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Overwrite Protection Period and Append Period Times Advice

Created: 12 Feb 2013 • Updated: 18 Feb 2013 | 6 comments
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Hi folks,

We're in a situation where I currently need the daily differential backup media used and the set wiped each day. We have one full backup a week on a Friday and differentialson other week days, and space is extremely limited.

On the daily differentials I've set the OPP to 12 hours and AP to 0 Hours, so that each media can be overwritten every day. I set the OPP to 12 hours so that the media can be free to be overwritten after that time - am i right in thinking this? And the AP is set to 0 so that it can be overwritten.

Fridays full backups have a different set so thats set to OPP 2 days (as these only run once a week) and AP to 0 hours. So after two days the whole set is ready to be overwritten without appending.

I'd be grateful if someone could confirm whether this is ok?



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pkh's picture

You are o.k.  Just make sure that your jobs specify overwrite and not append.

Colin Weaver's picture

Your settings look Ok for what you have asked for - although from a good practice point of view it is a very bad idea

Thinks of these scenarios:

1) Full Backup runs on Friday - keep for 2 days. Differential backup starts to run on Monday night and overwrites the full backup - this immediately loses you any ability to fully restroe the serevr and apart form maybe a small number of recoverable fiels in teh Monday differential makes teh differential useless as well.

2) Full backup runs on Friday and you eject the tape. Differential runs as an Overwrite on Monday - Wednesday and tape is always left in the drive. Differential starts to run on Thursday but then something happens to kill Backup Exec during the actual backup operation (Service crash, Power Cut, Flood etc) so after the tape has been Overwritten. OK your full from Friday still exists because you ejected it, but all your Differential data since the full will be toast - can you really afford to lose 4 days, especially if whatever takes down the backup server also takes down the production servers you are backing up.

Oh and if disk based backups (in BE 2010) the problem is kind of the same except you may not be able to eject media for safety. BE 2012 implments DLM which would need more disk space but does try and stop configurations similar to above losing your ability to restore (at least from the last sucessful backup)

evren's picture

Thanks Colin - I forgot to add, the full backups are separate to the diffs  and are in a separate media set so are only overwritten each Fri night byt the following full backup. They're also pushed to a separate set of tapes so are pretty much isolated from the diff tapes. 

Its definitely not a good scenario, but we have no budget for another SCSI drive and I need to optimise this as much as I can for the time being.



Colin Weaver's picture

Are you aware that if a specific tape is overwritable and Backup Exec finds that tape in a drive that it can be overwritten by any backup job that asks for an overwritable tape, not just jobs that use the same media sets - as such the 2 day OPP for your full is a really bad idea if the tape might be left in the drive.

Read the table in the media management section of the admin gudie to understand the exact sequence that BE might use to choose an overwritable tape and notice that recyclable media in any media set is somewhere in every possible sequence.

The mainr eason why BE works this was is that the product is designed on the understanding that if a job starts BE should make every possible effort to find a media to complete on before recording a job failure.

evren's picture

I've got two different backup sets & media sets with tapes assigned to each - both for hard-disk and tapes. Surely in that case it won't just use any tape it finds?

Colin Weaver's picture

yes it will, the default selection sequence for choosing an overwritable tape is:

  • Scratch media
  • Recyclable media in the targeted media set
  • Recyclable media in any media set
  • Imported media

The backup sets itself make no difference this is all about Overwriteable media which is purely media sets (at least as long as BE 2012 and disk storage is NOT involved)

Notice the 3rd line that I bolded.

Recyclable media means media which is already in a configured media set that is eligible for an overwrite (so already contains an older backup set).

You can change the order of the above by changing settings - but every single combination of settings always contains that 3rd line.