Problem Description
This issue occurs when the msdb database filegroup is full, preventing SQL Server from recording new backup metadata. As a result, differential and transaction log backups fail with the error SQL6, and backup reports stop updating.
Traceback
Differential and log backup jobs fail with:
SQL backup failed: Phoenix SQL agent found no instance/database on the server to backup or the SQL service is not running or the backup content is set to exclude all the databases. (#100030006) (Error Code: SQL6)
Logs show “Log chain broken” warnings for multiple databases.
Event Viewer logs show:
Event ID 1105 – Could not allocate space for object 'dbo.backupmediaset'
in database 'msdb' because the 'PRIMARY' filegroup is full.
No new entries appear in the Backup and Restore Events report after a specific date.
Differential backups revert to full backups, and transaction log backups fail.
Cause
The drive or mount point hosting the msdb database (e.g.,System) is completely full.
Because of this, SQL Server cannot allocate additional space in the msdb database to store backup metadata in the dbo.backupmediaset table.
This breaks the Log Sequence Number (LSN) chain, causing differential and transaction log backups to fail.
Impact
Backup metadata logging stops.
Differential backups run as full backups.
Transaction log backups fail with “Log chain broken.”
Backup and Restore Events report does not update.
Log Snippet Example
[INFO] Log chain broken
[WARNING] Skipping database during log backup
[ERROR] SqlAgent : SQL backup failed: Phoenix SQL agent found no instance/database on the server to backup
or the SQL service is not running or the backup content is set to exclude all the databases. (#100030006) (Error Code : SQL6)
Resolution:
1. Check Mount Point Free Space
Use the following query to check disk space utilization for all SQL mount points:
-- Query to check mount point space
SELECT DISTINCT vs.volume_mount_point,
vs.file_system_type,
vs.logical_volume_name,
CONVERT(DECIMAL(18,2), vs.total_bytes/1073741824.0) AS [Total Size (GB)],
CONVERT(DECIMAL(18,2), vs.available_bytes/1073741824.0) AS [Available Size (GB)],
CONVERT(DECIMAL(18,2), (vs.total_bytes - vs.available_bytes)/1073741824.0) AS [Used Size (GB)],
CONVERT(DECIMAL(18,2), vs.available_bytes * 1.0 / vs.total_bytes * 100.0) AS [Space Free %],
CONVERT(DECIMAL(18,2), (vs.total_bytes - vs.available_bytes) * 1.0 / vs.total_bytes * 100.0) AS [Space Used %]
FROM sys.master_files AS f WITH (NOLOCK)
CROSS APPLY sys.dm_os_volume_stats(f.database_id, f.file_id) AS vs
ORDER BY vs.volume_mount_point
OPTION (RECOMPILE);
Review the Space Free % column — if the mount point hosting msdb (e.g., System) is nearly full (less than 10% free), proceed to free up space.
2. Free Up Space on the Mount Point
Delete or move unnecessary .bak, .trn, or temporary files.
Ensure at least 10 GB of free space remains available on the volume hosting msdb.
3. Clean Up the msdb Database
Use SQL Server Management Studio (SSMS) or T-SQL to delete old backup history and shrink the database:
USE msdb;
GO
EXEC msdb.dbo.sp_delete_backuphistory @oldest_date = 'YYYY-MM-DD';
EXEC msdb.dbo.sp_delete_database_backuphistory @database_name = '<DatabaseName>';
DBCC SHRINKDATABASE (msdb);
GO
Tip: Choose a date (e.g., 30–60 days old) to safely remove outdated records.
4. Re-establish the Backup Chain
Run a new full backup after cleanup to create a valid LSN chain.
Verify that differential and log backups complete successfully afterward.
5. Implement Preventive Monitoring
Configure disk space alerts for the mount point hosting msdb.
Schedule regular cleanup of backup history to prevent future metadata space issues.
Verification
After performing the cleanup and running a new full backup:
The Backup and Restore Events report starts updating again.
Differential and transaction log backups complete successfully.
“Log chain broken” and “SQL6” errors no longer appear in job logs.
Additional Notes
Event ID 1105 indicates SQL Server failed to allocate space for a database object. When this occurs in msdb, it affects backup metadata logging.
If the msdb filegroup frequently runs out of space, consider increasing its file size or relocating it to a larger volume.