Yes, small files can fit in the metadata, which makes a super big difference when you have 300 million files of 405 bytes each. Also, inode handling is way better compared to ext4.
Oh neat! Thanks! I'm checking the docs and I guess you're referring to "Inline files". I don't have much knowledge about btrfs, so I didn't know...
The nearest thing (and it's not really really similar, just related) that ZFS has is special VDEVs.
When you have an array of disks (usually "slow", like regular HDDs) you can attach to it another array of disks (usually very fast, like NVMe) where you can store metadata (file information) and optionally small files up to a size that you can define.
So for example you have a 50TB (or 500, who knows) array of SATA disks, and a small but superfast array of NVMe drives. Lets say 128GB. Or 512, or 1TB depending on you want to do.
File metadata is saved there, so doing a find, ls, tree... operation is now very fast. And if you save, for example, all files smaller than 32KB there (it will depend on your needs, also) all the small file operations will be way faster.
Yes, the key thing about Btrfs is how it handles inodes and how it can store data with the metadata. In the OpenFreeMap image, 60% of the files are stored with the metadata, essentially taking up no space.
