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erofs: update the overview of the documentation

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Update the overview section to better reflect EROFS's design philosophy
as an immutable image filesystem, update the feature highlights with
recent capabilities, and remove outdated items.

The following detailed sections will be revised later since the overview
section is the most visible part of our documentation. Outdated or
ambiguous information could mislead new users and potential adopters.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
This commit is contained in:
Gao Xiang
2026-06-10 11:05:32 +08:00
parent 1ccc75909c
commit b05fb89900
1 changed files with 62 additions and 59 deletions
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121
Documentation/filesystems/erofs.rst
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@@ -7,83 +7,90 @@ EROFS - Enhanced Read-Only File System
Overview
========
EROFS filesystem stands for Enhanced Read-Only File System. It aims to form a
generic read-only filesystem solution for various read-only use cases instead
of just focusing on storage space saving without considering any side effects
of runtime performance.
EROFS (Enhanced Read-Only File System) is a modern, efficient, and secure
read-only kernel filesystem designed for various use cases including immutable
system images, container images, application sandbox images, and dataset
distribution.
It is designed to meet the needs of flexibility, feature extendability and user
payload friendly, etc. Apart from those, it is still kept as a simple
random-access friendly high-performance filesystem to get rid of unneeded I/O
amplification and memory-resident overhead compared to similar approaches.
An immutable image filesystem can be regarded as an enhanced archive format
which allows golden images to be built once and mounted everywhere -- images are
bit-for-bit identical across all deployments and can be verified, audited, or
shared without concerns about runtime modifications (in this model, all user
writes should be redirected into another trusted filesystem, for example, via
overlayfs for copy-on-write-style redirection, by design).
It is implemented to be a better choice for the following scenarios:
EROFS is a dedicated implementation of the image filesystem idea above, with a
flexible, hierarchical on-disk design so that needed features can be enabled on
demand. Filesystem data in the core format is strictly block-aligned in order
to perform optimally on all kinds of storage media, including block devices and
memory-backed devices. The on-disk format is easy to parse and purposely avoids
the unnecessary metadata redundancy found in generic writable filesystems, which
can suffer from extra inconsistency issues -- making it ideal for security
auditing and untrusted remote access. In addition, designs such as inline data,
inline/shared extended attributes, and optimized (de)compression provide better
space efficiency while maintaining high performance.
- read-only storage media or
In short, EROFS aims to be a better fit for the following scenarios:
- part of a fully trusted read-only solution, which means it needs to be
- As part of a secure immutable storage solution, where it needs to be
immutable and bit-for-bit identical to the official golden image for
their releases due to security or other considerations and
each individual copy, in order to meet security, data sharing, and/or
other requirements;
- hope to minimize extra storage space with guaranteed end-to-end performance
by using compact layout, transparent file compression and direct access,
especially for those embedded devices with limited memory and high-density
hosts with numerous containers.
- Minimizing storage overhead with guaranteed end-to-end performance
by using compact (meta)data layout, optimized transparent data compression,
deduplication and direct access, especially for those embedded devices with
limited memory and high-density hosts with numerous containers.
Here are the main features of EROFS:
Here is the list of highlights:
- Little endian on-disk design;
- Little endian on-disk design with 48-bit block addressing, supporting up
to 1 EiB filesystem capacity with 4 KiB block size;
- Block-based distribution and file-based distribution over fscache are
supported;
- Two compact inode metadata layouts for space and performance efficiency:
- Support multiple devices to refer to external blobs, which can be used
for container images;
======================== ======== ======================================
compact extended
======================== ======== ======================================
Inode core metadata size 32 bytes 64 bytes
Max file size 4 GiB 16 EiB (also limited by max. vol size)
Max uids/gids 65536 4294967296
Nanosecond timestamps no yes
Max hardlinks 65536 4294967296
======================== ======== ======================================
- 32-bit block addresses for each device, therefore 16TiB address space at
most with 4KiB block size for now;
- Support tailpacking inline data for better space efficiency and reduce
unneeded I/O amplification;
- Two inode layouts for different requirements:
- Block-based and file-backed distribution are both supported;
===================== ============ ======================================
compact (v1) extended (v2)
===================== ============ ======================================
Inode metadata size 32 bytes 64 bytes
Max file size 4 GiB 16 EiB (also limited by max. vol size)
Max uids/gids 65536 4294967296
Per-inode timestamp no yes (64 + 32-bit timestamp)
Max hardlinks 65536 4294967296
Metadata reserved 8 bytes 18 bytes
===================== ============ ======================================
- Multiple devices to reference external data blobs: inode data can be
optionally placed into external blobs, which enables image layering and data
sharing among different filesystems;
- Support extended attributes as an option;
- Inline and shared extended attributes with an optional bloom filter that
speeds up negative extended attribute lookups;
- Support a bloom filter that speeds up negative extended attribute lookups;
- POSIX.1e ACLs by using extended attributes;
- Support POSIX.1e ACLs by using extended attributes;
- Transparent data compression as an option: Supported algorithms (LZ4,
MicroLZMA, DEFLATE and Zstandard) can be selected on a per-inode basis.
Both the on-disk metadata and decompression runtime have been heavily
optimized to minimize the overhead for better performance.
- Support transparent data compression as an option:
LZ4, MicroLZMA, DEFLATE and Zstandard algorithms can be used on a per-file
basis; In addition, inplace decompression is also supported to avoid bounce
compressed buffers and unnecessary page cache thrashing.
- Merging tail-end data into a special inode as fragments;
- Support chunk-based data deduplication and rolling-hash compressed data
deduplication;
- Chunk-based deduplication and rolling-hash compressed data deduplication;
- Support tailpacking inline compared to byte-addressed unaligned metadata
or smaller block size alternatives;
- Direct I/O and FSDAX support on uncompressed inodes for use cases such as
secure containers, loop devices, and ramdisks that do not need page caching;
- Support merging tail-end data into a special inode as fragments.
- Page cache sharing among inodes with identical content fingerprints on
the same machine.
- Support large folios to make use of THPs (Transparent Hugepages);
For more detailed information, please refer to our documentation site:
- Support direct I/O on uncompressed files to avoid double caching for loop
devices;
- Support FSDAX on uncompressed images for secure containers and ramdisks in
order to get rid of unnecessary page cache.
- Support file-based on-demand loading with the Fscache infrastructure.
- https://erofs.docs.kernel.org
The following git tree provides the file system user-space tools under
development, such as a formatting tool (mkfs.erofs), an on-disk consistency &
@@ -91,10 +98,6 @@ compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs):
- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
For more information, please also refer to the documentation site:
- https://erofs.docs.kernel.org
Bugs and patches are welcome, please kindly help us and send to the following
linux-erofs mailing list: