If an NTFS volume of a Virtual Machine is expanded, subsequent NetBackup for VMware backups may be corrupt, leading to unrestorable files and a failure of the VM to boot.

Article:TECH160324  |  Created: 2011-05-18  |  Updated: 2011-06-24  |  Article URL http://www.symantec.com/docs/TECH160324
Article Type
Technical Solution


Issue



If an NTFS volume of a Virtual Machine is expanded, the $BITMAP file in the MFT (Master File Table) may become fragmented and become stored in non-contiguous locations on the volume.  This may lead to a condition where future VMware backups may be taken using the wrong information, leading to corruption, unrestorable files or a VM which can no longer be booted.


Environment



This issue affects the folowing versions of NetBackup for VMware:

  • NetBackup 7.1
  • NetBackup 7.0/7.0.1 when certain Emergency Engineering Binaries have been applied

This issue may occur if ALL of the conditions below are met:

  • VMware policies are configured with Exclude unused and deleted blocks enabled in the Snapshot parameters (see figure):

    Note: This parameter is enabled by default.
  • The virtual machine's guest OS uses NTFS volumes and has the capability for those volumes to be expanded
  • The volume has been expanded (volume size increased using a Windows volume manager)
  • The MFT index 6 ($BITMAP) is fragmented

 


Cause



When the $BITMAP is fragmented, the potential exists for incorrect sectors to be excluded from block level incremental backups.  However, no errors will be reported and the backup will appear successful.  If a restore is attempted, unexpected results may occur including:

  • Single-file restores may fail
  • Other files may restore, but not be usable
  • After a full VM backup, the VM may not boot properly and may request to be run in safe mode

 

 


Solution



A fix for this issue (Etrack 2382838) is included in the following version:

  • NetBackup for VMware 7.1 Maintenance Release 1 (7.1.0.1)

In addition, it is expected that currently available EEBs affected by this issue may be updated to include a fix for this issue.  Please subscribe to this document (instructions below) to be updated when a fix is available.

After applying a fix, new full backups should be taken as soon as is possible.

 

Workaround:

Either of the following actions will work around this issue until a fix can be applied:

  • Disable the Exclude unused and deleted blocks parameter from the Snapshot Client options (refer to figure above).  Note that this may increase the size of subsequent backups.
  • Do not extend a volume after it has been created.

Best Practices:
Symantec strongly recommends the following best practices:
1. Always perform a full DR backup prior to making 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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Supplemental Materials

SourceETrack
Value2382838
Description

Fragmented $BITMAP sets wrong blocks to be unallocated.



Article URL http://www.symantec.com/docs/TECH160324


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