HP StorageWorks XP Remote Web Console Software - Manual (Page 36)

in the secondary data volume according to the write sequence number of journal data. This will maintain
the data consistency between the primary and secondary data volumes. If the primary data volume write
operation fails, the primary storage system reports a unit check and does not create the journal data
for this operation. If the update copy operation fails, the secondary storage system suspends either the
affected pair or all URz pairs in the journal group, depending on the type of failure. When the suspended
URz pair or journal group is resumed (Resume Pair), the primary storage system and secondary storage
system negotiate the resynchronization of the pair(s). See “ Suspend Types ” on page 48 for further
information on URz suspend types.
During normal URz operations, the secondary storage system does not allow URz secondary data
volumes to be online (mounted), and therefore hosts cannot read from and write to secondary data
volumes. The URz secondary data volume write enable option allows write access to a secondary data
volume while the pair is split (see the next section). The secondary data volume write option can only be
enabled when you split the pair from the primary storage system.
When the DEVSERV command is issued to the URz secondary data volume, INDETERMINATE FAILING
UNIT is returned, if the status of URz secondary data volume is online. INTERVENTION REQUIRED
is returned, if the status of URz secondary data volume is offline.
Secondary Data Volume Write Option
For additional flexibility, URz provides a secondary data volume write option (S-Vol. Write) which enables
write I/O to the secondary data volume of a split URz pair. The secondary data volume write option can
be selected by the user during the Suspend Pair operation and applies only to the selected pair(s). The
secondary data volume write option can be accessed only when you are connected to the primary storage
system. When you resync a split URz pair which has the secondary data volume write option enabled,
the secondary storage system sends the secondary data volume track bitmap to the primary storage
system, and the primary storage system merges the primary data volume and secondary data volume
bitmaps to determine which tracks are out-of sync. This ensures proper resynchronization of the pair.
Secondary Data Volume Read Option
For additional flexibility, URz offers a special secondary data volume read option. The HP representative
enables the secondary data volume read option on the secondary storage system (mode 20). The
secondary data volume read option allows you to read a URz secondary data volume only while the pair
is suspended, that is, without having to release the pair. The secondary storage system will allow you to
change only the VOLSER of the suspended secondary data volume, so that the secondary data volume
can be online to the same host as the primary data volume while the pair is suspended. All other write
I/Os will be rejected by the secondary subsystem. The primary storage system copies the VOLSER of
the primary data volume back onto the secondary data volume when the pair is resumed. When the
secondary data volume read option is not enabled and/or the pair is not suspended, the secondary
storage system rejects all read and write I/Os to a URz secondary data volume.
Difference Management
The differential data (updated by write I/Os during split or suspension) between the primary data volume
and the secondary data volume is stored in each track bitmap. When a split/suspended pair is resumed
(Resume Pair), the primary storage system merges the primary data volume and secondary data volume
bitmaps, and the differential data is copied to the secondary data volume.
The number of bitmap areas affects the maximum possible number of pairs that can be
created in the storage system. For details on the maximum possible number of pairs, see
“ The Maximum Number of Pairs ” on page 59.
Journal Processing
The URz journal data contains the primary data volume updates and the metadata information
(associated control information), which enables the secondary storage system to maintain update
consistency of the URz secondary data volumes. URz journal processing includes:
36 Overview of Universal Replicator Operations