HP StorageWorks Business Copy XP user guide for the XP128/XP1024 (December 2005)

Business Copy XP user guide for the XP128/XP1024 25
The following figure shows the state of the S-VOL after the quick restore operation with or without the
Swap&Freeze option.
Figure 6 Quick Restore operation with or without Swap&Freeze option
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 ShadowImage SI390/ShadowImage for z/OS (SI390 and
z/OS) copying processing time. When the Host I/O Performance option is used, the disk array suppresses
execution of SI390/z/OS copying processing, and consequently host I/O responses improve.
When SI390/z/OS copying processing is suppressed, the time taken for copying increases. Also, if BC
pairs exist, host I/O responses might not be faster. In this case, set the Host I/O Performance option for BC
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 7 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.