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The many tiers of replication
One of the challenges related to replication is that there are so many choices and approaches. Selecting a given approach can have a significant impact on the architecture of the solution, as well as its management and support (see "Replication options," right). Let's review the non-host-based approaches:
Application-- Replication at the application level requires the application to offer this capability. There are potential benefits to this approach, most notably the opportunity for truly transparent failover and recovery from a user perspective. Only a few applications provide replication services, however, and these are built primarily around some type of transaction monitor or messaging service.
Database-- Major database vendors and several third parties offer database-level replication. Typically, the full database is replicated periodically and log entries are shipped to the remote standby database as they're created. If communication links are interrupted, transactions are buffered and sent when the links are restored. At the remote location, log entries are either applied immediately or after some scheduled delay (to avoid replicating a corruption of the database).
Network-- Replication within the network is a newer, but increasingly attractive approach. In a SAN environment, this is done via a dedicated appliance or an app running in an intelligent switch. The goal is to migrate the functionality available in top-tier storage systems to the network level to allow host-transparent synchronous and asynchronous replication across heterogeneous devices.
Storage-- A key goal of controller-based replication at the storage system level is to provide a service that's completely transparent to applications and databases, with little or no impact on hosts. Although expensive, it's generally viewed as the current best practice for protecting tier-one applications. Major features include:
Providing synchronous and asynchronous service. If a business requires local and remote copies of data to be continuously in sync on a real-time basis, then storage replication is probably the best solution.
Support for consistency groups. Enterprise apps can span multiple volumes assigned to several hosts, each handling different components of the application. If the volumes aren't replicated consistently, the target data may be unusable. Consistency groups ensure that interdependent and interrelated data is kept in sync.
Configuration flexibility. Storage-based replication usually provides many configuration options and automation capabilities, including a command-line interface (CLI) and a GUI. The CLI is useful for automation and standardization through scripting.
Centralized operation. The ability to manage and monitor all replication functionality across all storage from a single location simplifies operations and enables consistent protection policies across the enterprise.
This was first published in November 2005