HP XP P9500 Storage - Manual (Page 11)

Though the volumes are paired (PAIR 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.
Initial and update copy operations 11