A system that is being used on a regular basis might become physically difficult or impossible to recover in the aftermath of a disaster such as an earthquake or fire. A system configuration can be created that provides for operations to be resumed under such circumstances by starting a standby system that has been pre-positioned at a remote site. This is known as Real Time SAN Replication (RiSe). The site at which is located the system that is being used on a regular basis is called the main site, and the site at which the standby system is pre-positioned is called the remote site.

Data at both the main site and the remote site is stored on Hitachi disk array systems. When data at the main site is updated, the Hitachi disk array system's TrueCopy or Universal Replicator is used to mirror (update copy) the data to the remote site.

The following figure provides an overview of Real Time SAN Replication.

Figure 8-15 Overview of Real Time SAN Replication

Explanation

• The HiRDB at the main site is used normally to perform operations. When a file at the main site is updated, the updated data is update-copied to the remote site. This update copying maintains the same data at both the main site and the remote site.
• If a disaster such as an earthquake or fire strikes the main site and the system at the main site cannot be recovered quickly, operations can be resumed by restarting HiRDB at the remote site.
  

  Reference note

  • Update copy is performed by TrueCopy or Universal Replicator, which copy data directly between Hitachi disk array systems without going through the hosts.
  • RAID Manager is an optional program for a Hitachi disk array system, and provides commands for controlling and operating TrueCopy and Universal Replicator.

The files that are subject to update copying are listed below. When any of these files is updated, the updated information is copied to the same file at the remote site.

• Database files (HiRDB files comprising RDAREAs)
• System log files
• Synchronization point dump files
• Status files

Update copying can be performed by means of synchronous copy or asynchronous copy. The following table compares the features of synchronous copy and asynchronous copy.

Table 8-6 Features of synchronous copy and asynchronous copy

Item	Synchronous copy	Asynchronous copy
Processing method	Update processing is not complete at the main site until the matching update processing is completed at the remote site; update processing at the main site waits until update processing at the remote site is finished.	Update processing at the main site is completed without waiting for update processing to be performed at the remote site.
Data compatibility between main site and remote site	Data is always synchronized between the main site and the remote site.	Loss of data synchronization might occur. Therefore, data between the main site and the remote site might not always match.
Effect on performance	Transaction processing performance declines. The percentage of decline is proportional to the distance between the sites.#1	No effect on performance occurs.#1, #2

#1: This feature is consistent with logical values guaranteed by TrueCopy.

#2: This feature is consistent with logical values guaranteed by Universal Replicator.