As part of normal operations, XRC ensures that the changes on these
secondary volumes are identified to the SDM system. To accomplish
this, XRC issues commands on the SDM system to vary secondary volumes
offline and online. These actions occur at the following times:
- During initial volume copy, when XRC copies cylinder 0, track
0 to the target volume, the secondary volume is varied offline.
- After a volume pair becomes duplex, XRC varies the secondary volume
online.
- During the XRECOVER function, when the secondary volume's
serial number is clipped to the primary volume serial number, the
secondary volume is varied online to ensure that the volume's
in-storage copy of its VTOC and VTOC index is updated.
Other systems that share these secondary volumes will only have
their in-storage VTOC pointers updated when these commands are issued.
If I/O errors occur when accessing the VTOC from an attached system,
vary the secondary volumes offline and online to correct the condition.