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|
.. SPDX-License-Identifier: GPL-2.0
======================================
EROFS - Enhanced Read-Only File System
======================================
Overview
========
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.
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).
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.
In short, EROFS aims to be a better fit for the following scenarios:
- 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
each individual copy, in order to meet security, data sharing, and/or
other requirements;
- 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 is the list of highlights:
- Little endian on-disk design with 48-bit block addressing, supporting up
to 1 EiB filesystem capacity with 4 KiB block size;
- Two compact inode metadata layouts for space and performance efficiency:
======================== ======== ======================================
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
======================== ======== ======================================
- Support tailpacking inline data for better space efficiency and reduce
unneeded I/O amplification;
- Block-based and file-backed distribution are both supported;
- 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;
- Inline and shared extended attributes with an optional bloom filter that
speeds up negative extended attribute lookups;
- 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.
- Merging tail-end data into a special inode as fragments;
- Chunk-based deduplication and rolling-hash compressed data deduplication;
- 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;
- Page cache sharing among inodes with identical content fingerprints on
the same machine.
For more detailed information, please refer to our documentation site:
- 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 &
compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs):
- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
Bugs and patches are welcome, please kindly help us and send to the following
linux-erofs mailing list:
- linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
Mount options
=============
=================== =========================================================
(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled
by default if CONFIG_EROFS_FS_XATTR is selected.
(no)acl Setup POSIX Access Control List. Note: acl is enabled
by default if CONFIG_EROFS_FS_POSIX_ACL is selected.
cache_strategy=%s Select a strategy for cached decompression from now on:
========== =============================================
disabled In-place I/O decompression only;
readahead Cache the last incomplete compressed physical
cluster for further reading. It still does
in-place I/O decompression for the rest
compressed physical clusters;
readaround Cache both ends of incomplete compressed
physical clusters for further reading.
It still does in-place I/O decompression
for the rest compressed physical clusters.
========== =============================================
dax={always,never} Use direct access (no page cache). See
Documentation/filesystems/dax.rst.
dax A legacy option which is an alias for ``dax=always``.
device=%s Specify a path to an extra device to be used together.
directio (For file-backed mounts) Use direct I/O to access backing
files, and asynchronous I/O will be enabled if supported.
domain_id=%s Specify a trusted domain ID. Filesystems sharing the same
domain ID can share page cache across mounts when inode
page sharing is enabled. (not shown in mountinfo output)
fsoffset=%llu Specify block-aligned filesystem offset for the primary device.
inode_share Enable inode page sharing for this filesystem. Inodes with
identical content within the same domain ID can share the
page cache.
=================== =========================================================
Sysfs Entries
=============
Information about mounted erofs file systems can be found in /sys/fs/erofs.
Each mounted filesystem will have a directory in /sys/fs/erofs based on its
device name (i.e., /sys/fs/erofs/sda).
(see also Documentation/ABI/testing/sysfs-fs-erofs)
On-disk details
===============
Summary
-------
Different from other read-only file systems, an EROFS volume is designed
to be as simple as possible::
|-> aligned with the block size
____________________________________________________________
| |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
|_|__|_|_____|__________|_____|______|__________|_____|______|
0 +1K
All data areas should be aligned with the block size, but metadata areas
may not. All metadata can be now observed in two different spaces (views):
1. Inode metadata space
Each valid inode should be aligned with an inode slot, which is a fixed
value (32 bytes) and designed to be kept in line with compact inode size.
Each inode can be directly found with the following formula:
inode offset = meta_blkaddr * block_size + 32 * nid
::
|-> aligned with 8B
|-> followed closely
+ meta_blkaddr blocks |-> another slot
_____________________________________________________________________
| ... | inode | xattrs | extents | data inline | ... | inode ...
|________|_______|(optional)|(optional)|__(optional)_|_____|__________
|-> aligned with the inode slot size
. .
. .
. .
. .
. .
. .
.____________________________________________________|-> aligned with 4B
| xattr_ibody_header | shared xattrs | inline xattrs |
|____________________|_______________|_______________|
|-> 12 bytes <-|->x * 4 bytes<-| .
. . .
. . .
. . .
._______________________________.______________________.
| id | id | id | id | ... | id | ent | ... | ent| ... |
|____|____|____|____|______|____|_____|_____|____|_____|
|-> aligned with 4B
|-> aligned with 4B
Inode could be 32 or 64 bytes, which can be distinguished from a common
field which all inode versions have -- i_format::
__________________ __________________
| i_format | | i_format |
|__________________| |__________________|
| ... | | ... |
| | | |
|__________________| 32 bytes | |
| |
|__________________| 64 bytes
Xattrs, extents, data inline are placed after the corresponding inode with
proper alignment, and they could be optional for different data mappings.
_currently_ total 5 data layouts are supported:
== ====================================================================
0 flat file data without data inline (no extent);
1 fixed-sized output data compression (with non-compacted indexes);
2 flat file data with tail packing data inline (no extent);
3 fixed-sized output data compression (with compacted indexes, v5.3+);
4 chunk-based file (v5.15+).
== ====================================================================
The size of the optional xattrs is indicated by i_xattr_count in inode
header. Large xattrs or xattrs shared by many different files can be
stored in shared xattrs metadata rather than inlined right after inode.
