Often, there will be users who would complain that BE upset their carefully thought out tape naming scheme by using a tape not intended for that backup. For example, a daily backup job taking a tape from the weekly media set or Monday's backup data was written onto Tuesday's tape. This is by design. To complete the backup, BE will use any over-writeable tape in the tape library/partition. This is deemed to be preferable to having the job failed due to a lack of overwriteable tape.
You may or may not agree with this philosophy, but there is no way to prevent this from happening unless you partition your library. If you place your media sets in different partitions, then BE will not take a tape from another partition even if one is available. The backup job which needs an over-writeable tape will fail due to lack of one. You may think that there is no big deal about a backup failure; I will just make it up with the next backup. However, consider this scenario, last night’s daily backup failed and on mid-day, your server crashes. You got to restore it from the previous night’s backup. Now, the users need to recover 1.5 days of work instead of 0.5 day. Furthermore, the side-effects of such a recovery, like having 2 invoices, one before and one after the crash, with different serial number would magnify. This is why for some organizations; backup failure is not an option. A successful backup is preferable to having messed-up tapes.
Why must Monday’s data be on only a tape labeled Monday and not one labeled Tuesday? To go further, why must the tapes be labeled Monday to Friday? Why can’t they be labeled Tape1, Tape2 and so on? BE certainly does not care. Any tape would do, as long as it can write to it. I suspect that this practice is a carry-over from use of stand-alone tape drives and the organizations did not review and change the procedure when they switched over to tape libraries.
When one uses stand-alone tape drive, it is preferable to labeled the tapes as Monday, Tuesday, …, Week1, Week2, etc. The person feeding the tape into the tape drive can easily identify the tape needed. It would be more difficult to do so if the tapes were labeled Tape1, Tape2, etc.
In a tape library, it does not matter how the tapes are labeled. Suppose you have enough tapes in your library for your needs and you don’t have to add tapes to or remove tapes from the library. In this case, the tapes can be labeled as anything. It does not matter. BE will backup to the appropriate over-writeable tape and when you want to restore something, BE will load the correct tape. The tape label is transparent to you. In any case, if you are using pre-printed barcode labels, they would probably come in a series which is random, e.g. your first set of labels could start with 123456 and the next set of labels that you buy could start from 345678.
Some might argue that in a situation where tapes are removed from the library on a regular basis to some off-site storage and then returned to the library later, a tape label that shows the usage like Daily Monday is better. I say this is not so, it would be better to label the container for transporting the tape in this manner rather than the tapes themselves. By do so; it does not matter whether Monday’s data is on Tape1 or Tape5. Just take the tape and put it in the correct container, i.e, if Tape5 was used by the Monday job, then put it into the Monday container.
Let’s examine the situation where you are using a tape library and try to force the data onto the “correct” tape, i.e., Monday’s daily backup MUST be on a tape labeled Monday Daily, etc. What can throw is scheme off track?
1. You do not do any backup on holidays.
Suppose Tuesday is a holiday, when the Wednesday job runs, it will write to the Tuesday tape since it is the oldest over-writeable tape. It would not know that it is suppose to skip the Tuesday tape. Of course, you can remove the Tuesday tape from the library before the Wednesday job runs, but who can remember after coming back from a holiday.
2. Your Tuesday backup job failed.
Even if your job failed, the tape will still be protected because whatever data that were written to it may be important. When you try to re-run the job, the Wednesday tape would be used. There are ways around this problem. You can create an media set with an OPP (Overwrite Protection Period) of 0 hours, move the Tuesday tape to it and target your re-run job to this new media set. Again, manual intervention is required.
3. The wrong tape was brought back from off-site
For some reason, the Wednesday tape was brought back, instead of the Tuesday tape. It can be used, but it is the “wrong” tape.
The above are just some examples. There are many more scenarios which can throw this scheme off.
To force BE to use the “correct” tape, some installations resort to using partitions as a strait-jacket. They will create 5 partitions, one for each day of the week and then have 5 daily jobs/templates which target each of these partitions. The drawback of this scenario is that if for whatever reason, BE is unable to write to the tape in the targeted partition, the job will fail even if there are overwriteable tapes available in the other partitions. This is because the partitions form logical libraries and BE will not take tapes from other libraries, regardless of whether they are physical or logical.
There is also a security angle to using any tape. In the old days when tape capacity was small and was only good enough to hold the data from one application, they were often labeled as General Ledger 1, Accounts Payable 3, etc. When tape library management software became available, it was a security recommendation to use “anonymous” tapes, i.e., label the tapes as 00000001, etc. This was so that a thief after the GL data would not know which tape to swipe.
Hopefully by now, you are more amendable to the idea that it is not necessary to have a job write to a particular tape and that any tape would do. By doing so, you are letting BE does what it does best, i.e. manage your tapes. Don’t waste your time and effort managing your tapes, let the experts, BE and the tape library, work on your behalf.