Hyper-V VHD compression help

I'm not aware of any issues on size other than the time it will take to compress. Have a look at the zerodata tool mentioned here http://msmvps.com/blogs/anguslogan/archive/2004/10/03/14756.aspx

My experience under Hyper-V R2 is that precompaction isn't needed, at least for Windows VMs. Notice that the precompaction tool that shipped with Virtual Server 2005 isn't included with Hyper-V. My guess is that the VHD compaction routine looks at the NTFS structures within the VHD to find the actual data to be copied.  

That is inaccurate. Hyper V manager doesn't compress automatically a VHD that is setup for dynamic. You will have to turn off that VM and then use hyper v manager to compress the VHD to regain free space

I was talking about the pre-compaction utility that shipped with Virtual Server that was needed to defragment (and possibly zero out) the VHD before VHD compaction. You still need to turn off the VM in order to compact the VHD. There isn't a supported specification for compressing a VHD, but rather to compact a VHD, which essentially removes all of the white space in a VHD in order to make the file physically smaller. Nothing is actually compressed.

Regarding this topic I wanted to know if there is a certain rule of size of when and when not to compress to get my space. I haven't really found anything online that talks about it.

So basically there is no rule. You just look and the HDD from the VM and compare it to the VHD and see if its worth the time to compress?

Yes. Ask 100 admins you'll get 101 different answers. :-)

Thanks for your response. You clarified what I have always thought.