I/Os by the host. Update journals copied to the secondary disk array are stored in journal volumes in the
journal group to which the secondary data volume belongs. The secondary disk array restores the update
journals to the secondary data volumes in the order write I/Os are made, so the secondary data volumes
are updated just like the primary data volumes are updated.
Read and write I/O operations during Continuous Access XP Journal operations
When a primary disk array receives a read I/O for a Continuous Access XP Journal primary data volume,
the primary disk 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 disk array does not read the
Continuous Access XP Journal secondary data volume for recovery.
When a primary disk array receives a write I/O for a primary data volume with PAIR status, the primary
disk array performs the update copy operation and writes to the primary volume.
The primary disk array completes the primary data volume write operations independently of the update
copy operations at the secondary data volume. The secondary disk array updates data in the secondary
data volume according to the write sequence number of journal data. This maintains data consistency
between the primary and secondary data volumes. If the primary data volume write operation fails, the
primary disk array reports a unit check and does not create journal data for the operation. If the update
copy operation fails, the secondary disk 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 disk arrays
negotiate the pair resynchronization. See ” Continuous Access XP Journal suspend types ” on page 33 for
more information.
During normal Continuous Access XP Journal operations, the secondary disk array does not allow
Continuous Access XP Journal secondary data volumes to be online (mounted), and therefore hosts cannot
read from and write to secondary data volumes.
The secondary disk array does not allow a Continuous Access XP Journal secondary data volume to be
online (mounted) during normal Continuous Access XP Journal operations and rejects all host-requested
write I/O operations for a secondary data volume. The Continuous Access XP Journal secondary data
volume write enable option allows write access to a secondary data volume while the pair is split (see
” Secondary data volume write option ” on page 25). The secondary data volume write option can only be
enabled when you split the pair from the primary disk array.