As I'm in the Storage Xenmotion process, I stumbled over limitations like: Free space needed on both SRs.
Citrix XenServer 7.0(7.1) Virtual Machine User's Guides say:
6.1.3 (Page 26)
"For Storage XenMotion, the target storage must have enough free disk space (for the VM and its snapshot)
available for the incoming VMs. If there is not enough space, the migration will fail to complete."
You should be more precisely: How much space do I need in which condition?
As support wrote me: I do need 2 times the space of the corresponding VDI (I assumed 1 times for 1 snapshot).
Also: Is it the same for on- and offline migrations? Last one would not need any snapshot, as long as it prevents the machine from boot up (probably an option you could request when going through the init-process).
Since I won't ever get enough space for 2 times the VDI as free space, I'm kinda locked and would probably need to create a new VDI on target SR and use XenConvert to migrate the disk vom inside a VM (worked with an Exchange-DB disk already).
Also it turned out, that XenConvert inside a VM was about as double as fast, as Storage XenMotion was: ~420 to ~210 MB/s.