IBM TSM : my exchange database backups are twice the total size of my databases

Hello experts,

I am having an issue with my exchange backup.

I use tdp for exchange    . My exchange DAG is a cluster of  2 mbx servers. The total of the databases are supposed to be less than 5 terrabytes.

When Watching my tsm reports, I have noticed that the size of my DAG backups is around 9 terrabytes wile I do daily incremental backups and I only keep one version of each backups!

I also noticed that the preferDAGpassive option is set to false? Do I backup twice the dabases?? Whats the best practice, please I am lost. whats the real meaning of preferdagpassive?

preferdagpassive
Williams225System AdministatorAsked:
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MAS (MVE)EE Solution GuideCommented:
This could be because of exchange database's de-duplication feature. A message sent to multiple users will be stored once and each user will have a pointer to it. When a brick-level backup is performed this de-duplication is undone, and each mailbox is backed up separately.
Please check this as well
http://serverfault.com/questions/144739/exchange-backup-size-larger-then-file-size
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Will SzymkowskiSenior Solution ArchitectCommented:
Exchange as a whole it is always recommended to backup the passive node while doing backups for DAG environments (if your backup software supports it). Each backup method can slightly differ based on the vendor that you are using and the technologies that they use to backup the software.

However i am not familiar with your backup software but rule of thumb is backup the passive node. See the below link for additional detail.

https://technet.microsoft.com/en-us/library/dd876874.aspx

Will.
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Williams225System AdministatorAuthor Commented:
Ok you are right, I have just  noticed the preferdag passive is set to true in my backup script.

But What I don't understand is the fact that  the size of my exchange backup is twice the size of the exchange databases size while i only keep one active version of all exchange backups.
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