VSS Backup Modes in Proxmox

Overview

Volume Shadow Copy Service (VSS) provides multiple backup modes that determine how applications behave during backup.

The two most important modes for SQL Server and Exchange are:

• VSS Full Backup
• VSS Copy-Only Backup

The difference mainly affects transaction log handling.

VSS Full Backup

A VSS Full backup informs the application that a production backup is being performed.

After backup completion, the application may truncate transaction logs according to its backup policy.

For SQL Server and Exchange, this means:

• backup is registered as a primary backup
• transaction logs may be truncated
• backup chain state changes

This is useful when the backup system is the primary backup solution.

VSS Copy-Only Backup

Copy-Only creates an independent backup copy without affecting backup history.

In this mode:

• transaction logs are not truncated
• differential base is not reset
• backup chain remains unchanged

This backup behaves as if it does not exist from the application’s backup-chain perspective.

Why Copy-Only Matters

Copy-Only is important when multiple backup systems protect the same server.

Example:

• Backup system A handles production backups
• Backup system B creates additional test or migration copies

If system B performs VSS Full backup:

• transaction logs may be truncated
• backup chain may break
• production backups may fail

Copy-Only prevents this issue.

Typical Use Cases for Copy-Only

Use Copy-Only when:

• SQL Server already has native backups configured
• Exchange is backed up by another enterprise solution
• Multiple backup products protect the same VM
• You need a temporary consistent snapshot for testing

Hypervisor Defaults

Default VSS modes commonly used by hypervisors:

• VMware → VSS_BT_COPY
• Proxmox → VSS_BT_FULL
• Hyper-V → VSS_BT_FULL

This difference is important for SQL and Exchange administrators.

SQL Server Recommendation

For SQL Server:

• Use Copy-Only if databases are backed up separately using SQL-native jobs
• Use Full if Proxmox is the primary backup solution

Exchange Recommendation

For Exchange:

• Full backup triggers VSS BackupComplete event to Exchange writers
• Exchange may truncate logs after successful backup

Use:

• Full when Proxmox is primary backup system
• Copy-Only when Exchange is protected elsewhere