In Backup Exec 2010, during a backup of a Virtual Machine the AVVI agent is incorrectly identifying used blocks of data as unused, resulting in missing data from the backup and the inability to restore the missing data.

Article:TECH128232  |  Created: 2010-01-31  |  Updated: 2011-10-12  |  Article URL
Article Type
Technical Solution



An issue has been identified in Backup Exec 2010 (revision 2896) and ESX 4.0. Where a backup job of a thick disk using the AVVI agent incorrectly identifies a used block of data as unused, resulting in missing data on backup, and the inability to restore said missing data.


What is Affected:
At the time of this writing, the problem is only know to occur with the Backup Exec 2010 (revision 2896) AVVI agent when trying to protect ESX 4.0

Severity:   High
Probability:   Medium

(For information on severity and probability levels, refer to this technote:

How to Determine if Affected:
When backing up a thick disk unused blocks are not transferred to the media server, this is by design so as to reduce backup time and storage requirements.  The code that determines which blocks are used or unused contains an error that will incorrectly consider some used blocks as being unused.  Since these used blocks are not transferred to the media server, file/folder GRT may fail, but even if file/folder GRT succeeds, data is missing, which leads to "bad" restores.  It is often undetectable until a restore is attempted and you get missing data or corrupt files.

Formal Resolution:
This issue is addressed in the following hotfix for Backup Exec for Windows Servers 2010 (revision 2896)

Backup Exec 2010:

End users can protect their Virtual machines by doing one of the following:
1. Installing the Agent for Windows Servers onto the Virtual Machine and backing it up
2. Disable used block mapping by setting the following registry key to a value of zero (0) HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Backup Exec For Windows\Backup Exec\Engine\VMware Agent\Disable NTFS Used Sector Tracking

Warning: Incorrect use of the Windows registry editor may prevent the operating system from functioning properly. Great care should be taken when making changes to a Windows registry. Registry modifications should only be carried-out by persons experienced in the use of the registry editor application. It is recommended that a complete backup of the registry and workstation be made prior to making any registry changes.

Best Practices:
Symantec strongly recommends the following best practices:
  1. Always perform a full backup prior to and after any changes to your environment.
  2. Always make sure that your environment is running the latest version and patch level.
  3. Perform periodic "test" restores.
  4. Subscribe to technical articles.

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