Overview:
STATUS CODE: 156 "snapshot error encountered" occurs when utilizing VERITAS Volume Snapshot Provider (VSP) or Microsoft Volume Shadow Copy Service (VSS) for a client backup.

Troubleshooting:
If a backup on a Windows NetBackup (tm) 5.0 or later client fails with status code 156, this indicates that the client is using Windows Open File Backups to protect open or active files. The volume "snapshot" that occurs to facilitate open file backup has failed. This failure is often caused by the VSP/VSS cache file running out of allocated space.

Master Log Files:  N/A

Media Server Log Files:  N/A

Client Log Files:  
bpbkar
6:00:14.156 AM: [452.976] <2> tar_base::V_vTarMsgW: ERR - Snapshot Error while reading file: Volume{43877774-d2zz-4rar-r5id-fg9i49fuje7815}\Accounts\Akerfeldt\data\Amm.txt
6:00:14.171 AM: [452.976] <2> tar_base::V_vTarMsgW: FTL - Backup operation aborted!
6:00:14.218 AM: [452.976] <2> tar_base::V_vTarMsgW: INF - EXIT STATUS 156: snapshot error encountered


Resolution:
Firstly, if Open file backups are not desired for the client, disable them. This will decrease the resources used by the client during backup and resolve any Status 156 errors. The default NetBackup install options are to have open file backups enabled.

To confirm if VSS or VSP open file backups are activated:

1. Within the NetBackup Administration Console, expand Host Properties in the hand pane

2. Within Host Properties, click Master Server and then the double-click the name of the Master Server in the right pane

3. Click on the Client Attributes section

4. If the client in question is listed here, click on the client name. If it is not listed, add and click on the client name.

5. Click the Windows Open File Backup tab

6. Ensure that Enable Windows Open File Backups for this client is not selected if open file backups are not required

If open file backup functionality is needed, the most common cause for snapshot errors is cache file configuration. The steps on changing this depend on whether VSP or VSS is used as the Snapshot Provider.

If VSP is being used as the snapshot provider:
Access the Host Property settings for VSP:

1. Within the NetBackup Administration Console, expand Host Properties in the left pane

2. Within Host Properties, click Clients and then double-click the name of the client in the right pane

3. Expand the Windows Client section and click on VSP to display cache settings

Make sure the Customize the cache sizes check box is not selected. With this not selected, NetBackup automatically determines the best VSP cache file sizes. In most cases, NetBackup will be able to create a large enough VSP cache file for backups if this is not selected.

If errors still occur with this not selected, it is possible to increase the initial VSP cache size and maximum VSP cache size manually. These settings vary depending on the client's requirements. Use caution when manually specifying sizes for the Initial and Maximum Cache Size, since these settings will be used regardless of volume size.

Refer the NetBackup System Administrator's Guide for more information on changing the configuration of VSP cache file sizes. If backups still abort with error status 156 after making changes to the VSP cache file size configuration, there may not be enough free disk space on the affected client. Free up as much disk space on the drives of the affected client as possible.

For Volume Shadow Copy (VSS)  (Windows 2003 Only):
Use the following steps to increase the cache size when VSS is used on Windows 2003 Server clients.

1. In Windows, right-click My Computer and select Manage

2. In the console tree, right-click Shared Folders, select All Tasks, and then Configure Shadow Copies

3. Select the volume on which to make changes, and then click the Settings button

4. In the Settings dialog box, change the Maximum Size setting to either No Limit or a size large enough to suit the requirements of the installation and usage of VSS