Basic behavior – HP XP Continuous Access Software User Manual
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Figure 29 3DC cascading configuration (combining XP Continuous Access Journal with XP Continuous
Access Synchronous)
Data that XP Continuous Access Synchronous copies is stored in the XP Continuous Access Journal
primary data volume. If a failure occurs at the XP Continuous Access Synchronous P-VOL, you can
continue your business tasks using data in the XP Continuous Access Journal secondary data volume,
which contains data in the XP Continuous Access Synchronous P-VOL. Data in the XP Continuous
Access Synchronous P-VOL is restored through XP Continuous Access Journal to the secondary
data volume in the secondary site, with update sequence integrity ensured. If a failure occurs at
both the XP Continuous Access Synchronous P-VOL and S-VOL due to a disaster or any other
reason, you can perform disaster recovery using the XP Continuous Access Journal secondary data
volume.
NOTE:
You must create an XP Continuous Access Synchronous P-VOL in a local array. The XP
Continuous Access Synchronous P-VOL's fence level must be Data.
Basic behavior
This section explains the basic behavior of a 3DC cascading configuration under the following
conditions:
•
The XP Continuous Access Synchronous pair status is PAIR. The XP Continuous Access Journal
pair status is also PAIR.
•
A host in the primary site issues an I/O request to the XP Continuous Access Synchronous
P-VOL
As illustrated in
, if a host in the primary site issues an I/O request, data is
stored in the XP Continuous Access Synchronous P-VOL. Data is also stored in the XP Continuous
Access Synchronous S-VOL (that is, the XP Continuous Access 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 XP Continuous Access Journal primary data volume (that
is, XP Continuous Access 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 XP Continuous Access 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 XP Continuous Access Journal secondary data volumes in the
secondary site are made in the same order that updates to XP Continuous Access 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 XP Continuous
Access Synchronous P-VOL, data is restored asynchronously to the XP Continuous Access Journal
secondary data volume in the secondary site in a remote location.
Combining XP Continuous Access Journal with other data management functions
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