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Druva Documentation

Differential database backup of AG database gets converted to full backup

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This article applies to:

  • OS: Windows
  • Product edition: Phoenix

Problem description

The differential backups in Phoneix of the AG database get converted to full backup.

Causes and Resolutions

The possible causes of this issue are discussed below with their respective resolution.


Differential backups are not supported on secondary replicas of the AG. Refer Offload supported backups to secondary replicas of an availability group.


[2018-12-31 05:04:04,196] [INFO] roboSyncer: Sending log to Phoenix server with message : 'Backup type is : 'DIFF'.'
[2018-12-31 05:04:39,242] [INFO] Diff backup for availability group, converting to full
[2018-12-31 05:04:39,242] [INFO] roboSyncer: Sending log to Phoenix server with message : 'Converted to full backup.'


Switch the backup from secondary replica to primary replica to get the differential backup of the AG database.


Full backup will happen once the backup is configured from a secondary replica to the primary replica.


[2019-01-07 16:56:41,674] [INFO] [Tid:<_MainThread(MainThread, started 1632)>] SQLAgent: Downloaded metadata completed
[2019-01-07 16:56:41,674] [WARNING] [Tid:<_MainThread(MainThread, started 1632)>] SqlBackup :Instance-name and metadata-instance-name are not same so converting to FULL
[2019-01-07 16:56:41,674] [INFO] [Tid:<_MainThread(MainThread, started 1632)>] SQLAgent: Converting to FULL backup


After the full backup is complete, the subsequent backup will a differential backup.


Start time mismatch occurs when instance backup is taken by another third party tool. Backup is configured from Primary replica.


[2019-01-10 05:03:13,171] [INFO] [Tid:<_MainThread(MainThread, started 9796)>] Backup start time mismatch for [ORNATUS_DB01\DISCOVERY:ProGet] found
[2019-01-10 05:03:13,171] [INFO] [Tid:<_MainThread(MainThread, started 9796)>] Backup start time mismatch found, converting to full
[2019-01-10 05:03:13,171] [INFO] [Tid:<_MainThread(MainThread, started 9796)>] roboSyncer: Sending log to Phoenix server with message : 'Differential backup can't be taken.'~~~~~~~~~~~~~~~~~~~~~~


Follow any one of the following methods to resolve this issue:

  • Stop the backup from the third-party tool.
  • Configure backup from the third-party tool from the secondary replica and use the primary replica only for Phoenix backups.

Microsoft supports “copy only” and “transaction log” backups from secondary replica.