Chapter 2 About Universal Replicator Operations 2-23
Hitachi Universal Replicator for IBM /OS User’s Guide
Note: When you issue the DEVSERV command 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 Hitachi 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.
