HP StorageWorks XP Remote Web Console Software - Manual (Page 20)

the secondary data volume belongs to. The secondary storage system will restore the update
journals to the secondary data volumes in the order write I/Os are made, so that the secondary
data volumes will be updated just like the primary data volumes are updated.
Read and Write I/O Operations
When a primary storage system receives a read I/O for a URz primary data volume, the primary
storage system 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 storage system does
not read the URz secondary data volume for recovery.
When a primary storage system receives a write I/O for the primary data volume with PAIR status,
the primary storage system performs the update copy operation, as well as writing to the primary
data volume.
The primary storage system completes the primary data volume write operations independently of
the update copy operations at the secondary data volume. The secondary storage system updates
the data 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 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
20 Overview of Universal Replicator for Mainframe Operations