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We are having a problem with full backups taking several days to complete on a file server. We're considering going to synthetic fulls for this server. Could someone who has experience with synthetic fulls tell us if there are any pitfalls we should watch out for?

One paragraph in the documentation has us concerned:
"In a scenario where a conventional full backup is run only once for a given subclient, and incrementals (or differentials) with periodic synthetic full backups are run after that, files that never change may inadvertently be missed. Eventually, these files may be pruned, leaving no existing backups of the files. The problem of omissions may build up over time until the file changes or a conventional backup is executed."

I really don't understand how unchanged files would get missed...unless I'm not understanding how a synthetic full actually works. Isn't the synthetic full condensing down the incrementals and writing a new backup...using the old synthetic full as the source? How would unchanged files be dropped?

Thanks for your help.


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This was a problem with older software (old versions of commvault included) where a file that was moved/deleted wasn't properly recorded so you'd have a Synthetic full be a lot larger than what a real full would be. When I used Netbackup you had to have File Tracking turned on (which used a lot more CPU during the pre-backup phase), and only certain OS's supported File Tracking.

There's a checkbox on Synthetic Fulls in Commvault V8 (and I think it was there in v7 too). Try to run one manually, but click the advanced button and you'll see Verify Synthetic Full. That should ensure that there is no problem. It will perform a scan as if it was going to do a real full and then makes sure the Synthetic that gets created matches (at least that's how it was explained to me).

That said, I've never had any problems with my synthetic fulls used for restores.

Also, I'm actually going to be moving away from Synthetic Full (though the concept is similar). In V9, using Client-Side deduplication, they become DASH copies where the concept of the synthetic full is there (it's only pulling an incremental), but instead of rebuilding the full backup again, it just adjusts the dedupe pointers to the new date of the full. If you think that's a direction you're moving towards, you may just look into that.

"warrior_princesss_00" <bellevil < at > msu.edu> 5/18/2011 1:39 PM >>>

We are having a problem with full backups taking several days to complete on a file server. We're considering going to synthetic fulls for this server. Could someone who has experience with synthetic fulls tell us if there are any pitfalls we should watch out for?

One paragraph in the documentation has us concerned:
"In a scenario where a conventional full backup is run only once for a given subclient, and incrementals (or differentials) with periodic synthetic full backups are run after that, files that never change may inadvertently be missed. Eventually, these files may be pruned, leaving no existing backups of the files. The problem of omissions may build up over time until the file changes or a conventional backup is executed."

I really don't understand how unchanged files would get missed...unless I'm not understanding how a synthetic full actually works. Isn't the synthetic full condensing down the incrementals and writing a new backup...using the old synthetic full as the source? How would unchanged files be dropped?

Thanks for your help.




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