Duplicati: VSS Error ID 8194 in Windows Event Viewer

[Germs2004]

• [x] I have searched open and closed issues for duplicates.

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Environment info

• Duplicati version: 2.0.3.3_beta_2018-04-02
• Operating system: Windows Server 2016
• Backend: A different local Windows Server 2016, shared folder

Description

I run Duplicati as a Windows Service running as LocalSystem, and have snapshot-policy On and usn-policy On. It seems to do successful backups and restores. The Duplicati logs show no errors about VSS but there are errors in the Windows Event Viewer every time a Duplicati backup job runs.

Timeline:

Backup job started: 10:52:41
First error in Event Viewer: 10:52:46 AM
Second error in Event Viewer (duplicate of first): 10:59:15 AM
Begin time of Delete phase: 11:00:27 AM
Backup job End Time: 11:01:31 AM

Event Viewer error (2 of these):

Level = Error
Source = VSS
Event ID = 8194

Volume Shadow Copy Service error: Unexpected error querying for the IVssWriterCallback interface.  hr = 0x80070005, Access is denied.
. This is often caused by incorrect security settings in either the writer or requestor process. 

Operation:
   Gathering Writer Data

Context:
   Writer Class Id: {e8132975-6f93-4464-a53e-1050253ae220}
   Writer Name: System Writer
   Writer Instance ID: {2d2433ca-ba80-4d45-867b-aeebfb418c1e}

Duplicati backup job Result log:

DeletedFiles: 0
DeletedFolders: 0
ModifiedFiles: 2
ExaminedFiles: 280941
OpenedFiles: 2
AddedFiles: 0
SizeOfModifiedFiles: 73363456
SizeOfAddedFiles: 0
SizeOfExaminedFiles: 376658058583
SizeOfOpenedFiles: 73363456
NotProcessedFiles: 0
AddedFolders: 0
TooLargeFiles: 0
FilesWithError: 0
ModifiedFolders: 0
ModifiedSymlinks: 0
AddedSymlinks: 0
DeletedSymlinks: 0
PartialBackup: False
Dryrun: False
MainOperation: Backup
CompactResults:
    DeletedFileCount: 0
    DownloadedFileCount: 0
    UploadedFileCount: 0
    DeletedFileSize: 0
    DownloadedFileSize: 0
    UploadedFileSize: 0
    Dryrun: False
    MainOperation: Compact
    ParsedResult: Success
    EndTime: 5/25/2018 11:01:20 AM (1527260480)
    BeginTime: 5/25/2018 11:01:05 AM (1527260465)
    Duration: 00:00:14.9378394
    BackendStatistics:
        RemoteCalls: 9
        BytesUploaded: 31531335
        BytesDownloaded: 135107031
        FilesUploaded: 3
        FilesDownloaded: 3
        FilesDeleted: 1
        FoldersCreated: 0
        RetryAttempts: 0
        UnknownFileSize: 0
        UnknownFileCount: 0
        KnownFileCount: 4504
        KnownFileSize: 232152427832
        LastBackupDate: 5/25/2018 10:52:41 AM (1527259961)
        BackupListCount: 6
        TotalQuotaSpace: 12000539176960
        FreeQuotaSpace: 8154772398080
        AssignedQuotaSpace: -1
        ReportedQuotaError: False
        ReportedQuotaWarning: False
        ParsedResult: Success
DeleteResults:
    DeletedSets: [
        Item1: 1
        Item2: 5/25/2018 10:38:44 AM (1527259124)
    ]
    Dryrun: False
    MainOperation: Delete
    ParsedResult: Success
    EndTime: 5/25/2018 11:01:20 AM (1527260480)
    BeginTime: 5/25/2018 11:00:27 AM (1527260427)
    Duration: 00:00:53.2314182
RepairResults: null
TestResults:
    MainOperation: Test
    Verifications: [
        Key: duplicati-20180525T145241Z.dlist.zip.aes
        Value: [],
        Key: duplicati-i6b4a390ca4da402f9457d51dddd476e1.dindex.zip.aes
        Value: [],
        Key: duplicati-b7c8038aa788e44a28fa84ad204f92b2c.dblock.zip.aes
        Value: []
    ]
    ParsedResult: Success
    EndTime: 5/25/2018 11:01:31 AM (1527260491)
    BeginTime: 5/25/2018 11:01:28 AM (1527260488)
    Duration: 00:00:02.6094123
ParsedResult: Success
EndTime: 5/25/2018 11:01:31 AM (1527260491)
BeginTime: 5/25/2018 10:52:41 AM (1527259961)
Duration: 00:08:50.2555864
Messages: [
    [Retention Policy]: Start checking if backups can be removed,
    [Retention Policy]: Time frames and intervals pairs: 7.00:00:00 / 1.00:00:00, 28.00:00:00 / 7.00:00:00, 365.00:00:00 / 31.00:00:00,
    [Retention Policy]: Backups to consider: 5/25/2018 10:38:44 AM, 5/24/2018 2:21:24 PM, 5/10/2018 1:57:47 PM, 5/3/2018 1:00:47 PM, 3/23/2018 7:00:03 PM, 2/16/2018 5:27:37 PM,
    [Retention Policy]: Backups outside of all time frames and thus getting deleted: ,
    [Retention Policy]: All backups to delete: 5/25/2018 10:38:44 AM,
...
]
Warnings: []
Errors: []

Steps to reproduce

1. Global setting "snapshot-policy" = On
2. Global setting "usn-policy" = On
3. Run a backup job
4. The backup job successfully finishes without any problems mentioned in the Duplicati logs.
5. Check the Windows Event Viewer and see the above mentioned errors.

• Actual result:
  Several errors about VSS in the Windows Event Viewer

• Expected result:
  No errors in the Windows Event Viewer. If VSS problems occur, they should at least be logged in Duplicati's logs too.

[mnaiman]

Use this to get rid of error
--vss-exclude-writers={e8132975-6f93-4464-a53e-1050253ae220}

I think this should be in global config of duplicati set as default. It is System Writer - when you want to image whole running operating system, which is not purpose of Duplicati anyway.

[Germs2004]

I'm not a VSS expert so I'd appreciate it if you can help me with a few questions: Will excluding that GUID cause Duplicati to not use VSS at all? How do we know that the System Writer is only for doing full system images? Which VSS Writer is Duplicati supposed to use for its backups? How can we verify that VSS is being used properly for Duplicati backup jobs?

[mnaiman]

https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2003/cc785914(v=ws.10)

Will excluding that GUID cause Duplicati to not use VSS at all? - No still be backed up using VSS.
How do we know that the System Writer is only for doing full system images? - It its purpose.
Which VSS Writer is Duplicati supposed to use for its backups? - Dont need writer for files, using Hyper-v writer and MSSQL writer if you backup those.
How can we verify that VSS is being used properly for Duplicati backup jobs? - Lock the file for exclusive access and if it is backed up properly than it using VSS.

[Germs2004]

Thanks for the educational info! I now understand much better and I can agree that Duplicati should exclude that writer by default unless and until it gets the ability to back up a system state.