With a 'backup only files changed in last 'n' days' approach does not give you is... a complete recovery point.
All I can ask is... what about files that get backed-up once, and never change again? How would these be restored? You must be keeping tapes forever? No?
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Assuming that you had all of your tapes, and that they will all still be good, how long would it actually take to restore 64TB, especially when you factor in all the manual media changing. Why then even bother with backups for a 64TB data-set. You would be better off with some kind of archiving as Marianne suggest, and/or with some form of mirroring or replication at the file system or volume layer - which, whilst you won't have any point-in-time copies (which you get with backups), you'll at least have an alternate copy. Maybe you could implement some kind of snapshot at the storage layer, and mirror/replicate those snapshots which is very easy with NetApp - but even this has subtleties and niceties to be aware of.
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What you could try is a highly structured approach of multiple backup policies...
1) Work out how big your various sub-folders and trees are, and try to devise 12 distinct unique lists of folder names which are each roughly equal in size, i.e. each list contains the names of one twelth of the total 64TB - i.e. each list of volumes/drives/mount-points/folders/sub-folders would be about 5.3TB.
2) Setup twelve different backup policies, one for each list.
3) Schedule an "Annual_Full" schedule in each of the twelve backup policies, each to run only once a year but each during a different month to each of the other eleven "Manual_Full" schedules in each of the other eleven backup policies..
4) In each of the twelve backup polciies, schedule one "Monthly_Cinc" cumulative incremental schedule to run monthly (in eleven months of the year), and you could even space these out so that out of every four weeks of the month, only three will trigger.
5) In each of the twelve backup polcies, schedule one "Daily_Diff" differential incremental schedule to run daily (but not on the days that the Monthly_Cinc or Annual_Full schedules will run).
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Then, for any given restore of any one of the twelve lists - you would then need to restore from one full + up to eleven cummulative + up to say 24 (assuming working days (Mon-Fri))... but you won't always have to restore from that many backups. Do you see how the number of backups that you would have to restore from only increases through the month and through the year.
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I'm sure that with a spreadsheet you could easily model this and so you could then know, on any given day, how many backup images you need and maybe even calculate how many tapes you might need.