EROFS

EROFS

Developer(s)	Huawei originally, Alibaba Cloud, Bytedance, Coolpad, Google, OPPO, Red Hat, Shanghai Jiao Tong University, VIVO, Uniontech, South China University of Technology
Full name	Enhanced Read-Only File System
Introduced	November 24, 2019 (2019-11-24) with Linux 5.4
Limits
Max volume size	1 EiB since 6.15
Max file size	4 GiB (compact) 16 EiB (extended, also limited by volume size) [1]
Max no. of files	Depends on volume size
Max filename length	255 bytes
Features
Dates recorded	File change time (extended only) [1]
Date resolution	1 ns
Attributes	POSIX, Extended file attributes
File system permissions	POSIX, ACL
Transparent compression	Yes (LZ4; LZMA since 5.16; DEFLATE since 6.6) [1]
Data deduplication	Yes (extent-based)
Other
Supported operating systems	Linux

EROFS (Enhanced Read-Only File System) is a lightweight read-only file system initially developed by Huawei, originally for the Linux kernel and now maintained by an open-source community.

EROFS aims to form a generic read-only file system solution for various read-only use cases (embedded devices, containers and more) instead of just focusing on storage space saving without considering any side effects of runtime performance. ^[1]

For example, it provides a solution to save storage space by using transparent compression as an option for scenarios that need high-performance read-only requirements on their devices with limited hardware resources, e.g. smartphones like Android and IoT operating systems such as HarmonyOS alongside its HarmonyOS NEXT core system iteration. ^{[2] [3]} All of Huawei's new products shipped with EMUI 9.0.1 or later used EROFS, ^[4] and it was promoted as one of the key features of EMUI 9.1. ^[5] Oppo, Xiaomi and some Samsung products also use EROFS. ^{[6] [7]}

Also, it provides a content-addressable chunk-based container image solution together with lazy pulling feature to accelerate container startup speed by using new file-based fscache backend since Linux kernel v5.19 and file-backed mounts since v6.12. ^{[8] [9]}

The file system was formally merged into the mainline kernel with Linux kernel v5.4. ^[10]

Features

The file system has two different inode on-disk layouts. One is compact, and the other is extended. ^[1]

• Little-endian on-disk design ^[1]
• 48-bit block addressing, which currently limits the total possible capacity of an EROFS filesystem to 1 EiB of 4 KiB block size.
• Metadata and data can be mixed by design for on-disk flexibility together with tail-packing inline data technology ^[1]
• Support POSIX attributes and permissions, extended file attributes and ACL ^[1]
• Fixed-size output transparent compression with LZ4, MicroLZMA (since Linux 5.16 ^[11] ), zstd and/or DEFLATE (since Linux 6.6 ^[12] ) for relative higher compression ratios ^[1]
• In-place decompression for higher sequential read ^{[13] [14]}
• Big pcluster feature allowing up to 1 MiB big pclusters for better compression ratios since Linux 5.13. ^{[15] [16]}
• Direct I/O, Direct Access (DAX) support, chunk-based data deduplication for uncompressed files since Linux 5.15. ^{[1] [17]}
• Multiple device support for multiple layer container images since Linux 5.16. ^[1]
• Ztailpacking support since Linux 5.17. ^[18]
• File-based Fscache backend support since Linux 5.19 with "on-demand mode". ^{[19] [20]}
• Rolling hash deduplicated data compression and fragment support since Linux 6.1. ^[1]

See also

• cramfs
• ext4 - previously Huawei used this file system for its system partitions
• SquashFS

References

1. Gao Xiang. "erofs.html". www.kernel.org. Retrieved 2022-08-04.
2. Michael Larabel (2018-11-19). "There Is Finally A User-Space Utility To Make EROFS Linux File-Systems". Phoronix. Retrieved 2018-12-13.
3. Xiang, Gao (2018-05-31). "erofs: introduce erofs file system". Linux kernel (Mailing list). Retrieved 2018-12-13.
4. Xiang, Gao (2019-07-04). "erofs: promote erofs from staging". Linux kernel (Mailing list). Retrieved 2019-09-03.
5. "EMUI 9.1, the Intelligent Android OS". HUAWEI Global. Huawei Technologies. Retrieved 2022-01-10.
6. "Google's plan to use EROFS on Android". 20 May 2022. Retrieved 2022-08-04.
7. "The Galaxy S23 might be Samsung's first Android device to use EROFS". 2023-02-07. Retrieved 2023-07-22.
8. "The Evolution of the Nydus Image Acceleration". 6 June 2022. Retrieved 2022-08-04.
9. "The 6.12 merge window begins [LWN.net]". lwn.net. Retrieved 2025-04-13.
10. corbet (2019-11-25). "The 5.4 kernel has been released". LWN.net. Retrieved 2019-12-01.
11. Michael Larabel. "EROFS File-System Adding LZMA Compression Support Via New MicroLZMA". Phoronix. Retrieved 2022-01-10.
12. Michael Larabel. "EROFS File-System Adding DEFLATE Compression Support". Phoronix. Retrieved 2023-09-03.
13. "staging: erofs: introduce LZ4 decompression inplace" . Retrieved 2020-12-03.
14. "staging: erofs: integrate decompression inplace" . Retrieved 2020-12-03.
15. Michael Larabel. "EROFS Sends In "Big Pcluster" Support For Linux 5.13, Other Improvements". Phoronix. Retrieved 2021-07-11.
16. "erofs: add big pcluster compression support" . Retrieved 2021-07-11.
17. Michael Larabel. "XFS & EROFS File-Systems Have Big Changes For Linux 5.15". Phoronix. Retrieved 2021-11-02.
18. "EROFS-Utils 1.5 Released With ZTailPacking, FSCK Extraction" . Retrieved 2022-08-04.
19. Jonathan Corbet. "5.19 Merge window, part 1" . Retrieved 2022-08-04.
20. Marius Nestor (31 July 2022). "Linux Kernel 5.19 Officially Released, Linus Torvalds Teases Linux 6.0 as Next Kernel Series" . Retrieved 2022-08-04.

External links

• Official website
• EROFS Linux kernel repo - git.kernel.org
• erofs-utils - git.kernel.org
• ATC '19 - EROFS: A Compression-friendly Readonly File System for Resource-scarce Devices
• An introduction to EROFS - LWN.net
• EROFS, an Introduction and our Smartphone Practice on YouTube
• EROFS vs. SquashFS: A Gentle Benchmark - blog.sigma-star.at
• Finding the Best Block Filesystem for Your Embedded Linux System - Michael Opdenacker, Bootlin on YouTube