When performing storage xenmotion (moving of disks from one SR to another while it is attached to a VM that is running), it fails with insufficient space.

1. For storage live migration, the target storage must have enough free disk space available for the incoming VMs. The free space required can be three times the VDI size (without snapshots). If there is not enough space, the migration fails to complete.
2. Storage live migration works by moving a VM’s virtual disks prior to performing a traditional migration. To support this, we have introduced a new internal operation: snapshot and mirror. Each of a VM’s disks are snapshotted, and from the point of the snapshot onwards, all of the disk’s writes are synchronously mirrored to the destination storage repository. In the background, the snapshotted disk is copied to the destination location. Once a snapshot has finished copying, the next disk to be migrated is snapshot/mirrored. This operation is repeated until all of the VM’s disks are in the process of being synchronously mirrored.
3. It’s important to recognize that a VM snapshot could require as much as 100% of the virtual size (i.e. allocated size) of the base VDI(s) when using volume-based storage.
4. During storage live migration, the VDI gets mirrored on the source SR  and a temporary VDI gets created to record any I\O for the disk, then the mirror gets copied to destination SR.
5. Hence it is recommended to have minimum 2x free space on source SR for storage live migration and 3x free space on destination SR. 

Problem Cause

The VDI move failed due to insufficient space on the SR