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On Symantec Backup Exec, I see there is a Media set with 26 Weeks Overwrite protection period and 1 year appendable period. On all the help information I was able to find the appendable period is usually smaller than the overwrite period.


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Backup Exec "protects" media from being overwritten with the "Overwrite Protection Period". So, in your question, a media will not be permitted to be overwritten until 26 weeks have elapsed since the completion of the last write operation on that media.

Because you might want to continue to append to a media after the initial write (but not actually overwrite what's already there), Backup Exec tracks a separate "Appendable" period. For 1 year after the last write operation completes on a given media, in your question, additional data can be appeneded to the media.

Bear in mind that any appending causes additional writes to the media, and any future checks as to whether the media is able to be appended-to or overwritten will be based on that last write time.

Typically, where you see this used is in libraries where you have, say, multiple tape cartridges loaded and you want to do "one tape per day" type rotations. Let's imagine a library with 5 tapes in it and a once-per-weekday backup rotation. You'd put the media into a set with an "Overwrite Protection Period" of 6 days.

You load the library with tapes "1" thru "5", all of which are brand new and are overwritable.

  • On Monday evening at 7PM the library chooses tape "1" (though it won't necessarily choose the numerically lowest number-- I'm just doing that to make the example easier) and writes to it. That tape is now overwrite-protected for 6 days-- 'til sometime after 7PM on the next Sunday.

  • On Tuesday evening at 7PM the library choose tape "2" and writes to it. That tape is now overwrite-protected until sometime late on the following Monday. The library can't choose tape "1" for this operation because it's overwrite protected.

  • This continues until Friday, when all the tapes in the library are overwrite protected except "5", forcing the library to choose that one.

  • When the following Monday's backup rolls around, the library is forced to use tape "1" because all the other tapes are still overwrite protected. From this time on, the tapes come off of overwrite protection less than 24 hours before they're actually needed, but no sooner.

You can imagine how this could apply to multiple sets of medias (rotating an entire week in and out of the library, etc) with a little mental extrapolation.

The append period is a bit more involved. Take the same scenario above, but say that you run an "incremental" backup during the day each day, Tuesday thru Saturday at noon, and want that incremental backup to be appended to the tape used for the prior night's full backup. You'd set an append period of, say, 20 hours.

  • On Monday at 7PM the backup runs normally. Tape "1" is used and marked overwritable on the following Sunday evening. It remains appendable until late afterooon on Tuesday.

  • At noon on Tuesday the incremental backup runs. Since this is an append job, the library choose a tape that's "appendable". Because tape "1" is still appendable (the append period has expired for all the other tapes), the library chooses tape "1" and appends to it.

  • Tuesday night's full backup runs on tape "2", just like we'd expect.

  • Wedneday at noon, the incremental backup runs. Since tape "1" became non-appendable during Tuesday afternoon it cannot be used. The library chooses tape "2" since it became appendable after the completion of the Tuesday night backup.

There's a crash course in Backup Exec media protection. >smile<

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My guess is that:

  • Overwrite protection period is the lenth of time that needs to expire before a bit of data is overwritten
  • Appendable period is how long you can "add on" to the backup set. Note that "adding on" to the backup set does not overwrite any of the data on the set, it just adds to it.

The of the backup/media set as a file. To do the backup the first time it will write all data to that file. To save disk and processing time, it only adds changes to those original files to the backup set, it does not actually change the backup media.

This means that if someone creates a file and then deletes it, you can still restore it from the backup set.

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