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CAUTION:
Even if a copy operation is completed without any host I/O, the data in the P-VOL and S-VOL may not
beidentical.WhethertheP-VOLandtheS-VOLhavethesamedatadependsontheconditionofthe
disk array. To make the P-VOL and S-VOL data the same, split the pair and make the pair status PSUS.
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.
• If shared memory is lost (for example, offline micro exchange or volatile PS on), the differential
bitmap is also lost. In this case, the disk array treats the entire BC P-VOL (S-VOL for COPY(RS) pairs)
as difference data and recopies all data to the S-VOL (P-VOL for COPY(RS) pairs) to ensure proper
pair resynchronization. For pairs with COPY(SP) or PSUS(SP) status, the disk array changes the status
to PSUE due to the loss of the differential bitmap, also ensuring proper resynchronization of these
pairs. If shared memory is lost, allow extra time for BC operations.
Figure 3 Update copy operation
Cascade function
This function creates a second layer of BC volume pairs (L2) underneath the first layer of BC pairs (L1).
Cascade
pairs are created by clicking Cascade instead of Set in the Paircreate window (see Creating
BC pair
s(Paircreate)). This cascade layering scheme creates up to nine copies of one BC P-VOL. See
Cascad
efunctionfor further information on cascade pairs and the cascade function.
Pairsplit operation
The BC split capability, by making the BC copies (S-VOLs) available for host access, provides a
point-in-time copy that can be used for data backup or to provide real data for test purposes. The BC
pairsplit operation copies all P-VOL data or differential data to the S-VOL to make the S-VOL identical to
theP-VOL.IftherearealotofhostI/OsfortheP-VOL,itmighttakelongertosynchronizetheP-VOL
and S-VOL.
Use the Pairsplit window to split existing BC pairs or to add and split new BC pairs in one step. When
you split an existing pair, only the differential data is copied to the S-VOL. When you create and split a
Business Copy XP user guide
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