Designing Disaster Recovery Clusters using Metroclusters and Continentalclusters, Reprinted October 2011 (5900-1881)

Continuous Access Journal Overview
Continuous Access Journal is an asynchronous data replication between two HP P9500, HP
XP10000, HP XP12000, HP XP20000 or HP XP24000 storage disk arrays. As depicted in
Figure 29, Continuous Access Journal uses two main features, “disk-based journaling” and “pull-style
replication”. These two features reduce XP12000 internal cache memory consumption, while
maintaining performance and operational resilience.
Figure 29 Journal Based Replication
Continuous Access Journal performs remote copy operations for data volume pairs. Each Continuous
Access Journal pair consists of primary data volumes (PVOL) and secondary data volumes (SVOL)
which are located in different storage arrays. The Continuous Access Journal PVOL contains the
original data, and the SVOL contains the duplicate data. During normal data replication operations,
the PVOL remains available to all hosts at all times for read and write I/O operations. During
normal data replication operations, the storage array rejects all host-requested write I/Os for the
SVOL. The SVOL write enable option allows write access to a secondary data volume while the
pair is split and uses the SVOL and PVOL track maps to resynchronize the pair.
Journal Volume
When Continuous Access Journal is used, updates to PVOL can be stored in other volumes, which
are called journal volumes. The update data that will be stored in journal volumes are called journal
data.
Figure 29 depicts Continuous Access Journal data replication for disk-based journaling in which
the data volumes at the primary data center are being replicated to a secondary storage array at
the remote data center.
When collecting the data to be replicated, the primary P9000 or XP array writes the designated
records to a special set of journal volumes. The remote storage array then reads the records from
the journal volumes, pulling them across the communication link as described in the next section
“Pull-Based Replication”. By writing the records to journal disks instead of keeping them in cache,
Continuous Access Journal overcomes the limitations of earlier asynchronous replication methods.
Writes to the journal are cached for application, but are quickly de-staged to disk to minimize
cache usage. The journal volumes are architected and optimized for keeping large amounts of
host-write data in sequence.
In addition to the records being replicated, the journal contains metadata for each record to ensure
the integrity and consistency of the replication process. Each transmitted record set includes both
time stamp and sequence number information, which enables the replication process to verify that
all the records are received at the remote site, and to arrange them in the correct write order for
storage. These processes build on the proven algorithms of P9000 or XP Continuous Access
Asynchronous Data Replication. The journaling and replication processes also support consistency
across multiple volumes.
Overview of Continuous Access P9000 and XP Concepts 159