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Dell replication technologies can generate a restartable or recoverable copy of the data. The difference between the two types of copies can be confusing. A clear understanding of the differences is critical to ensure that the recovery goals for a vSphere environment can be met.
A recoverable copy of the data is one in which the application (if it supports it) can:
The recoverable copy is most relevant in the database realm where database administrators use it frequently to create backup copies of database, for example, Oracle. If a failure of the database occurs, the following abilities are critical to most business applications:
If a failure occurs, an unacceptable loss of all transactions that occurred since the last backup happens without these capabilities.
Creating recoverable images of applications running inside virtual machines using Dell replication technology requires that the application or the virtual machine be shut down when it is copied. If the application supports a mechanism to suspend writes when the copy of the data is created, a recoverable copy of an application can also be created. Most database vendors provide functionality in their RDBMS engine to suspend writes. This functionality must be invoked inside the virtual machine when Dell technology is deployed to ensure that a recoverable copy of the data is generated on the target devices.
If a copy of a running virtual machine is created using Dell Consistency technology without any action inside the virtual machines, the copy is a crash-consistent, restartable image of the virtual machine.
Note: See Copying virtual machines with TimeFinder for more detail.
This means that when the data is used on cloned virtual machines, the operating system, or the application goes into crash recovery. The exact implications of crash recovery in a virtual machine depend on the application that the machine supports:
Most applications and databases cannot perform roll-forward recovery from a restartable copy of the data.
Note: More recent versions of the Oracle database can roll-forward from a crash-consistent copy.
Therefore, a restartable copy of data that is created from a virtual machine that is running a database engine, is inappropriate for performing backups. However, applications that use flat files or virtual machines that act as file servers can be backed up from a restartable copy of the data. This scenario is possible since none of the file systems provide a roll-forward logging mechanism that enables recovery.