Differential Backup (TrueDiff) — Administrator Guide

This document explains how the TrueDiff differential backup process works in Xackup Core. It is written for system administrators and avoids deep technical detail while providing the operational information you need to run, monitor, and troubleshoot backups.

At a glance

• TrueDiff creates one full baseline backup (called a Master) and then stores only changes (Deltas) on subsequent runs.
• Masters are larger and occur less frequently; Deltas are smaller and faster to create and transfer.
• The processor enforces retention rules so only a configured number of Master+Delta sets are kept.

Simple flow

1. On each job run, the processor checks for an existing Master snapshot for the VM.
2. If no valid Master exists (or a new Master is forced), the processor creates a new Master:
   • A Xen snapshot is created and exported as a full XVA file (Master).
   • The Master is saved to the backup storage location defined in the job.
3. If a valid Master exists, the processor creates a Delta:
   • Short-lived snapshots capture the VM disks' current state.
   • Only the differences between the Master and the current state are exported to Delta files.
   • Temporary snapshots are removed after export.
4. Each Master has a timestamp-based set-id; Deltas for that Master are numbered so their order is explicit.
5. Retention rules remove oldest Master+Delta sets when the configured retention count is exceeded.

What this means for operations

• Storage layout: backups are organized as Master files (full exports) and Delta files (differentials) under the job's backup folder.
• Performance: most runs will be fast and low bandwidth because they create Deltas only. Creating a Master is I/O intensive.
• Snapshots: the process creates snapshots on the hypervisor and removes them when finished. Short-lived snapshots are expected.
• Retention: configure retention count and maximum Master age to control when full Masters are re-created.

Important admin considerations

• Do not manually delete Master files unless you understand the implications — removing a Master may trigger a full Master re-creation on the next run.
• If a Master becomes too old (per job settings), the processor will discard it and create a fresh Master.
• Compression: client-side compression can reduce transfer size but increases CPU usage on the backup host.
• Bandwidth: Deltas reduce bandwidth usage, but occasional Masters will require more network and storage throughput.

Monitoring and logs

• Look for log entries such as:
  • "Generating Master Snapshot" — Master creation in progress
  • "Generating Master File" or "Generating Difference File" — export activity
  • Snapshot cleanup messages — confirm temporary snapshots were removed
• Leftover snapshots on the hypervisor indicate a failed or interrupted run and should be investigated.

Troubleshooting

• Backups unexpectedly large:
  • Check if a new Master was created (new set-id and Master file present).
  • Verify the Master and Delta files match (Master set-id and Delta numbering).
• Leftover snapshots on host:
  • Inspect job logs for errors during export or cleanup.
  • Only remove snapshots manually after confirming no active backup is running.
• Delta creation fails for a specific disk:
  • Check VDI/disk health and storage connectivity.
  • Confirm permissions and network access for export operations.
• Missing Master file:
  • If the Master file was deleted but the hypervisor snapshot exists, the system may attempt to re-export the Master from that snapshot.
  • If both are missing, the next run will create a new Master.

Best practices

• Configure sensible retention and a maximum Master age so chains of deltas do not grow indefinitely.
• Ensure backup storage has capacity for occasional full Master exports.
• Schedule occasional manual full backups if you prefer more frequent baselines.
• Regularly review job logs for any repeated errors or leftover artifacts.

Quick checklist for incident handling

• If a run fails: check logs, confirm snapshot cleanup, do not immediately delete snapshots.
• If you see leftover snapshots: review the last run's logs, then clean up snapshots only when safe.
• If activation or exports fail consistently: check network, permissions, and host storage availability.

Glossary

• Master: the full baseline export (XVA) for a VM.
• Delta: the differential export containing changes since the Master.
• Set-id: a timestamp-based identifier tying a Master to its Deltas.
• VDI: virtual disk image associated with the VM.

For additional help or to view example logs, see the Xackup job output in the server's log directory or contact Fungusware support with the job ID and timestamps of the problematic run.