In a VMware based data center, commercial backup software is likely to access virtual disks remotely, perhaps from a backup proxy. The proxy can be a virtual machine or a physical machine with backup-restore software installed and access to alternate storage such as a tape autochanger or equivalent.

At a given point in time, during the backup window, backup software:

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Snapshots virtual machines in a cluster, one by one, or in parallel. Virtual machines run off the snapshot.

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Copies the quiesced base disk, or (for incremental backup) only changed blocks, to backup media.

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Records the configuration of virtual machines.

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Reverts and deletes snapshots, so virtual machines retain any changes made during the backup window.

In the above procedure, the virtual disk library is used in the second step only. The other steps use a portion of the vSphere API (called VADP) to snapshot and save configuration of virtual machines. The virtual disk in a cluster is “managed” by vSphere.

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