HP XP P9500 Storage - Manual (Page 11)

Update copy
If the host issues an update after the initial copy is complete, the data is written to the P-VOL and
copied to the S-VOL. Subsequent updates are held until the current update is completed in both
volumes. This keeps the pair synchronized.
Update copy has a higher priority than initial copy. However, if an initial copy is in progress when
updates are sent by the host, the update copy must wait until the initial copy’s copy pace completes.
For example, if the copy pace is 15 tracks, the update copy may wait up to 15 tracks (1 cylinder).
Overview of initial, update copy operations 11