EROFS

EROFS (Enhanced Read-Only File System) is a lightweight read-only file system initially developed by Huawei for the Linux kernel.

EROFS
Developer(s)Huawei, Alibaba Cloud
Full nameEnhanced Read-Only File System
IntroducedNovember 24, 2019 (2019-11-24) with Linux 5.4
Limits
Max. file size
Max. number of filesDepends on volume size
Max. filename length255 bytes
Features
Dates recordedFile change time (extended only)[1]
Date resolution1 ns
AttributesPOSIX, xattr
File system permissionsPOSIX, ACL
Transparent compressionYes (LZ4; LZMA since 5.16)[1]
Other
Supported operating systemsLinux

EROFS aims to form a generic read-only filesystem 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 for scenarios which need high-performance read-only requirements on their devices with limited hardware resources, e.g. smartphones like Android and HarmonyOS.[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 and Xiaomi products also use EROFS.[6]

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.[7]

The file system was formally merged into the mainline kernel with Linux kernel v5.4.[8]

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]
  • 4 KB block size and 32-bit block address, which limits the total possible capacity of an EROFS filesystem to 16 TB.[1]
  • Metadata and data could be mixed by tail-packing inline data technology[1]
  • Support POSIX attributes and permissions, xattr and ACL[1]
  • Fixed-output transparent compression with LZ4 for relative higher compression ratios[1]
  • In-place decompression for higher sequential read [9][10]
  • Big pcluster feature allowing up to 1 MiB big pclusters for better compression ratios since Linux 5.13.[11][12]
  • Direct I/O, Direct Access (DAX) support, chunk-based data de-duplication for uncompressed files since Linux 5.15.[1][13]
  • Multiple device support for multiple layer container images since Linux 5.16.[1]
  • MicroLZMA algorithm support since Linux 5.16.[14]
  • Ztailpacking support since Linux 5.17.[15]
  • File-based Fscache backend support since Linux 5.19 with "on-demand mode".[16][17]

See also

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. 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 Evolution of the Nydus Image Acceleration". 6 June 2022. Retrieved 2022-08-04.
  8. corbet (2019-11-25). "The 5.4 kernel has been released". LWN.net. Retrieved 2019-12-01.
  9. "staging: erofs: introduce LZ4 decompression inplace". Retrieved 2020-12-03.
  10. "staging: erofs: integrate decompression inplace". Retrieved 2020-12-03.
  11. Michael Larabel. "EROFS Sends In "Big Pcluster" Support For Linux 5.13, Other Improvements". Phoronix. Phoronix. Retrieved 2021-07-11.
  12. "erofs: add big pcluster compression support". Retrieved 2021-07-11.
  13. Michael Larabel. "XFS & EROFS File-Systems Have Big Changes For Linux 5.15". Phoronix. Phoronix. Retrieved 2021-11-02.
  14. Michael Larabel. "EROFS File-System Adding LZMA Compression Support Via New MicroLZMA". Phoronix. Phoronix. Retrieved 2022-01-10.
  15. "EROFS-Utils 1.5 Released With ZTailPacking, FSCK Extraction". Retrieved 2022-08-04.
  16. Jonathan Corbet. "5.19 Merge window, part 1". Retrieved 2022-08-04.
  17. 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.
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