New article: Inside the XFS Filesystem.
It splits the disk into independent Allocation Groups so multiple cores can work in parallel without blocking each other.
XFS uses B+ trees for almost everything. Check it out here: https://internals-for-interns.com/posts/xfs-filesystem/

XFS | Internals for Interns
In the previous article , we explored NTFSโa filesystem where everything is a file, from your documents to the Master File Table itself. NTFS centralized all metadata into the MFT, using attribute-based records and a single journal to keep Windows volumes consistent and feature-rich. Now let me introduce you to XFS, the filesystem designed for extreme scale. Originally built by Silicon Graphics in 1993 for their high-end IRIX workstations, XFS was engineered to handle filesystems measured in terabytes when most systems still counted in megabytes. Its core idea is to divide the disk into independent regions called Allocation Groupsโeach with its own free space tracking, its own inode management, and its own locks. This simple design choice is what allows XFS to scale linearly with the number of CPU cores and support filesystems up to 8 exabytes.




