HP StorageWorks XP Remote Web Console Software - Manual (Page 33)

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 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.
Logical DKC (LDKC)
The XP storage system controls the CU (Control Unit) by dividing the CUs in to groups of 255 CUs. Each
group is a storage system that logically exists in XP disk array (logical storage system). These groups are
called a “logical DKC” or an “LDKC (Logical disk controller)”. There are 2 LDKCs in the XP storage system
and number “00” and “01” is assigned to each LDKC.
Each LDKC controls 255 CUs, however the number of CUs that can be used for XP disk array program
products is up to 255. Therefore, the maximum number of volumes that can be used for XP disk array
program products is 130,560 (65,280 volumes for an LDKC).
Initiator Ports and RCU Target Ports
The initiator port and the RCU target port are required at both the primary storage system and secondary
storage system. The initiator port at the primary storage system is connected to the RCU target port
at the secondary storage system via the fibre channel interface. The initiator port at the secondary
storage system is connected to the RCU target port at the primary storage system. The initiator port at the
secondary storage system issues a "read journal" command to the primary storage system, and then the
RCU target port at the primary storage system sends journal data to the secondary storage system in
response to the "read journal" command.
Any fibre-channel interface port of the XP disk array can be configured as an initiator port. The initiator
ports cannot communicate with the host processor channels. The host channel paths must be connected to
the fibre-channel interface port other than the initiator port.
Hitachi Universal Replicator for z/OS user guide for XP12000/XP10000 Disk Arrays and SVS 200 33