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why increase all of my diff backup size after 3rd day


Dear all


i have  created many  job in backupexec that get  one full backup(vmdk) in a month and after that get diff backup every day(vmdk) in most of my backup 2 of my diff backups size are so low for example first day is 25M , second day is 30M  but after second day all af my diff backups size so increased to 1.5G and 2G and 2.2G , .... i am sure after second day no changes doing on my source servers .

All of my backups Diff size will increase after second day i have attached backup set from 3 server

why ?  Is this Backupexec architecture?

How can solve this problem?

Warm regards



3 Replies

Re: why increase all of my diff backup size after 3rd day

OK so as you mention VMDK I am going to assume are using our virtual agent to backup VMs


We use the VMware CBT (Change Block Tracking) and Snapshot processes to backup VMware so  the architecture that is causing this  step change is probably within VMware and nothing to do with Backup Exec.


Are you running any 3rd party software that might also be touching CBT or VMware snapshots as that could be the cause and you may need to disable such software.


I am sure not installed any

I am sure not installed any software or any changed after 2 days Is your means that tjs is related to vmware function??

Whilst we use Change block

Whilst we use Change block tracking and reset it during backup processes, change block tracking itself is a VMware offered function which can be affected by other things in the VMWare environment.

If this step change always happens on the same day of the week, then you need to be looking at something that is scheduled to run between the last differential that seems to show a reasonable size and the one that shows that large difference. If you make the Full backup run on a different day of the week does it still go wrong after the 3rd day or does the problem stay to a specific day (which would be indicative of something external.) You will have to edit your current job schules to do this test.

Of course you might want to check that an end user or application is not actually modifying a large amount of data inside the VM on the same day every week as that would have the same effect and mean that CBT is working.