27.1 Overview of the UFS File System

UFS (UNIX File System) is the native file system of FreeBSD, based on the Berkeley Fast File System (FFS), first introduced by Kirk McKusick, Bill Joy, and others in 1983 with 4.2BSD. The current version is UFS2. Historically, macOS also used this file system as its root file system. This chapter covers five types of operations: adding disks, expanding disks, creating and mounting snapshots, managing quotas, and encrypting disks.

The UFS file system discussed in this section must be distinguished from the UFS storage used in mobile phones and other devices, as they belong to entirely different technical domains. The latter stands for Universal Flash Storage, a hardware storage standard that has evolved to version 5.0. FreeBSD has provided basic MMC/SD support through the mmc/mmcsd driver since 7.x, achieved complete eMMC support in FreeBSD 10.4 (including partitioning, TRIM, and HS400 bus speed mode), and added UFS storage support via the ufshci driver in FreeBSD 15.0.

As a file system, UFS is currently at version 2. Android systems based on Linux do not support the UFS file system; such devices typically use ext4 as their root file system, with some newer devices adopting F2FS, while Linux's read-write support for UFS remains incomplete.

References

• Jaeyoon Choi. Universal Flash Storage on FreeBSD[EB/OL]. [2026-04-16]. https://freebsdfoundation.org/our-work/journal/browser-based-edition/freebsd-15-0/universal-flash-storage-on-freebsd/. This article introduces the development process and current status of the FreeBSD UFS driver.
• McKusick M K. 4.4BSD Operating System Design and Implementation[M]. Li Shanping, Liu Wenfeng, Ma Tianchi, trans. Beijing: China Machine Press, 2012. Chapter 7 of this book describes the design and history of FFS in detail.
• McKusick M K, Joy W N, Leffler S J, et al. A Fast File System for UNIX[J]. ACM Transactions on Computer Systems, 1984, 2(3): 181-197. The original FFS paper, which first systematically described the design and implementation of the Berkeley Fast File System.