Early versions of Unix filesystems were referred to simply as "FS". FS only included the boot block, superblock, a clump of inodes, and the data blocks. This worked well for the small disks early Unixes were designed for, but as technology advanced and disks grew larger, moving the head back and forth between the clump of inodes and the data blocks they referred to caused thrashing. Marshall Kirk McKusick, then a Berkeley graduate student, optimized the BSD 4.2's FFS (Fast File System) by inventing cylinder groups, which break the disk up into smaller chunks, with each group having its own inodes and data blocks.