Unix File System

From Academic Kids

UNIX file system (UFS) is a file system used by many unix and Unix-like operating systems. It is derived from the Berkeley Fast File System (FFS), which itself was originally developed from FS in the first versions of UNIX developed at Bell Labs.

Nearly all BSD unix derivatives including FreeBSD, NetBSD, OpenBSD, NeXTStep, and Solaris use a variant of UFS. In Mac OS X it is available as an alternative to HFS. In Linux, partial UFS support is available and the native Linux ext2 filesystem is inspired from UFS.



A UFS filesystem 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. The first several inodes are reserved for historical reasons, followed by the inode for the root directory.

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.

History and evolution

Early versions of unix used a filesystem they 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 got larger, moving the head back and forth between the clump of inodes and the data blocks they referred to caused thrashing. BSD optimized this in FFS by inventing cylinder groups, breaking the disk up into smaller chunks, each with its own inode clump 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 near by 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 off of alternating tracks while waiting for the platter to spin around.

As disks grew larger and larger, sector level optimization became obsolete (especially with disks that used linear sector numbering and variable sectors per track), and fragmented reads became more of a problem. To combat this, BSD increased the filesystem block size from one sector to 8k. This has several effects. The chances of a file's sectors being contiguous is much greater. The amount of overhead to list the file's blocks is reduced. The number of blocks representable in a fixed bit width block number is increased (allowing for larger disks).

With larger block sizes, disks with many small files would waste a lot of space, so BSD added block level fragmentation, where the last partial block of data from several files may be stored in a single "fragment" block instead of multiple mostly empty blocks.


Most vendors adapted UFS to their own uses, adding proprietary extensions that may not be recognized by other vendors' versions of unix. Surprisingly, many have continued to use the original block size and data field widths as the original UFS, so some degree of (read) compatibility remains across platforms.

FreeBSD 5.0 introduced UFS2, which adds support for volumes over 1TB in size. It has since been ported to NetBSD.

The Linux ext2 file system was rewritten from scratch based on concepts from UFS. ext2 allows block sizes to be set at filesystem-creation time to allow for larger disks. It also has 64 bit data fields in the inode to allow for larger files. Ext3 adds journaling. ReiserFS was the first journaling file system to be included in the Linux kernel, and is faster when dealing with smaller files. Linux includes a UFS implementation for binary compatibility at the read level with other unices, but since there is no standard implementation for the vendor extensions to UFS, Linux does not have full support for writing to UFS.

As of Solaris 7, Sun Microsystems included UFS Logging in the Solaris Operating Environment, which brought filesystem journaling to UFS. Solaris UFS also has extensions for large files and large disks and other features.


  • Little UFS2 FAQ (http://lists.freebsd.org/pipermail/freebsd-current/2003-April/001444.html)
  • Unix System V documentation
  • Man pages from multiple flavors of unix
  • /usr/include/sys/fs/ufs_inode.h and other include files from multiple flavors of unix

See also

External links

  • McKusick, 2002. Running "fsck" in the Background html (http://www.usenix.org/publications/library/proceedings/bsdcon02/mckusick/mckusick_html/index.html) pdf file (http://www.usenix.org/publications/library/proceedings/bsdcon02/mckusick/mckusick.pdf)de:Unix File System

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