Datto Windows Agent: "Error reading from snapshot. Error: 23"

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Issue

When you try to start a backup for a system protected by the Datto Windows Agent, you receive the error message "Error reading from snapshot. Error: 23."

Environment

  • Datto Windows Agent

Cause

Error: 23 is a Windows error code indicating that the specified device is encountering a Data Error (cyclic redundancy check or CRC). Common causes are:

  • Disk errors.
  • Filesystem errors
  • Degraded RAID arrays

Resolution

Identify which volume is failing by navigating to the agent's logs

To see agent logs:

1. Navigate to the web UI on the device.

2. Select the Protect tab.

3. Click the Backup Logs link for the agent.

4. Select Show Agent Logs.

5. Press CTRL+F to search for "Error: 23". 

Find the error in the agent log

Fri 19/01/18 4:22:07 am - Error reading from snapshot. Error: 23
Fri 19/01/18 4:22:07 am - Backup exception: Could not backup volume snapshots.
Fri 19/01/18 4:22:07 am - Unsuccessful DBD backup.
Fri 19/01/18 4:22:07 am - Destroy snapshots successful.
Fri 19/01/18 4:22:07 am - Query snapshots: 0 existing snapshots.
Fri 19/01/18 4:22:07 am - == Backup Failed ==

Troubleshooting 

Identify the specific volume failing in the snapshot operation

1. Identify the volume snapshot GUID for this backup attempt in the lines preceding the error in the agent logs:

Figure 1: The volume snapshot GUID in the agent log

2. Search the backup log for the first line containing the volume snapshot GUID from the previous step. Identify and note the related volume GUID.

Figure 2: The corresponding volume ID

3. To identify what drive letter this volume GUID is associated with, you can use one of the following methods.

Methods 3b and 3c require terminal access to the Datto appliance, if you do not have this access and cannot login to the protected agent for method 3a, contact Datto Technical Support for assistance.

3a. On the protected system, open the Windows start menu.

Type CMD, then click the Command Prompt item that appears.

In the prompt, type "mountvol". It will display each connected volume with it's GUID and Drive letter pairing. Note which drive letter matches the GUID you noted before and proceed to the next section titled "Resolving the Issue".

3b. Check the agent's '/datto/config/keys/[agentID].agentInfo' file for the volume GUID and its corresponding drive letter. Note which drive letter matches the GUID you noted before and proceed to the Resolving the Issue section of this article.

3c. Alternatively, you can run snapctl update [agentID] and search for the volume GUID to find the corresponding drive letter:

Figure 3: The drive letter and its corresponding GUID

Note which drive letter matches the GUID you noted before and proceed to the next section of this article.

Resolving the Issue

This error is generally caused by a problem with the volume found in the above section.  A few common causes and solutions:

  • Check for disk errors by investigating the server’s event logs at the time of the backup failure. Look for events containing the Volume GUID and/or the volume snapshot GUID for clues about disk errors.  
    • If any are found, run chkdsk /r followed by a reboot of the system, in order to try and repair them.
    • If a chkdsk /r fails to resolve the errors, replacement of faulty hardware, may be required to repair the protected system.
  • A specific common cause are bad blocks on the volume - when this occurs you will see errors generated in the System event log, like this:
Log Name:      System
Source:        Disk
Date:          5/14/2018 8:55:10 AM
Event ID:      7
Description:
The device, \Device\Harddisk0\DR0, has a bad block.
  • If a chkdsk /r fails to resolve the bad block errors in the event log, you may need to do a chkdsk /B in order re-evaluate the bad clusters on the volume.
  • If the volume is using a RAID array, check the array's health, ensure that it is not in a degraded state, and resolve any errors the RAID controller reports.
  • Verify disk health of the volume using utilities like SMART. 

Additional Resources


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