Standby continuous replication (SCR) is a new feature being introduced in Service Pack 1 for Microsoft Exchange Server 2007. As its name implies, SCR is designed for scenarios that use standby recovery servers. SCR extends the existing continuous replication features and enables new data availability scenarios for Exchange 2007 Mailbox servers. SCR uses the same log shipping and replay technology as local continuous replication (LCR) and cluster continuous replication (CCR) to provide added deployment options and configurations. SCR is very similar to LCR and CCR, but it has unique characteristics of its own:
- SCR supports multiple targets per storage group. LCR and CCR support only one replication target per storage group (the passive copy).
- SCR includes a built-in delay for replay activity, and allows enables an administrator to specify an additional delay. This is useful in a variety of scenarios. For example, in the event of logical corruption of an active database, the built-in and additional admin-configured delay could be used to prevent logical corruption of an SCR target database.
- In the RTM version of Exchange 2007, rules are enforced so that in a continuous replication environment a log file is not deleted unless and until it has been backed up and replayed into the copy of the database. This rule is modified when using SCR. SCR (which introduces the concept of multiple database copies) allows log files to be truncated at the SCR source as soon as they are inspected by all SCR target machines. Log truncation at the SCR source server does not wait till all logs have been replayed into all SCR targets because SCR target copies can configured with large log replay lag times.
- Unlike CCR and LCR, you cannot back up an SCR copy. When using SCR, the database headers for SCR copies are updated, and the log files are truncated, when supported backups are taken against the source storage group (or, in the case of LCR and CCR, when backups are taken against either the active or passive copies of the source storage group).
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