Anatomy of A Backup Chain: Snapshots and the Live Dataset

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1. Background

This article discusses a backup chain.

2. ShadowSnap

ShadowSnap is a block level backup agent that uses a live dataset and snapshots through dual backup engines. ShadowSnap streamlines the BMR process by supporting dissimilar hardware.

3. Live Dataset vs. Snapshots

The live dataset of an agent is the most recent backup image with incremental changes from backup to backup. This represents the most current image that we have for a device. It's essentially the last backup of the agent presented without a snapshot.

After the backup is completed, the live dataset is updated and presented to snapshot. Once that snapshot takes place, you are given a point-in-time backup.

3.1 Removing the Live Dataset

There are certain times where it will be necessary to remove the live dataset of a server.

  • When an agent has a particular volume resized, we need to remove the sparse image of the disk to generate a new full image.
  • This accommodates the newly changed volume size. You'll typically see this paired with an error Destination Volume Too Small.
  • A Differential Merge is used during troubleshooting in order to try and repair a backup image.
  • If the image is still not being correctly processed, remove the entire image and process a new one. This will result in a new full image of the volume to be generated and processed.

3.2 What removing the Live Dataset won't do

  • Removing a live dataset will not clear up space unless the image has not been completely processed by the device. An example would be when a full backup gets cancelled or interrupted in the middle of the backup attempt.  This is uncommon and typically requires tech support intervention.
  • Removing a live dataset will not affect your previous backups.
  • The backups that you already have on the device will not be affected by this process.
For more information on the storage space used by a ShadowSnap backup chain, please see Explaining the Inverse Chain and Investigating Large Backups.

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