Unix File System
| Developer(s) | CSRG |
|---|---|
| Full name | UNIX file system |
| Introduced | with 4.2BSD |
| Structures | |
| Directory contents | tables |
| Limits | |
| Max. volume size | 273 bytes (8 ZiB) |
| Max. file size | 273 bytes (8 ZiB) |
| Max. filename length | 255 bytes |
| Other | |
| Supported operating systems | A/UX, DragonFlyBSD, FreeBSD, FreeNAS, NAS4Free, HP-UX, NetBSD, Linux, OpenBSD, Solaris, SunOS, Tru64 UNIX, UNIX System V, and others |
The Unix file system (UFS; also called the Berkeley Fast File System, the BSD Fast File System or FFS) is a file system used by many Unix and Unix-like operating systems. It is a distant descendant of the original filesystem used by Version 7 Unix.
A UFS volume is composed of the following parts:
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 is kept in the inode.
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.
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 sectors, disk rotation speed, head speed, and alignment of the sectors between tracks. In a fully optimized system, the head could be moved between close tracks to read scattered sectors from alternating tracks while waiting for the platter to spin around.
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