Section 8.19. Volume Verification

8.19. Volume Verification

Another often-ignored area of data protection software is its ability to verify its own backups. There are plenty of horror stories out there about people who did backups for years or months assuming that they were working just fine. However, when they went to read the backup volumes, the backup software told them that it couldn't read them. The only way to ensure that this never happens to you is to run regular verification tests against your media. There are several different types of verification:

Reading part of volume and comparing it

There is at least one major vendor that works this way. If you turn on media verification, it forwards to the end of the volume and reads a file or two. It compares those files against what it believes should be there. This is obviously the lowest level of verification.

Comparing table of contents to index

This is a step up from the first type of verification. It is the equivalent of doing a tar tvf. It does not verify the contents of the file; it verifies only that the backup software can read the header of the file.

Comparing contents of backup against contents of filesystem

This type of verification is common in low-end PC backup software. Basically, the backup software looks at its backup of a particular filesystem, then compares its contents against the actual contents of the filesystem. Some software packages that do this will automatically back up any files that are different from what's on the backup or that do not exist on the backup. This type of verification is very difficult, because most systems are changing constantly.

Comparing checksum to index

Some backup software products record a checksum for each file that they back up. They then are able to read the backup volume and compare the checksum of the file on the volume with the checksum that is recorded in the index for that file. This makes sure that the file on the backup volume will be readable when the time comes.

Verify, Verify, Verify!

We were using commercial backup software to back up our file servers and database servers. One day, a multimillion-dollar client wanted some files back that were archived about a year and a half before. We got the tapes and tried a restore. Nothing. We tried other tapes. Nothing. The system administrator and her manager were both fired. The company lost the client and got sued. The root cause was never identified, but they had definitely never tried to verify their backups.

Eugene Lee