HP XP P9000 Business Copy for Mainframe Systems User Guide (AV400-96572, July 2013)

Though the volumes are paired (DUPLEX status), data in the P-VOL and S-VOLs may not be the
same because update copying occurs asynchronously, and therefore later data exists in the P-VOL.
You can ensure that P-VOL and S-VOLs data are the same at a certain point in time, by splitting
pair.
Also, data in the P-VOL and the data in the S-VOL may not be the same if a copy operation is
completed without host I/O. As mentioned above, the P-VOL and S-VOLs can be made the same
by splitting the pair.
10 Business Copy Z overview