EROFS

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

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 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