Update copy operations – HP StorageWorks XP Remote Web Console Software User Manual
Page 27
Continuous Access XP Journal user guide
27
suspended. When a data volume pair is suspended, a representation of host data writes to the primary
and secondary data volumes is recorded in difference bitmaps.
The primary array’s base-journal data is sent to the secondary array journal volume after a read
command is issued from the secondary array. After that, base-journal data is restored from the journal
volume to the secondary data volume. The initial copy operation finishes when all base-journals have
been destaged to the 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
copy operation is performed. During an update copy operation, update data written to the primary data
volume is obtained as an update journal. The update journal is copied to the secondary array, and then
restored to the secondary data volume.
The primary array packages 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 array issues read journal commands, update journals are copied
from the primary array to the secondary array asynchronously with regards to completion of write I/Os
by the host. Update journals 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 way as the primary data volumes are updated.
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 ”