HP XP Continuous Access Software - Manual (Page 19)

Continuous Access XP user guide for the XP1024/XP128 19
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:
• A write command whose data length is short enough not to pass the track-end
• Write data to the track-end, whose total length is long enough to pass the track-end
• Write data to the last track for the command
NOTE: No matter how long the write data is, update copy is executed for each track.