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Synthetic Backups should I use Cummulative Incrementals or Differentials?

Les_Denniss
Level 4

Hello I hope someone can clarify

I am using Optimized Synthetic Full Backups and I am doing the following

1. One Full Backup

2. Differential Incremental backup during the week with ' collect true image restore information ' and ' with move detection ' checked in the policy.

3. Synthetic Full at the weekend which is then duplicated then replicated.

The question I have is, should I be using cummulative incrementals or differentials incrementals during the week?

I understand that differentials are better for restores, but what is the difference and what should I be using?

Thanks in advance!

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Accepted Solutions

Marianne
Moderator
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Differential is best for you if you have a small backup window and lots of clients.

Yes, all Diffs since the last Full must be present to create the new Synthetic Full. 
So, if you are backing up to tape only, you cannot send tapes offsite on a daily basis.

Disk is a good choice for Daily Diffs. 
It ensures quick restores and much faster duplication when Synth Full is created, because there is no tape mount/dismount to read each Diff.

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9 REPLIES 9

NBU35
Level 6

I think incrementals would be fine, as taking diffrentials will increase the overhead (large data so large finger priniting) , also large catalog size.

correct me if I am wrong.

Marianne
Moderator
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Have a look at this post that explains the difference between Differential and Cummulative:

Difference between Incremental and Differential back up 

So, Diffs will be smaller and quicker to run, but for a restore, you will need the last Full and all subsequent Diffs. Same when next Synthetic Full runs - last Full and all subsequent Diffs are needed.

Cummulative Inc will be bigger each day and take longer, but for a restore, you will need the last Full and the last Cummulative.

Please also read Chapter 21 - Synthetic backups in NetBackup Administrator's Guide, Volume I 

Les_Denniss
Level 4

Hi ChAmp35 thanks for you respose much appreciated!

Les_Denniss
Level 4

Hi Marianne as I have a short backup window and a large number of clients would you suggest differentials are the way to go? they also collect all the changes and files that have moved?

Also I wish to add this backup method to a server that holds a vast amout of images that is being continuously added to every day. Its current size is 7TB would diffs be able to collect these file types also?

Thanks for you info by the way, much appreciated!

NBU35
Level 6

My bad, i used diffrent terms (diffrential & incremental)  .. I actually meant diffrential incr would be better than cummulative incr :)

Les_Denniss
Level 4

So differentials seem the best option all round? but must ensure all exsist since the last full, before synthetic full starts?

Marianne
Moderator
Moderator
Partner    VIP    Accredited Certified

Differential is best for you if you have a small backup window and lots of clients.

Yes, all Diffs since the last Full must be present to create the new Synthetic Full. 
So, if you are backing up to tape only, you cannot send tapes offsite on a daily basis.

Disk is a good choice for Daily Diffs. 
It ensures quick restores and much faster duplication when Synth Full is created, because there is no tape mount/dismount to read each Diff.

NBU35
Level 6

for normal synthetic backups you should have large number of slots in your tape library and number of tape drives to support it, bcz it requires atleast one drive for read and one for write operation.

in case of optimized Syntehtic, it works on dedup. so there is no such limitation.

Les_Denniss
Level 4

Thank you ChAmp35 and Marianne for your help I will continue to use Diffs!