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Creating A Differential Backup

Created: 07 Jan 2014 | 5 comments

I am currently using a full backup that runs Mon - Fri.  The amount of data is starting to exceed the original amount of tapes I had set aside, so I have decided to create a differential backup.  I would run the differential on Monday - Thursday and I would run the full backup on Friday. I would like to keep the data for 90 days (this is set in stone by the powers that be), the same as my full backup.  I estimated that I could use no more than seven tapes, four differentials and one full, with two for scratches.  I have a few questions.

1.  Would BE let me use the incremental tapes over until they are full?  I assume yes since I have multiple full backups on any given tape now.

2.  I set the OPP for 90 days and the APP for Infinite - Allow Append.  Would this be the correct settings (BE defaulted to this when I choose my 90 days)?

3.  Would seven tapes be sufficient?  The amount of data that actually changes daily is less than 15 gigabytes.  I would rotate four of the daily tapes (Mon - Thurs) for the differential.  With the full backup on Friday, I would have three tapes to use, two for the normal rotation and one for a scratch.

I have never done a differential backup scenario, so any help would be greatly appreciated.

Operating Systems:

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lmosla's picture

Hello whitehurstge,

Differential backups will increase in size until the next full backup  see: Should I use differential or incremental backups?     and whether 7 tapes will be sufficient depends on how much data is being backed up.

here is a great video explaining this topic The Basics of Overwrite Protection Period in Backup Exec

What version of Backup Exec is this?

pkh's picture

1. Yes, if you append to the tapes.

2. Yes. You can set the OPP and AP this way.

3. Nobody can tell you this because nobody knows how much of your data changes each day. You would just have to start the back rolling, observe and adjust accordingly

whitehurstge's picture

Well I have the backups setup. 

I have a differential setup for Mon - Thur using one tape for each day and a full backup setup for Friday using three tapes, one every week.  I have the OPP set for 3 days so I can make sure my Monday tape is ready for the following Monday.  I'm still skittish on keeping my data for the 90 days that is set in stone. 

I've been checking my data usage for about two weeks and it seems I would have about 50GB of data to backup on my incrementals.  Using a DLT3 tape compressed to store 800GB, I should be fine with the 90 day backup.  For my fulls, I have 121.5GB of data.  using the same tapes, I get roughly six full backups per tape, with three full tapes to use, that is about 18 weeks (90days).

Does my logic/math seem to be correct?

I appreciate all the help and insight you guys have offered.

pkh's picture

Using a DLT3 tape compressed to store 800GB

A DLT3 tape has a native capacity of only 10GB

I think you mean that you have a LTO3 tape which has a native capacity of 400GB.  The 800GB compressed capacity is a marketing thing which assumes a 2:1 compression ratio.  This is hardly achievable with real data.  Typically, the compression ratio for real data is around 1.2 - 1.3

whitehurstge's picture

I swear this backup is going to be the death of me.  LOL  I think I should be fine with the differentials Mon - Thurs.  I'll see about getting funds for a few more tapes for my full backups and that should do the trick. *crosses fingers*