2. Shared xattrs metadata space
Shared xattrs space is similar to the above inode space, started with
a specific block indicated by xattr_blkaddr, organized one by one with
proper align.
Each share xattr can also be directly found by the following formula:
xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
::
|-> aligned by 4 bytes
+ xattr_blkaddr blocks |-> aligned with 4 bytes
_________________________________________________________________________
| ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ...
|________|_____________|_____________|_____|______________|_______________
Directories
-----------
All directories are now organized in a compact on-disk format. Note that
each directory block is divided into index and name areas in order to support
random file lookup, and all directory entries are _strictly_ recorded in
alphabetical order in order to support improved prefix binary search
algorithm (could refer to the related source code).
::
___________________________
/ |
/ ______________|________________
/ / | nameoff1 | nameoffN-1
____________.______________._______________v________________v__________
| dirent | dirent | ... | dirent | filename | filename | ... | filename |
|___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
\ ^
\ | * could have
\ | trailing '\0'
\________________________| nameoff0
Directory block
Note that apart from the offset of the first filename, nameoff0 also indicates
the total number of directory entries in this block since it is no need to
introduce another on-disk field at all.
Chunk-based files
-----------------
In order to support chunk-based data deduplication, a new inode data layout has
been supported since Linux v5.15: Files are split in equal-sized data chunks
with ``extents`` area of the inode metadata indicating how to get the chunk
data: these can be simply as a 4-byte block address array or in the 8-byte
chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more
details.)
By the way, chunk-based files are all uncompressed for now.
Long extended attribute name prefixes
-------------------------------------
There are use cases where extended attributes with different values can have
only a few common prefixes (such as overlayfs xattrs). The predefined prefixes
work inefficiently in both image size and runtime performance in such cases.
The long xattr name prefixes feature is introduced to address this issue. The
overall idea is that, apart from the existing predefined prefixes, the xattr
entry could also refer to user-specified long xattr name prefixes, e.g.
"trusted.overlay.".
When referring to a long xattr name prefix, the highest bit (bit 7) of
erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole
represent the index of the referred long name prefix among all long name
prefixes. Therefore, only the trailing part of the name apart from the long
xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if
the full xattr name matches exactly as its long xattr name prefix.
All long xattr prefixes are stored one by one in the packed inode as long as
the packed inode is valid, or in the meta inode otherwise. The
xattr_prefix_count (of the on-disk superblock) indicates the total number of
long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start
offset of long name prefixes in the packed/meta inode. Note that, long extended
attribute name prefixes are disabled if xattr_prefix_count is 0.
Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4),
where len represents the total size of the data part. The data part is actually
represented by 'struct erofs_xattr_long_prefix', where base_index represents the
index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for
"trusted.overlay." long name prefix, while the infix string keeps the string
after stripping the short prefix, e.g. "overlay." for the example above.
Data compression
----------------
EROFS implements fixed-sized output compression which generates fixed-sized
compressed data blocks from variable-sized input in contrast to other existing
fixed-sized input solutions. Relatively higher compression ratios can be gotten
by using fixed-sized output compression since nowadays popular data compression
algorithms are mostly LZ77-based and such fixed-sized output approach can be
benefited from the historical dictionary (aka. sliding window).
In details, original (uncompressed) data is turned into several variable-sized
extents and in the meanwhile, compressed into physical clusters (pclusters).
In order to record each variable-sized extent, logical clusters (lclusters) are
introduced as the basic unit of compress indexes to indicate whether a new
extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now
fixed in block size, as illustrated below::
|<- variable-sized extent ->|<- VLE ->|
clusterofs clusterofs clusterofs
| | |
_________v_________________________________v_______________________v________
... | . | | . | | . ...
____|____._________|______________|________.___ _|______________|__.________
|-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-|
(HEAD) (NONHEAD) (HEAD) (NONHEAD) .
. CBLKCNT . .
. . .
. . .
_______._____________________________.______________._________________
... | | | | ...
_______|______________|______________|______________|_________________
|-> big pcluster <-|-> pcluster <-|
A physical cluster can be seen as a container of physical compressed blocks
which contains compressed data. Previously, only lcluster-sized (4KB) pclusters
were supported. After big pcluster feature is introduced (available since
Linux v5.13), pcluster can be a multiple of lcluster size.
For each HEAD lcluster, clusterofs is recorded to indicate where a new extent
starts and blkaddr is used to seek the compressed data. For each NONHEAD
lcluster, delta0 and delta1 are available instead of blkaddr to indicate the
distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is
also a HEAD lcluster except that its data is uncompressed. See the comments
around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
If big pcluster is enabled, pcluster size in lclusters needs to be recorded as
well. Let the delta0 of the first NONHEAD lcluster store the compressed block
count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy
to understand its delta0 is constantly 1, as illustrated below::
__________________________________________________________
| HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD |
|__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_|
|<----- a big pcluster (with CBLKCNT) ------>|<-- -->|
a lcluster-sized pcluster (without CBLKCNT) ^
If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
but it's easy to know the size of such pcluster is 1 lcluster as well.
Since Linux v6.1, each pcluster can be used for multiple variable-sized extents,
therefore it can be used for compressed data deduplication.
|
