HP StorageWorks Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide (AA-RPHZF-TE, March 2004)

Remote Copy Set Features
34 Data Replication Manager HSG80 ACS Version 8.7P Configuration Guide
Remote Copy
DRM uses the peer-to-peer remote copy function of the HSG80 controller to achieve data
replication. The HSG80 dual-controllers at the initiator site are connected to their partner
HSG80 controllers at the target site. Remote copy sets are mirrors of each other and are
created from units at the initiator and target sites. As data is written to a unit at the initiator
site, it is mirrored to its remote copy set partner unit at the target site.
The remote copy feature is intended not only for disaster recovery but also to replicate data
from one storage subsystem or physical site to another subsystem or site. It also provides a
method to perform a backup at either the local or remote site.
With remote copy, user applications continue to run while data movement goes on in the
background over a separate interconnect. Data warehousing, continuous computing, and
enterprise applications all require remote copy capabilities. The remote copy feature is the
major component in the DRM solution.
Remote Copy Sets
A remote copy set is a bound set of two units—one at the initiator site and the other at the
target site—for long-distance mirroring. The term unit is defined as a single disk, storageset,
mirrorset, or RAIDset. The local controller is designated the initiator. The initiator acts as the
director of the replication process. The corresponding remote controller is designated the
target. The target receives I/O requests from the initiator to replicate the data at its location.
Remote copy sets are created only at the initiator site. There can be up to 12 remote copy sets
per controller.
Nonremote Copy Sets
Nonremote copy sets can exist on the same subsystem at the initiator or the target site, or both,
and are generally used for local storage at each site. Clones and snapshots of existing remote
copy sets are nonremote copy sets and can be created for activities like testing and backup.
Because the nonremote copy sets are unique to the specific controller pair, data is not site
disaster tolerant, but can use the various RAIDset types for failure tolerance.
Operation Modes
There are two possible remote copy operation modes: synchronous and asynchronous.
Figure 10 shows the timeline differences between the two.
Synchronous Operation Mode
In synchronous operation mode, data is simultaneously written to the cache of the initiator
subsystem and the cache of the target subsystems. The I/O completion status is not sent to the
host until all members of the remote copy set are updated. Synchronous operation ensures the
highest possible level of data consistency, which makes this process especially appropriate for
business applications that require a high level of currency. The default setting is synchronous.
Asynchronous Operation Mode
In asynchronous operation mode, the write operation is reported to the host as complete before
the data is written to the remote unit of the remote copy set. Asynchronous mode can provide
improved response time, but the data on all members of the remote copy set cannot be
assumed to be the same at all times.