So, the btrfs-convert for the smaller drive is done... after near 5 
days. Which stats can I give you ?

It's a 340GB LVM block device, and "btrfs filesystem df /backup/" say that :

Data: total=225.97GB, used=181.94GB
System: total=32.00MB, used=24.00KB
Metadata: total=111.00GB, used=91.56GB


If I mount the ext2_saved/image copy, I can see 257GB of data (78% of 
the block device used), with 17M of inodes.


The other btrfs-convert stay running.