HP XP Business Copy Software - Manual (Page 17)

Business Copy XP user guide for the XP128/XP1024 17
Initial copy operation
This operation occurs when you create a new volume pair (see Figure 2). The BC initial copy operation
copies all data on the P-VOL to the associated S-VOL. The P-VOL remains available to all hosts for read and
write I/Os throughout the initial copy operation. After the initial copy is complete, the update copy
operation duplicates write operations performed on the P-VOL during the initial copy operation at the
S-VOL.
Before the initial copy operation, the status of a pair is SMPL. The status of the pair is COPY(PD)
(PD = pending) while the initial copy operation is in progress. The status changes to PAIR when the initial
copy is complete. Write operations performed on the P-VOL during the initial copy operation are
duplicated at the S-VOL by update copy operations after the initial copy is complete.
NOTE: When creating pairs, you can select the pace for the initial copy operation: slower, medium, and
faster. The slower pace minimizes the impact of BC operations on disk array I/O performance, while the
faster pace completes the initial copy operation as quickly as possible.
Figure 2 Adding a pair in BC
Update copy operation
This operation updates the S-VOL of a BC pair after the initial copy operation is complete. Since a P-VOL
remains available to all hosts for read and write I/Os during the initial copy operation, the P-VOL might be
updated during the operation. Therefore, when the status of the pair changes to PAIR after the initial copy
operation is complete, the update copy operation takes place and the P-VOL’s differential data is copied to
the S-VOL. Update copy operations take place only for duplex pairs (status = PAIR). As write I/Os are
performed on a duplex P-VOL, the disk array stores a map of the P-VOL’s differential data, and performs
update copy operations periodically based on the amount of differential data present on the P-VOL and the
elapsed time between update copy operations. Figure 3 illustrates an update copy operation in a BC pair
with only one S-VOL.
NOTE:
•
Update copy operations are not performed for pairs with the following status: COPY(PD) (pending
duplex), COPY(SP) (steady split pending), PSUS(SP) (quick split pending), PSUS (split), COPY(RS)
(resync), COPY(RS-R) (resync-reverse), and PSUE (suspended).
• Update copy operations do not occur every time a host issues a write I/O operation to the P-VOL of a
BC pair. BC update copy operations are performed asynchronously according to the differential
bitmap, which is stored in shared memory.