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So what does "verify" actually do?

Level 4

I guess I've been misunderstanding the meaning of "verify" in the context of Backup Exec.  For all this time, I'd been under the impression that a verify compared the data just written to the data on the device or media it had just backed up from.

Yet I recently ran a job to duplicate all of the backup sets from one tape to another (2-drive library).   I noticed that in the verify phase, only the destination tape was loaded into a tape drive!   This was about 500 GB of data and the only place the data existed before the duplication job was the source tape.

The only thing I can think of is that Backup Exec is doing some sort of checksum validation, either across the entire media or at some finer-grained unit (block? file? backup set?).


Level 6

In Backup Exec, verify does NOT compare backed up data to the original data.  If you think about it a little, that doesn't make much sense if you are talking about an active server, whether a File Server or a Database Server or whatever.

What it does is read the backed up data, compute checksums and compare those to checksums written on the backup media.  I'm not sure at what level, but I think it is at the block level

If they match, the verify is successful