If you want to use replication, you should check "Thin provision" in the storage creation menu. Otherwise, when you create a zfs snapshot, it will take up as much space as the source. In your case, you need to create a backup of virtual machines to external storage, then select "Thin provision" and import the virtual machines back. Read about the creation of a sparse file systems zfs. Meybe direct setting of file system properties via the command "zfs set reservation=none <zfsname>"may help. In my case, backup and reverse import solved the problem.