Duplicated images using OpenStorage technologies (OST) direct copy to tape may fail to restore the last file in the image when the SIZE_DATA_BUFFERS_DISK setting is greater than 64K.

Article:TECH180202  |  Created: 2012-01-30  |  Updated: 2014-11-04  |  Article URL http://www.symantec.com/docs/TECH180202
Article Type
Technical Solution

Product(s)

Environment

Subject

Issue



Document history:
June 15, 2012: Document updated to reflect fix availibility in a current release (see Solution section below)

Non-NDMP client-type images duplicated using OpenStorage technologies (OST) direct copy to tape may be incomplete.  This problem can occur when SIZE_DATA_BUFFERS_DISK has been set to a value greater than 65536 (64K).  The part of the last block of the backup image after the first 64K is not duplicated.  If the last block of the image is 64K or smaller, the image copied is intact.  NDMP client-type images are not affected by this issue.

Note: By default, the value of the SIZE_DATA_BUFFERS_DISK setting is greater than 64K, which means this problem could occur.  When the touch file has not been created to override the setting, its default value is used.  That default value is 262144 (256K).

The SIZE_DATA_BUFFERS_DISK touch file can be found in the following directory/folder:
Windows: <install_path>\VERITAS\NetBackup\db\config
UNIX/Linux: /usr/openv/netbackup/db/config


Error



There are no errors reported during the duplication.


Environment



This issue will be seen when ALL of the following criteria are met:

  • Read side media server running NetBackup 6.5.2 and above
  • Image being duplicatied is not of NDMP client type
  • OpenStorage direct to tape has been configured to copy images from disk to tape
  • The SIZE_DATA_BUFFERS_DISK touch file on the original backup media server has not been created
    OR contains a value larger than 65536.
  • Duplication of images from the OST disk pool to tape has completed AND the images on disk have expired

The possibility of data loss can occur within the last file of the image.  If the last block of the backup is larger than 64K and the SIZE_DATA_BUFFERS_DISK setting is larger than 64k, the block will be truncated to its first 64K when written to tape.

Note: This file may still be restorable from an available unexpired disk copy.  The data loss could occur if there is no longer a copy of this image on disk.  The last part of the data could be missing from the image on tape and it will not be available for restore.


Cause



 A block-size miscalculation on the last block of data leads to the loss of data.


Solution



The formal resolution to this issue (Etrack 2673602) is scheduled included in the following release:

  • NetBackup 7.5 Maintenance Release 3 (7.5.0.3)

More information on NetBackup 7.5.0.3 can be found by accessing the Related Article linked below.

Workaround:
To prevent this condition, perform either of the two workarounds below.

1. Manually set the value of the touch file to 65536 (64K):

echo 65536 > /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS_DISK
# cat /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS_DISK
65536

Note: After changing these settings, it is recommended to perform new duplications to tape of images still on the disk pool.

2. Add the NDMP_OST_DISABLE setting to the ndmp.cfg file.

ndmp.cfg can be found in the following directory/folder:
Windows: <install_path>\VERITAS\NetBackup\db\config
UNIX/Linux: /usr/openv/netbackup/db/config

This will lead to the data being transported through the media server instead of directly from the OST device to the tape media server.

Please note that both of these workarounds may cause a degradation in performance.

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
Value2673602
Description

Cannot restore from copy 2 on physical tape. Also if image is duplicated thru Media manage STU then it works




Article URL http://www.symantec.com/docs/TECH180202


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