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How to calculate tapes required?

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Hello, I am creating a new backup script and want to backup every day and keep a monthly tape as an archive. Funds are limited so I am trying to calculate what is the least number of tapes I would need for a year to do the above? I am a bit confused at the point in the script where is says weekly or daily rotation. Can anyone offer any advice? Thanks.

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The frequency of rotating your backup sets will have little effect on the number of tapes used. Rotating more frequently is less convenient, but will shorten the period of lost data should one of your tape members become lost or unreadable.


As for the number of tapes you'll use, there are so many variables that your best bet is to try an actual backup and see what you achieve given your setup. Besides the obvious variables of tape type, length, and type of compression you're using, tape use will depend on the sizes and types of files being backed up, how compressible they are, your specific hardware configuration, whether you are backing up networked clients and what the network traffic is like at the time of a given backup, etc., etc.


In our particular DDS setup, we achieve nominal overall compression rates of -20% (where the files from a networked client are incompressible, the client computer is probably in use at the time, and network traffic is high) to +50% (where the files are highly compressible and data transfer rates are also high).


Tape use will also, of course, depend on whether and how often you perform a recycle backup of a given backup set.


Remember, though, that however expensive the tapes may be, they're a lot cheaper than the cost of data loss. Whatever backup scheme you eventually settle on, don't skimp by having only one backup set destination. Have at least two backup sets; consider even three; and plan on rotating one off-site in the event of fire or other disaster.

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I appreciate your response and would prefer to have more tapes. If I was to do a script running daily using recycle Backup Set A tapes and then do a normal backup monthly to a Set B tape (to archive). Would I be able to introduce a recycle Backup Set C set of tapes later on?

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You can create new backup sets at any time, and you can add them to your scripts as new destinations simply by editing the script. However, if you're trying to save money, I'd suggest rather that you incorporate two rotating daily backup sets from the start, and wait to implement your monthly archive.


You probably don't want to perform recycle backups at every backup session because you then lose the ability to maintain a history of modified files. It can be very valuable to retrieve a file from several days or weeks earlier when a user belatedly discovers they've modified or deleted something they'd rather not have.


I would instead recommend that you perform normal backups for your daily backup sets. Then, when you find that a set is spanning too many tape members, you could perform a recycle backup for just one set at a time, so that you always maintain a series of prior backup sessions in your other backup set. You might, for example, do your recycle backups just after you perform your monthly archive backups, recycling the two daily backup sets on alternate months so that you always retain between one and two months of daily backup sessions.


In general, some redundancy in your backup scheme is a Good Thing.

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