HP StorageWorks XP Remote Web Console Software - Manual (Page 16)

suspended due to an error condition, the MCU generates sense information that should be
transferred to the remote site using the host failover software for effective disaster detection and
recovery.
XP Continuous Access Remote Copy Operations
Figure 2 (page 16) illustrates two types of XP Continuous Access remote copy operations: initial
copy and update copy. To reduce the overhead associated with these remote copy activities and
maximize data transfer, the storage system uses a special write command that is allowed only for
XP Continuous Access initial and update copy operations. This command transfers the control
parameters and the FBA-format data for consecutive updated records in a track using a single
write operation. The special XP Continuous Access write command eliminates the overhead required
for performing FBA-to-CKD and CKD-to-FBA conversions.
Figure 2 Remote Copy Operations
This section describes the following aspects of XP Continuous Access remote copy operations:
•
Initial copy operations
•
Update copy operations
•
Read and write I/O operations for XP Continuous Access volumes
•
S-VOL write option
•
Difference management
Initial Copy Operation
The initial copy operation synchronizes the P-VOL and S-VOL independently of host I/O processes.
The initial copy operation is the same for XP Continuous Access Synchronous and XP Continuous
Access Asynchronous pairs. An XP Continuous Access initial copy operation takes place when you
add a pair (paircreate) or resynchronize a split/suspended pair (pairresync). When a new pair
is created, the entire contents of the P-VOL is copied to the S-VOL cylinder by track, except for the
diagnostic and unassigned alternate tracks. You can also select None for the initial copy mode. If
None is selected, the user is responsible for ensuring that the P-VOL and S-VOL are already identical
because the MCU cannot verify the contents of the volumes. When a split/suspended pair is
resynchronized (pairresync), only cylinder #0 and out-of-sync tracks (updated by write I/Os during
split/suspend) are copied to the S-VOL.
NOTE: For further information on cylinder/track difference management, see “Difference
Management” (page 19) .
When the S-VOL write enable pairsplit option is used, the RCU keeps track of S-VOL updates and
sends the S-VOL differential data to the MCU when the split pair is resynchronized. In this case,
the MCU merges the P-VOL and S-VOL differential data to identify the out-of-sync tracks.
For additional flexibility, XP Continuous Access provides the following options for initial copy:
•
The number of tracks option allows you to specify how many tracks are copied simultaneously
by the XP Continuous Access initial copy operation when creating/resynchronizing an XP
16 About XP Continuous Access Operations