HP StorageWorks XP24000 Continuous Access Journal Software User and Reference Guide, v01 (T5278-96001, June 2007)

long. This combination is also intended to ensure that the secondary site stores data that has been stored
in the
primary site even when a failure occurs in the primary site. These intentions will be fullled if remote
copy operations are performed using cascading connections and a three d ata center (3DC) c on guration.
In a
3DC conguration, an intermediate site is located between the primary a nd secondary sites.
Figure 24 on page 59 shows an example of a 3D C cascading conguration in which Continuous Access
Journal and Continuous Access Synchronous are combined. In this conguration, the volume in the
primary site is a Continuous Access Synchronous primary volume (P-VOL). The corresponding Continuous
Access secondaryvolume(S-VOL) is thevolumeatthe intermediate site, whichisashortdistancefrom
the
primary site. This Continuous Access Synchronous secondary volume is also used as a Continuous
Access Journal primary data volume. The corresponding Continuous Access Journal secondary data
volume is the volume in the secondar y site, which is a long distance from the intermediate site. If a host
issues an I/O to the Continuous Access Synchronous primary volume at the primary site, data will be
transferred via Continuous Access Synchronous and Continuous Access Journal to the secondary site and
will be restored to the secondary data volume.
Figure 24 3DC Cascading Conguration (C om bining Con tin uous Access Journ al wit h
Continuo
us
Access Synchronous)
Data that is copied by Continuous Access Synchronous is stored in the Continuous Access Journal
primary data volume. If a failure occurs in the Continuous Access Synchronous P-VOL, business tasks
can
cont
inue by using data in the Continuous Access Journal secondar y data volume containing data
in the Continuous Access Synchronous P -VOL. Data in the Continuous Access Synchronous P-VOL is
restored via Continuous Access Journal to the secondary data volume at the secondary site with update
sequenc
e
integrity ensured. If a failure occurs at both the Continuous Access Synchronous P-VOL and
S-VOL d
ue
to a disaster or some other reason, disaster recovery can be performed using the Continuous
Access Journal secondar y data volume.
Note:
A Continuous Access Synchronous P-VOL must be created in a storage system. The fence level of
the
Con
tinuousAccessSynchronous P-VOLmustbe Data.
Basic Behavior
This section explains the basic behavior of a 3DC cascading conguration under the following conditions:
The status of the Continuous Access Synchronous pair is PAIR. The status of the Continuous Access
Journal pair is also PAIR.
A host in the primary site issues an I/O request to the Continuous Access Synchronous P-VOL.
As illustrated in Figure 25 on page 60, if a host at the primary site issues an I/O request, data will be
stored in the Continuous Access Synchronous P-VOL. The data will also be stored in the Continuous
Access Synchronous S-VOL (that is, the Continuous Access Journal primary data volume). After that, the
fact that data has been stored will be reported to the primary site, and then the fact that I/O has been
nished will be reported to the host. Also, data in the Continuous Access Journal primary data volume
(that is, the Continuous Access Synchronous S-VOL) will be stored as journal data in the master journal
volume in the intermediate site. Sequence numbers, which indicate the order that data will be written, will
be added
to the journal da ta. Also, journal data in the master journal volume at the intermediate site
will be transferred, independently from and asynchronously with the behavior mentioned above, to the
restore journal volume in the secondary site. Next, the journal data in the restore journal volume will be
restored to the Continuous Access Journal secondary data volume at the secondary site. The journal
data will be restored in the order of the sequence numbers that are added to the journal da ta. These
sequence numbers ensure that updates to Continuous Access Journal secondar y data volumes at the
XP24000 Continuous Access Journal Software User a nd Reference Guide
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