HP XP Continuous Access Software - Manual (Page 75)

Basic behavior
This section explains the basic behavior of a 3DC cascading configuration under the following conditions:
• The Continuous Access XP Synchronous pair status is PAIR. The Continuous Access XP Journal
pair status is also PAIR.
• A host in the primary site issues an I/O request to the Continuous Access XP Synchronous P-VOL
As illustrated in Figure 30 , if a host in the primary site issues an I/O request, data is stored in the
Continuous Access XP Synchronous P-VOL. Data is also stored in the Continuous Access XP Synchronous
S-VOL (that is, the Continuous Access XP Journal primary data volume). After that, the fact that data
is stored is reported to the primary site, and then the fact that I/O is finished is reported to the host.
Also, data in the Continuous Access XP Journal primary data volume (that is, Continuous Access XP
Synchronous S-VOL) is stored as journal data in the master journal volume in the intermediate site.
Sequence numbers, which indicate the order data is written, are added to the journal data. Also,
journal data in the master journal volume in the intermediate site is transferred, independently from and
asynchronously with the behavior mentioned previously, to the restore journal volume in the secondary
site. Next, journal data in the restore journal volume is restored to the Continuous Access XP Journal
secondary data volume in the secondary site; journal data is restored in the order of sequence numbers
added to the journal data. These sequence numbers ensure that updates to Continuous Access XP Journal
secondary data volumes in the secondary site are made in the same order that updates to Continuous
Access XP Synchronous P-VOL in the primary site are made.
As explained previously, when the host in the primary site issues an I/O request to the Continuous Access
XP Synchronous P-VOL, data is restored asynchronously to the Continuous Access XP Journal secondary
data volume in the secondary site in a remote location.
Figure 30 Basic behavior in 3DC cascading configuration
If a Continuous Access XP Synchronous pair is suspended in a 3DC cascading configuration, you can
resynchronize the pair using a differential copy. If a Continuous Access XP Journal pair is suspended, you
can resynchronize the pair using a differential copy.
NOTE:
To perform disaster recovery from a failure in the primary site in a 3DC cascading configuration, ensure
that the Continuous Access XP (synchronous) P-VOL's fence level is Data. If this fence level is specified,
you can perform disaster recovery using the Continuous Access XP Journal secondary data volume
containing data stored in the Continuous Access XP Synchronous P-VOL.
Hardware configuration
This section explains the hardware configuration for a 3DC cascading configuration using Continuous
Access XP (synchronous) and Continuous Access XP Journal. In a 3DC cascading configuration, three
arrays are required. HP recommends that you install RAID Manager XP on hosts in the primary,
intermediate, and secondary sites. For Command View XP, a management station is required for these
sites. The following explains hardware configuration for these sites.
Continuous Access XP Journal user guide 75