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How Incremental Forever Backup Retention Works - Understanding .DAT File Pruning

Learn how NovaBACKUP's Incremental Forever retention removes old data through intelligent pruning of .DAT files rather than simple deletion.

Many customers expect that when retention rules are applied to Incremental Forever backups, the system simply deletes old .DAT files. However, NovaBACKUP uses a more sophisticated approach called pruning that maintains backup integrity while efficiently managing storage space.

How Incremental Forever Retention Actually Works

The Pruning Process

When files or file versions fall outside your retention window, NovaBACKUP doesn't just delete entire .DAT files. Instead, it uses a three-step pruning process:

Step 1: Read Existing Data

  • The backup engine reads your existing .DAT files
  • It identifies which files and file versions need to be removed based on your retention settings

Step 2: Create New .DAT Files

  • New .DAT files are created containing only the data that should be kept
  • Data outside the retention window is excluded from these new files
  • This maintains the integrity of your backup chain

Step 3: Remove Old Files

  • Only after the new .DAT files are successfully created are the old .DAT files removed
  • Space is then reclaimed on your backup destination

Why This Approach?

This method ensures:

  • Data Integrity: Your backup chain remains intact throughout the process
  • Efficient Storage: Only necessary data is retained
  • Safe Removal: Old data is only removed after new archives are verified

Common Retention Issues

"Retention Not Working" - Storage Full

Symptom: Retention is set to X days, but old backups aren't being removed and storage keeps filling up.

Cause: The pruning process requires temporary free space to create new .DAT files before removing old ones. If your backup destination is at or near capacity, pruning cannot complete.

Solution:

  1. Free up 10-20% of space on your backup destination (temporarily)
  2. Allow the next backup job to run
  3. The pruning process should complete and reclaim space
  4. Monitor subsequent backups to confirm retention is working

Error Messages Related to Space Issues

If you see errors like:

  • wrn*0x0000:4015*squ: fat table not found, invalid document
  • You are currently over quota
  • Backup jobs failing with I/O errors

These often indicate your destination is full and pruning cannot complete.

Retention Settings Best Practices

  1. Always maintain 10-20% free space on your backup destination
  2. Monitor backup reports regularly to ensure pruning is completing
  3. Adjust retention settings if storage constraints are frequent