Convert btrfs

https://btrfs.wiki.kernel.org/index.php/Conversion_from_Ext3


# Always run fsck first
fsck.ext3 -f /dev/xxx

# Convert from Ext3/4->Btrfs
btrfs-convert /dev/xxx

# Mount the resulting Btrfs filesystem
mount -t btrfs /dev/xxx /btrfs

# Mount the ext3/4 snapshot
mount -t btrfs -o subvol=ext2_saved /dev/xxx /ext2_saved

# Loopback mount the image file
mount -t ext3 -o loop,ro /ext2_saved/image /ext3