The Unix file system (UFS) is a family of file systems supported by many Unix and Unix-like operating systems. It is a distant descendant of the original filesystem used by Version 7 Unix. Design A UFS volume is composed of the following parts: A few blocks at the beginning of the partition reserved for boot blocks (which must be initialized separately from the filesystem) A superblock, containing a magic number identifying this as a UFS filesystem, and some other vital numbers describing this filesystem's geometry and statistics and behavioral tuning parameters A collection of cylinder groups. Each cylinder group has the following components: A backup copy of the superblock A cylinder group header, with statistics, free lists, etc., about this cylinder group, similar to those in the superblock A number of inodes, each containing file attributes A number of data blocks Inodes are numbered sequentially, starting at 0. Inode 0 is reserved for unallocated directory entries, inode 1 was the inode of the bad block file in historical UNIX versions, followed by the inode for the root directory, which is always inode 2 and the inode for the lost+found directory which is inode 3. Directory files contain only the list of filenames in the directory and the inode associated with each file. All file metadata are kept in the inode. History and evolution Early 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 V7 FS layout to create 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. The intent of BSD FFS is to try to localize associated data blocks and metadata in the same cylinder group and, ideally, all of the contents of a directory (both data and metadata for all the files) in the same or nearby cylinder group, thus reducing fragmentation caused by scattering a directory's contents over a whole disk. Some of the performance parameters in the superblock included number of tracks and