DWA: Error Reading from Snapshot. Error: 21

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Issue

When you try to start a backup with the Datto Windows Agent, you receive the error message "Error Reading from Snapshot. Error: 21."

Environment

  • Datto Windows Agent

Cause

Error: 21 is a Windows error code denoting that the specified device is not ready. Common causes are:

  • Disk errors.
  • Filesystem errors.
  • Insufficient free space on affected disk of protected system.
  • Insufficient available RAM on protected system.
  • Disk fragmentation on affected disk of protected system.
  • Excessive disk I/O (Possibly due to fragmentation.)
  • Failed VSS transactions (Possibly due to the above problems.)

Resolution

Identify which volume is failing

The error in the agent log:

Fri 19/01/18 4:22:07 am - Error reading from snapshot. Error: 21
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 will involve identifying the specific volume failing in the snapshot operation. Follow these steps for verification:

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. Check the agent's datto/config/keys/[agentID].agentInfo file for the volume GUID and its corresponding drive letter.

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

Resolving the issue

  • Disk errors and/or Filesystem errors.
    • 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, a chkdsk /r, or replacement of faulty hardware, may be required to repair the protected system.
  • Insufficient free space on affected disk of protected system.
  • Insufficient available RAM on protected system.
    • Make sure the requirements are met in Getting Started With the Datto Windows Agent.
    • The partner will need to investigate if any processes are using excessive RAM, or if a RAM upgrade would be required to meet the minimum requirements for backup.
  • Disk fragmentation on affected disk of protected system.
    • Windows disk optimizer can tell you the fragmentation percentage.
    • Utilities like Defraggler can make visualizing and managing the fragmentation easier.

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