How do the "verify" and "write checksums to media" processes work and why are they necessary?

Article:TECH18698  |  Created: 2010-01-05  |  Updated: 2014-07-16  |  Article URL http://www.symantec.com/docs/TECH18698
Article Type
Technical Solution

Product(s)

Issue



How do the "verify" and "write checksums to media" processes work and why are they necessary?


Solution



Important Note: Symantec Technical Services strongly recommends that both the Verify and Write checksum to media options should be used for all backups that are performed.  If Verify after backup completes is not selected, a successful backup only confirms that data was written to the tape or disk; it does not verify that the tape/disk can be read or restored from the media. As long as Write checksums to media is selected, a verify can be run at a later time. Tape verification errors can be the first indication that a tape, drive, SCSI controller, or SCSI cable are going bad. It may also be indicative of outdated or incompatible firmware on the device.  
 
It is recommended to run the verify on all kind of backups . Tape media is much more subject to physical damage than a hard drive, so data can be written incorrectly. Simple dirt or dust can affect a tape backup too but that doesn't mean that disk are not prone to physical damage. Disk backups can have this issue but rarely.
 

 
By not running a verify, data errors may not become apparent until a restore or catalog operation is attempted from tape media. For this reason, running backups without the verification and checksum options is not advised.

Verify after backup completes is selected by default (Figure 1), and when this option is selected, Backup Exec will run an operation against the tape to read the data that has been written. This does not ensure the integrity of the files,  it only ensures that the tape can be read. If the data that is being backed up on the hard disk is corrupt data, the data residing on the tape will be corrupt as well. This is true even with a successful verify operation.

Figure 1
 

Write checksums to media is also selected by default (Figure 1) and goes hand in hand with the verify operation. During all tape write operations (backup, archiving, etc.), Backup Exec for Windows generates and records checksums for all data placed on tape. During all tape read operations (catalog, restore, verify, etc.), these checksums are recalculated from the data read from the tape, and compared to the originally calculated checksums. The checksums that are created are used in the verify process to make sure that the data is viable on the tape. If this check box has been selected, backup performance may slightly be degraded because checksums are being calculated for each data stream and then written to tape immediately after the data stream. If there is an error in the data, the checksums will not match and the software will report an error.  

 
Data is not compared to the original file on the volume or other source because many files will have changed between the time the backup of a specific file ends and the verify operation of that file begins. This will cause many "false alarms" in verify, possibly causing real failures to be masked. Also the verify time to reread the files would be about the same as the original backup time, making it impossible for many businesses to complete a backup and verify in the available time window. Verifying the data on tape by comparing it to the checksum calculated during the backup will complete in less time since it is only reading the tape, (and not reading the volume backed up or writing the data to tape or disk ) and also will not place extra load on the disk channel by rereading the data that was just backed up.
 

In Backup Exec, these options can be accessed in the Backup Job Properties dialog box under Settings | General (Figure 2) or Destination | Device and Media (Figure 3).

Figure 2
 

Figure 3
 


 



Legacy ID



245177


Article URL http://www.symantec.com/docs/TECH18698


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