Description of problem:

We recently planned migrating from an existing storage backend (glusterfs) to a new one (iSCSI). On glusterfs, all our disks are thin provisioned, however, when moving disks to iSCSI, the following warning is shown:

   The following disks will become preallocated, and may consume considerably more space on the target: local-disk 

Indeed, after migration disks are preallocated.

Thin provisioning is especially valuable to us because we use our infrastructure for teaching and a lot of VM pools are created for students, most of them with enough storage to not need to extend them afterwards, but practically only 10% of disk space is used actually (around 600GB), so if we migrated all machines we'd be using around 6TB, which is a lot of wasted space.

This have been reported on list and Pavel Gashev suggested moving the file to a file based storage and then return it the block storage:

> Please note that while disk moving keeps disk format, disk copying changes 
> format. So when you copy a thin provisioned disk to iSCSI storage it's being 
> converted to cow. The issue is that size of converted lv still looks like
> preallocated. You can decrease it manually via lvchange, or you can move it 
> to a file based storage and back. Moving disks keeps disk format, but fixes 
> its size.

Also, please consider adding an option to move storage of all machines in a VM pool at once (maybe allowing to specify a maximum of VMs to migrate at a time?).