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LTO verification

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Not sure if this is the right part of the forum for this question, but here I go.


The other day I had a discussion with our system engineer about our backup verification process. Usually we always do some sort of verification. For disk-to-disk we do the MD5 verification and for LTO-4 backups we do a regular verification run after the actual backup.


However LTO drives have a feature that's called read-after-write where the backup in progress is immediately read by a specialized read head. In short it should be possible to do a backup without another verification run. Thus cutting backup time roughly in half and doubling your drive and drive head life span.


The only thing we don't know is if Retrospect actually uses that hardware feature of LTO. So can we safely uncheck verify on our scripts for LTO backups or not?

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Did a little more digging. It seems this technology is native to LTO drives: http://www.ultrium.c...gy/primer3.html


It seems that the LTO just does this as part of its function and it can't be disabled.


The question remains if this method is as reliable as running a verify. Probably not, but it can be sufficient for most needs. Another way of looking at it is this: The drive makes sure the data that it has received and caches in its memory is written to tape. This is what it actually verifies. However it can't know for sure if the data it received (and caches) is exactly the same as the backup program has sent to it.


So, if you don't do a verify run you're for the most part protected from bad tape errors this way. It checks for this. But it doesn't check the whole dataflow from the backup program to LTO drive, just the same as MD5 verification isn't 100% reliable, but maybe 99,999%


So yes, it is less reliable, but only a little bit. Just the same as MD5 verification is. If you want 100% reliability you need to do full verification. It's up to the admin to make that choice and do a trade off between risk and efficiency.

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I also have faith in LTO read after write technology, so it shouldn't be necessary to verify a tape just to be sure the data is actually on the tape. The drive does that for you. But as you say that doesn't check the rest of the data path.

I am a great fan of the, often overlooked, Verify Script (Automate > Manage Scripts > New > Verify). You can set up a verify script, specify all backups sets, and it will only verify outstanding sessions not already verified. {Set the Option "verify only backups not previously verified")

For maximum benefit, ensure the MD5 Sums are enabled in Prefs > Media > Verification > Generate MD5 Digests.

Set Backup scripts to No Verification so no time is wasted during your backup window with verification.

Set your Verify job to run during the day while the backups sets are idle. Retrospect will work its way through the previous night's sessions, verifying them against the MD5 sums in the catalog files, all without touching your clients.


How cool is that! :)

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