HP XP Business Copy Software - Manual (Page 31)

Business Copy XP user guide for the XP10000/XP12000 31
The Quick Restore pairresync operation on a pair with status of PSUS exchanges data in the pair’s P-VOL
and S-VOL. If you use the Swap&Freeze option when performing the Quick Restore pairresync operation,
the update copy operation will not take place after the Quick Restore pairresync operation is complete.
Therefore, data on the P-VOL and S-VOL is kept exchanged. If you do not use the Swap&Freeze option
when performing the Quick Restore pairresync operation, the update copy operation will take place after
the Quick Restore pairresync operation is complete, and data on the P-VOL overwrites data on the S-VOL.
Host I/O Performance option
This option improves host I/O responses over BC copy processing time. When you use the Host I/O
Performance option, the disk array suppresses execution of BC copy processing, and consequently host
I/O responses improve.
When BC copy processing is suppressed, the time taken for copying increases. Also, if ShadowImage for
z/OS pairs exist, host I/O responses might not be faster. In this case, set the Host I/O Performance option
for ShadowImage for z/OS as well.
Cascade function
This function creates a second layer of volume pairs underneath the first layer of BC pairs. Pairs created in
the first and second layer are called cascaded pairs. Figure 12 illustrates cascaded BC volume pairs. The
first cascade layer (L1) is the original BC pair, consisting of one P-VOL (root volume) and up to three
S-VOLs (node volumes). In the second cascade layer (L2), the S-VOLs of the original BC pair function as the
P-VOLs of the L2 pairs, and each L2 P-VOL can have up to two S-VOLs (leaf volumes). This layering scheme
can create up to nine copies of one BC primary volume (P-VOL).
Every volume can have up to three Mirror Unit (MU) numbers associated with it. The MU number
designates a pair for which the volume is a member. The MU number is not the number of the pair, but the
number of the volume. A pair is made up of two volumes of which each pair member could have the same
or different MU numbers.
• MU=0 is the number of a volume for its first pair.
• MU=1 is the number of the same volume for its next pair.
• MU=2 is the number of the same volume for its third pair.
• The MU number of an S-VOL is always MU=0.
For Level 1 (L1) pairs, the root volume could have MU=0, MU=1, and MU=2. For L1 pairs, the associated
S-VOLs all have MU=0 because this is the first pair of which they are a member.
For the Level 2 (L2) pairs, the L1 S-VOLs are now all acting as L2 P-VOLs. Because they are already a
member of one pair, the next MU number for each of them is MU=1 for their first L2 pair and MU=2 for