In NetBackup 7.5, when a "Backup From Snapshot" operation is defined in a Storage Lifecycle Policy where multistreaming is also enabled, some duplications may fail with status code 156 reported.

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Problem

When Backup From Snapshot replication jobs are defined as part of a Storage Lifecycle Policy (SLP), there is a potential for premature expiration of snapshots when multistreaming is enabled.  Status code 156 failures may be reported for some streams, resulting in incomplete backups and loss of particular "point in time" data.

Error Message

Status code 156 (snapshot error encountered) may be seen in the Activity Monitor for some duplication jobs.

 

Cause

NetBackup's process of snapshot rotation is intended to identify images/snapshots which are eligible for expiry/deletion based on the following parameters:

  1. The retention limit of the snapshot
  2. Whether or not all dependent copies of a snapshot have completed

Due to multistreaming being enabled, multiple jobs may run using the same snapshot.  It is possible for some jobs utilizing a certain snapshot to have completed while other jobs utilizing that same snapshot may still be in progress - and still other jobs may not have even started.  The snapshot rotation logic was incorrectly considering the snapshot to have all of its dependent copy completed because of some jobs being complete, rather than all jobs.  This resulted in a premature deletion of the snapshot.

Solution

The formal resolution to this issue (Etrack 2712437) is included in the following release:

  • NetBackup 7.5 Maintenance Release 1 (7.5.0.1)

NetBackup 7.5.0.1 download information is available via the Related Article linked below.

Best Practices:
Veritas 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.

Applies To

This issue may be seen when ALL of the following conditions are met:

  • A storage lifecycle policy (SLP) is configured containing a Backup from Snapshot operation
  • Multistreaming is enabled
  • New backups are started from the policy while previous duplications from that same policy are ongoing
  • A snapshot still in use by duplications is selected for rotation and is expired although it is still required for ongoing duplication jobs
  • Status code 156 failures are then reported for these jobs

This issue only affects NetBackup 7.5.  Other versions (including the current version, NetBackup 7.5.0.1) are not affected by this issue.

In a typical multistreamed snapshot-based backup configured as described above, it may occur that there will be more than one Backup From Snapshot jobs.  These jobs can take longer to complete, based on the amount of data that is being backed up as part of the corresponding job.

While these jobs are in progress, the same policy may be executed again  - either due to its scheduled execution time or via manual execution.  If the configured maximum snapshot limit in the policy is reached, a snapshot may be deleted, even though it is still in use.  This can happen when some but not all jobs utilizing a snapshot have completed and NetBackup erroneously believes that it is "done" with the snapshot.

With the snapshot now gone, jobs which were still using it fail reporting status code 156.

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