Description of problem:
Need a COW for VM cloning.

Additional info:
This can be done, with the support of COW in the underlying filesystems.

If you use BTRFS for your bricks, then when a COW request hits a file, a link can be made at the destination brick for the new copy, then instigate a COW action on the local file.

Then a rebalance can finish the actual copy process to the real destination brick, in the background.