HP XP P9500 Storage - Manual (Page 8)

Volume pairs (primary volumes and secondary volumes)
A pair consists of the P-VOL and secondary data volume. Each P-VOL can be paired with up to
three S-VOLs. Thus, you can create three pairs using one P-VOL.
During most operations, the P-VOL remains available to the host for I/O operations. Updating the
P-VOL regularly copies the new data, the differential data, to the S-VOL. Because S-VOLs are
updated asynchronously, the P-VOL and S-VOLs may not be identical. S-VOLs become available
for host access only after you split or delete the pair.
Cascaded pairs
You can pair each BC S-VOL with a second level S-VOL. You can pair a first level (L1) S-VOL with
two second level (L2) S-VOLs. Thus, up to nine S-VOLs can be available for one P-VOL.
L2 pairs are referred to as cascaded pairs.
8 Business Copy overview