jorgene wrote:If i backup this VM with Veeam and highest compression, how will Veeam hadle all those blank pages in the VHD files?
If you already had data written to these disks and then you removed it, Veeam backup job will read the entire virtual disk image since blocks on these will not be blank, the will be marked as deleted which is not the same. I believe you'd better to run compact operation first and then run Veeam backup job.
Btw, what Hyper-V version are you at? Just found this article that describes how to reclaim virtual disk space, might help: Working with and resizing Windows Server 2012 VHDs