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What does the MD5 verification exactly do ? does it only check whether  the file has been copied or not OR does it also check whether the file has been exactly copied or not ? We are copying LTOs thru Transfer set backup (All MOV files) in LTO 5; say around 90 files for 1400 GB each file being of 15-16 gb. So being only 90 files they can be counted easily. So can we say just by seeing memory status that the files hv been copied without any problem ? (i.e. when we have our verification option off) Do we need this MD5 verification at all ? Can anybody suggest ?

 

 

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It's an alternative method to verify that your backup data is good. It reads all the data back from a backupset and compares it with an MD5 checksum.

 

With LTO Drives Verification is not really necessary, their Read heads are physically positioned after the Write head and the drive internally reads the data back after the write and checks it against the original data while it is still in the memory buffer. Having Retrospect perform additional verification just increases tape and drive wear.  See a nice animation here http://lto.org/technology/primer3.html

 

If you are transferring backupsets or snapshots, from one tape backupset to another, then if they complete successfully you have verified the source tapes by definition.

 

However for backup to disk you need to do some form of verification.

Full Verification reads ALL the data back from the Backupset, AND the Source data, and compares and verifies it. This has a performance impact on the source system while verification is taking place.

An alternative is to use the MD5 Verification.

During Backup Retrospect calculates MD5 checksums on the source data and stores them in the Catalog. 

After the backup the data can be read back from the Backupset, and compared with the MD5 checksum WITHOUT re-reading the source data, thus relieving the strain on the source system.

AND you can schedule these verifications as separate Jobs (Backup > Verification) or set the flag on your backup job to have them done immediately after.

I like using separate jobs, as these can be run during the day since they don't affect source systems, which saves a lot of time during your backup window, AND there is an option to only verify data not already verified. So you can run a job against a whole backupset, and only the newest data is verified.

 

Check the user guide for Verification - Backup Script Preferences - Scripted Verification - Execution Options > Verification

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Thanks for your reply Mr. JoTraGo. but as I said I have only Mov files which are big in size quite identical in size & type to each other. Meanwhile I have done 1 backup without MD5 verification in Transfer Backup process. It looks like there isn't any problem. It simply saves my time.

But for those who have files of various types & of diff. sizes may find the verification process helpful/necessay/a must.

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Verification has nothing to do with the Type, Size, or Number of files being backed up. It's purpose is to verify whether the data that has been backed up or transferred to the target BackupSet has been done exactly and without error and to ensure the the target BackupSet is readable for a restore or recovery.

The decision on whether and how to verify depends on how sure you want to be that you have a good copy of your data.

 

It sounds like you may be Transferring Backupsets from older LTOx tapes to newer LTO5 tapes, perhaps you have a new Tape Drive or Library and are moving all your data with a view to retiring the old LTO.

 

In this case you may rely on the built in, Read After Write Verification feature of the LTO drive itself. However if you will be retiring the original tapes you will definitely want to be sure your copies are good. I would then recommend you perform a MD5 verification to be sure that the new target tapes are readable.

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ok Thanks. But to tell you that I have transfer backed up as many as 38 LTO 5 tapes; as expected I didn't receive a single error (when MD5 verification option was ON). So i put the verification option OFF for next 6 of the LTOs. It saved a lot off time for me (5 hours for 1400 GB < 8 hours for same). I restored some of the data - I didn't see any problem. What do you suggest ?

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Yes LTO is very reliable technology, and yes Reading back a tape is time consuming. In the end it is your choice. Balancing time savings against assurance of successful copy.

 

If your source tapes will still be available in the future should one of your LTO5s turn out to be unreadable, then you could probably take a chance and skip the verify.

But if the source tapes will be retired or disposed of, leaving your LTO5s as the ONLY copy of your data, then I would do the MD5 Verification, just to be sure.

 

Good Luck

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