HP XP Continuous Access Software - Manual (Page 37)

Figure 8 Delta resync activation in a 3DC multi-target configuration (after failure
occurs)
In Figure 8 , because a failure occurs in primary site A (DC1), the horctakeover command is used at DC2 to
change the former Continuous Access XP Synchronous secondary site (DC2) to be the primary site. If you
perform a DC2-to-DC3 delta resync operation in this situation, the contingency DC2-to-DC3 Continuous
Access XP Journal pair for the delta resync operation in Figure 7 becomes synchronized and usable.
When performing a DC2-to-DC3 delta resync operation, first the journal data in DC2 primary site B
is copied to the Continuous Access XP Journal secondary site via a journal copy. In this journal copy
operation, only journal data that is not yet sent to the secondary data volume in the DC3 Continuous
Access XP Journal secondary site is copied in chronological order. When the journal copy operation
completes, the journal restore operation takes place in the DC3 Continuous Access XP Journal secondary
site.
In a DC2-to-DC3 delta resync operation, the Continuous Access XP Journal pair's status does not change
to COPY, but changes to PAIR. This is because the delta resync operation copies only the necessary data
using the journal copy operation. Compared to the usual way that copies all the data in the primary
data volume, the delta resync operation requires much less time for recovering the Continuous Access
XP Journal pair after a failure occurs.
When the stored journal data's total capacity exceeds 80% of the journal volume of the Continuous
Access XP Synchronous secondary site, old journal data is automatically deleted. Therefore, if the total
capacity of journal data that is not restored to the Continuous Access XP Journal secondary data volume
exceeds 80% of the journal volume, the secondary data volume cannot be completely restored by
copying the journal data to the restore journal volume in the Continuous Access XP Journal secondary
site. In that case, according to the setting of the journal group option, all data in the primary data
Continuous Access XP Journal user guide 37