HP StorageWorks XP Remote Web Console Software - Manual (Page 61)

Continuous Access XP Journal user guide 61
operations are performed using cascading connections and a three data center (3DC) configuration. In a
3DC configuration, an intermediate site is located between the primary and secondary sites.
Figure 27 shows an example of a 3DC cascading configuration in which Continuous Access XP Journal
and Continuous Access XP (synchronous) are combined. In this configuration, the volume in the primary
site is a Continuous Access XP (synchronous) primary volume (P-VOL). The corresponding Continuous
Access XP secondary volume (S-VOL) is the volume in the intermediate site, which is a short distance from
the primary site. This Continuous Access XP (synchronous) secondary volume is also used as a Continuous
Access XP Journal primary data volume. The corresponding Continuous Access XP Journal secondary data
volume is the volume in the secondary site, which is a long distance from the intermediate site. If a host
issues an I/O to the Continuous Access XP (synchronous) primary volume in the primary site, data is
transferred using Continuous Access XP (synchronous) and Continuous Access XP Journal to the secondary
site and is restored to the secondary data volume.
Figure 27 3DC cascading configuration (combining Continuous Access XP Journal with Continuous
Access XP [synchronous])
Data that Continuous Access XP (synchronous) copies is stored in the Continuous Access XP Journal
primary data volume. If a failure occurs at the Continuous Access XP (synchronous) P-VOL, you can
continue business tasks by using data in the Continuous Access XP Journal secondary data volume, which
contains data in the Continuous Access XP (synchronous) P-VOL. Data in the Continuous Access XP
(synchronous) P-VOL is restored through Continuous Access XP Journal to the secondary data volume in the
secondary site, with update sequence integrity ensured. If a failure occurs at both the Continuous Access
XP (synchronous) P-VOL and S-VOL due to a disaster or any other reason, disaster recovery can be
performed using the Continuous Access XP Journal secondary data volume.
NOTE: A Continuous Access XP (synchronous) P-VOL must be created in a local disk array. The fence level
of the Continuous Access XP (synchronous) P-VOL must be Data.
Basic behavior
This section explains the basic behavior of a 3DC cascading configuration under the following conditions:
• The status of the Continuous Access XP (synchronous) pair is PAIR. The status of the Continuous Access
XP Journal pair 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 28 , 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 has
been stored is reported to the primary site, and the fact that I/O has been 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 above, 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.