5.1.3 Number of Database Copies
Before determining the number of database copies required by the organization, it is important to understand the two types of database copies.
• High availability database copy – Has a log replay time of zero seconds. When a change is made in the active database copy, changes are immediately replicated to passive database copies.
• Lagged database copy – Has a preconfigured delay built into the log replay time. When a change is implemented in the active database copy, the logs are copied over to the server hosting the lagged database copy, but are not immediately implemented. This provides point-in-time protection which can be used to recover from logical corruption of a database (logical corruption occurs when data has been added, deleted, or manipulated in a way the user or administrator did not expect). Lagged database copies allow up to 14 days of lagged data.
If vSphere did not provide workload mobility features, another design factor that would require consideration is database copies for hardware servicing. If only one high availability mailbox database copy is at the primary site, the operations team is required to switch over to a database copy hosted at a secondary data center when the host server needs to be powered off for servicing. Traditionally, to prevent this, you would be required to maintain a passive database at the same geographic location as the active database to support hardware maintenance. However, with DAGs, this is mitigated because vSphere vMotion and DRS are fully supported by VMware.
Despite this, a further architectural consideration is that Microsoft recommends having a minimum of three high availability database copies before removing traditional forms of backup. As Rainpole.com has chosen to forgo traditional forms of backup, they require at least three copies of each database to be implemented as part of the design. In addition, Rainpole.com’s IT organization has chosen to add a fourth, lagged database copy, to protect against logical corruption.