HP StorageWorks Continuous Access XP user guide for the XP10000/XP12000 (December 2005)

Continuous Access XP user guide for the XP10000/XP12000 19
depending on your installation requirements and standards. Command View XP or XP Remote Web
Console does not provide host failover functions.
When Continuous Access XP is used as a data migration tool, host failover is recommended but not
required. When Continuous Access XP is used as a disaster recovery tool, host failover is required to
ensure effective disaster recovery operations. When a Continuous Access XP pair is suspended due to an
error condition, the MCU generates sense information that should be transferred to the remote site via host
failover software for effective disaster detection and recovery.
Remote copy operations
There are two types of Continuous Access XP remote copy operations: initial copy and update copy.
Figure 3 Remote copy operations
Initial copy operations
Initial copy operation synchronize P-VOLs and S-VOLs independently of host I/O processes. Initial copy
operations are the same for Continuous Access XP Sync and Continuous Access XP Async pairs.
Continuous Access XP initial copy operations occur when adding a pair (paircreate) or resuming a
split/suspended pair (pairresync). When a new pair is created, the entire contents of the P-VOL are copied
to the S-VOL cylinder by cylinder, except for diagnostic and unassigned alternate tracks. You can select No
Copy for the Initial Copy field of the initial copy parameters, which copies only cylinder #0 to the S-VOL. If
No Copy is selected, you are responsible for ensuring that P-VOLs and S-VOLs are already identical. The
MCU cannot verify the contents of volumes. When a split/suspended pair is resumed (pairresync), only
cylinder #0 and out-of-sync cylinders (updated by write I/Os during split/suspension) are copied to the
S-VOL. When the Continuous Access XP S-VOL write enable function is used, the RCU tracks S-VOL
updates and sends the S-VOL cylinder bitmap to the MCU when the split pair is resumed. In this case the
MCU merges the P-VOL and S-VOL bitmaps to identify out-of-sync cylinders.
For additional flexibility, Continuous Access XP provides the following options for initial copy:
• Use the Initial Copy Pace field of the initial copy parameters to specify how many tracks the Continuous
Access XP initial copy operation copies simultaneously when creating/resuming a Continuous Access
XP pair. This option can only be specified using Continuous Access XP.
• Use the Priority field of the initial copy parameters to specify the order in which initial copy operations
are performed when creating/resuming multiple Continuous Access XP pairs. This option can only be
specified using Continuous Access XP.
• Use the Maximum Initial Copy Activity option to specify maximum number of concurrent initial copy
operations each MCU can perform (not pair-specific). This option can only be specified using
Continuous Access XP.
Update copy operations
Update copy operations occur when the host issues a write I/O operation to the P-VOL of an established
Continuous Access XP pair. Update copy operations duplicate P-VOL write I/Os at the S-VOL to keep the
volume pair synchronized. Continuous Access XP provides two modes for update copy operations:
synchronous and asynchronous. Update copy mode is specified when you add a Continuous Access XP
pair and is unchangeable for the life of the pair.
For synchronous update copy mode, the MCU ensures the P-VOL and S-VOL are synchronized at all times.
When using fence level “data,” the MCU does not return final ending status for the P-VOL write I/O until
the outcome of the P-VOL write and its associated update copy operation at the RCU are known. For
synchronous mode, the MCU starts the update copy operation when it receives: