Building Disaster Tolerant Serviceguard Solutions Using Metrocluster with Continuous Access XP
Overview of Continuous Access XP Concepts
Chapter 3164
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 3-3 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 XP12000 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 XP Continuous Access Asynchronous
Data Replication. The journaling and replication processes also support
consistency across multiple volumes.