linux - Understanding metadata usage behavior in a newly converted btrfs filesystem

I have a 5 TB hard drive with 3.3 TiB (as reported by du) of media files in 119k files and directories; the average file size was about 28 MiB. I converted the ext4 partition into btrfs with btrfs-convert. The process took 10.4 hours and was CPU-bound. Right after the conversion:

# btrfs fi df /mnt/btrfs/
Data, single: total=3.03TiB, used=2.21TiB
System, single: total=32.00MiB, used=236.00KiB
Metadata, single: total=1.52TiB, used=1.10TiB

btrfs allocated all space on the hard drive (3.03 + 1.52 TiB ≈ 5 TB), and it apparently put ~1 TiB of file content into metadata chunks (1.10 TiB used). This is unexpected because very few of my files would fit into the leaf nodes.

The standard solution to the huge metadata allocation is to re-balance the metadata. btrfs balance start -m took 8.0 hours and was I/O-bound. Afterwards, it appears that btrfs not only released unused metadata chunks, but also moved file content from metadata chunks into data chunks.

# btrfs fi df /mnt/btrfs/
Data, single: total=3.32TiB, used=3.31TiB
System, single: total=32.00MiB, used=104.00KiB
Metadata, single: total=3.00GiB, used=2.18GiB

Could someone explain what is happening? Why would "metadata" consume 1.10 TiB initially, and why would balancing reduce it to 2.18 GiB? Does btrfs balance move file content from metadata chunks to data chunks?

I used Linux kernel 3.13.0-46. My files have no hard links, xattrs, SELinux contexts, nor ACLs. I have not turned on compression, and have not deleted the ext4 roll back image.

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