Ethos,

Can you point me to the documentation that states the LTO drive you are using is performing a verify on the drive as it is writing data?

For most drives a checksum is writen to the tape when the data is written, this is not a verify operation.  When the verify is run those BE has the tape drive check those checksums to make sure they are correctly written.  With the verify turned off Symantec will not be able to guarantee you data can be restored.

It's built into LTO I believe:

en.wikipedia.org/wiki/Linear_Tape-Open

"LTO uses an automatic verify-after-write technology to immediately check the data as it is being written, but some backup systems explicitly perform a completely separate tape reading operation to verify the tape was written correctly. This separate verify operation doubles the number of end-to-end passes for each scheduled backup, and reduces the tape life by half."

LTO drives verify that the data written to the tape is the data that was sent to the tape drive.
I believe (though I may be wrong) that the verify that BE does ensures that the data of the backup itself is consistent.
I leave verify turned on.

I would suggest contacting your hardware vendor to verify that the drive being used does do this before turning off the verify option.  I did a little bit of digging around on HP's and IBM's sites and could not find any documentation on this from them.  The documentation you provided seems to be a direct copy from siegmann.org, but this was the only place I could find this and I don't know how reliable this source is.  When checking the LTO Technology website, I did not see this listed either.

Thanks for the replies.

Ben I found this on the LTO website: www.lto.org/technology/primer3.php which states:

"The head is positioned over data band two. The write element is shown in yellow. In this illustration, as the tape moves beneath the head, the elements write the data to the tape.
When the data passes read/verify element, shown in red, it is immediately checked for errors. If any errors are found, the block of data is rewritten farther down the tape. "

So what that is saying is that the drive is making sure the data is good when it is written. i.e. hard / soft write errors.  That does not verify the consistency of the data that was written to tape, this is where the verify option from Backup Exec comes in.

I'm intrigued to know the answer to this as we've got a very small backup window and turning off verificaion (if it wasn't needed) would really help.

There's also the fact that I've been using Backup Exec for the best part of 10 years (started on version 8.5 I think) and I have NEVER had a verify job come up with a failure, even though I have had one or two instances when I wasn't able to restore data from the same tapes a few weeks later. So I've always been left wondering if the Verify actually does anything useful or is it just a placebo to keep us backup administrators happy?!

By consistency I guess you mean integrity?

I'm still having issues understanding the different methods here. LTO will check for any soft/hard errors and that the data has been written right.

What does BE do on top of this? Because to me that sounds like all would be needed to verify a backup tape!

Canard, I have disabled it for tonight just to see the difference in speeds- I'm not 100% sure convinced it's needed yet.

By consistency I guess you mean integrity? - correct

The verify option in Backup Exec and the verify the LTO drive is doing are two completely different things.

Tape drive auto verify - checking for bad sectors on the tape, marking them bad if they are then writing the data to the next block on the tape. When it marks a block as bad Backup Exec keeps track of this, you can view the number of errors a tape has in the devices tab by highlighting the tape. There are considered hard write errors. The tape drive will try to write the data to a block twice, the hard write error only occurs after both attempts fails. If only the first try fails and the second works, this is considered a soft write error. Now just because the data was put on the tape, does not mean it is good data.

Backup Exec verify - Verifying that the data can be read/restored from tape. If the data cannot be verified, the job will fail. As I mentioned previously a checksum is written with every dataset that is written to the tape (this will happen even if verify is turned off). When the verify part of the job runs, Backup Exec has the tape drive read all of the data and these checksums to verify that the data can be read from the tape and the freshly computed checksum matches the checksum on tape. If either of these cannot be verified the job will fail.

Now you can always turn off Verify in the backup job, then run the verify at a different time.