STATUS CODE 85: A "media read error" message is reported when restoring data from a backup using MSEO (media server encryption option).

Overview:
Attempted restore results in status 85- "The system device driver returned an I/O error while NetBackup reads from tape".  This can result when attempting to do a restore of data encrypted using the NetBackup add-on feature called Media Server Encryption Option(MSEO).

Troubleshooting:
Was the SIZE_DATA_BUFFERS filein place when the backup occurred?
Was MSEO being used by the media server when the backup occurred?

Log Files:
The bptm log mayshow entries similar to the following:
<4>mpx_issue_begin_msg: begin reading backup id test_1220824929 (restore), copy 1,fragment 7 from media id ABC123 on drive index6
<2> mpx_read_data:ReadFile returned FALSE, More data is available. (234);bytes =0
<2>is_possible_recoverable_error: not attempting error recovery, errno =234
<16> mpx_read_data:cannot read image from media id ABC123, drive index 6, err =234
<2> check_error_history:just tpunmount: called from bptm line 876, EXIT_Status =85

The value of SIZE_DATA_BUFFERS can be determined by examining the value stored in the file found at the following locations:
Windows: <install_path>\NetBackup\db\config\SIZE_DATA_BUFFERS
UNIX: /usr/openv/netbackup/db/config/SIZE_DATA_BUFFERS

If this file is not present, the default value of 65536 (64 kilobytes) is used.

Resolution:
This is a common error encountered after initial implementation of MSEO.  It is common practice to tune NetBackup Media Servers using the SIZE_DATA_BUFFERS file up to the highest performing buffer size for a media server.  On Windows media servers, this parameter is commonly equal to 262144 (256 kilobytes).  It is important to understand that MSEO adds encryption overhead to each buffer sent to tape.  The amount of encryption overhead starts at around 3072 bytes and increases depending on the amount of configured keys and policy data configured within MSEO.  NetBackup sends the full SIZE_DATA_BUFFERS to the operating system.  The OS then sends the data buffer to the tape device driver.  MSEO performs driver-level encryption and adds its encryption overhead to the data buffer, then sending the data buffer to the tape drive.

When a subsequent read operation is performed, the combined value of SIZE_DATA_BUFFERS plus the encryption overhead exceeds an environmental threshold greater than what the media server can process.  The environmental threshold is a combination of operating system limitations, tape device driver limitations, HBA/SCSI card hardware and driver limitations, and tape drive hardware limitations.

It is documented within the MSEO Administration Guide in the Troubleshooting section that when creating the SIZE_DATA_BUFFERS file, the value should be reduced from the maximum buffer size for the environment by a factor of no less then 3072bytes.  

If the SIZE_DATA_BUFFERS file does not exist, thenthe media server is using the default buffer value of 65536 (64 KB).  It is recommended to create the SIZE_DATA_BUFFERS file and set the value to59392 (58 KB), thereby reducing the value below the default.

Note:It is critically important to test both backups and restores afterc onfiguring MSEO before encrypting critical production data.  If there store job fails as described above, tune the SIZE_DATA_BUFFERS file downward and re-test the backup and restore until a successful value is discovered.  It is important to understand that the backup must be re-run using the new buffer size, as the buffer size used during the backup is enforced during the restore.

If a situation has occurred where production data was successfully backed up using MSEO and the restore is failing as documented above, it may still be possible to restore the data.

These are several suggestions for restoring the data:
1. Create the MaximumSGList registry key as documented in article 270178 (linked below)
2. Find the bottleneck and fix it.  This can be one or more of the following: