Backups of virtual machines begin by quiescing the virtual machine and taking a VMware software snapshot. While the backup is in progress the virtual machine disk (VMDK) is frozen and changes are written to a temporary file (redo log). Once the backup completes, the VMware snapshot is removed and the virtual machine disk is updated with changes from the redo log (reconciled). For high transaction VMs the redo log can grow significantly if the backup takes a long time to complete, approaching the size of the entire VMDK for the source virtual machine. For other VMs, the changes recorded in the redo log are minimal.
VADP provides protection through an initial full backup supplemented by daily incremental backups and a periodic synthetic full backup to provide a single daily recovery point. This approach can include the following elements:
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VMware‘s Changed Block Tracking (CBT) feature identifies changed blocks for incremental backups.
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A synthetic full backup merges incremental changes from backup media to create a complete point-in-time VM backup without accessing the production VM.
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A secondary copy of a virtual machine can be updated regularly using the DASH Copy feature with deduplication. Only changed data is written to the secondary copy. DASH Copy creates a new full backup by updating reference counters on deduplicated blocks that exist on disk. DASH Copy can be run on a frequency dictated by your backup retention policy, ensuring that old data blocks that are no longer needed are deleted from the system (known as data aging).
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Advanced transport modes for VMs (SAN or HotAdd) minimize the impact of backup operations on local area networks (LANs).
This approach meets short term and long term retention needs with minimal impact on VMs, and can be used when a limited number of ESX proxies (required for IntelliSnap Backups) are available.
Benefits
- You can recover a full VM or any individual file from an incremental point-in-time backup (primary copy or any secondary copies). There is no need to consolidate daily backups into a synthesized full backup.
Considerations
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For high transaction VMs, this approach might not be optimal, because high change rates in VMs require a long reconciliation process.
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For high transaction VMs, incremental changes using CBT can take almost as long as a full backup because changes are written across the disk.
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If hardware snapshots are required for some virtual machines, those can be configured in a separate backup set.