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erofs-utils =========== Userspace tools for EROFS filesystem, currently including: mkfs.erofs filesystem formatter erofsfuse FUSE daemon alternative dump.erofs filesystem analyzer fsck.erofs filesystem compatibility & consistency checker as well as extractor EROFS filesystem 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. Typically EROFS could be considered in the following use scenarios: - Firmwares in performance-sensitive systems, such as system partitions of Android smartphones; - Mountable immutable images such as container images for effective metadata & data access compared with tar, cpio or other local filesystems (e.g. ext4, XFS, btrfs, etc.) - FSDAX-enabled rootfs for secure containers (Linux 5.15+); - Live CDs which need a set of files with another high-performance algorithm to optimize startup time; others files for archival purposes only are not needed; - and more. Note that all EROFS metadata is uncompressed by design, so that you could take EROFS as a drop-in read-only replacement of ext4, XFS, btrfs, etc. without any compression-based dependencies and EROFS can bring more effective filesystem accesses to users with reduced metadata. For more details of EROFS filesystem itself, please refer to: https://www.kernel.org/doc/html/next/filesystems/erofs.html For more details on how to build erofs-utils, see `docs/INSTALL.md`. For more details about filesystem performance, see `docs/PERFORMANCE.md`. mkfs.erofs ---------- Two main kinds of EROFS images can be generated: (un)compressed images. - For uncompressed images, there will be no compressed files in these images. However, an EROFS image can contain files which consist of various aligned data blocks and then a tail that is stored inline in order to compact images [1]. - For compressed images, it will try to use the given algorithms first for each regular file and see if storage space can be saved with compression. If not, it will fall back to an uncompressed file. Note that EROFS supports per-file compression configuration, proper configuration options need to be enabled to parse compressed files by the Linux kernel. How to generate EROFS images ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Compression algorithms could be specified with the command-line option `-z` to build a compressed EROFS image from a local directory: $ mkfs.erofs -zlz4hc foo.erofs.img foo/ Supported algorithms by the Linux kernel: - LZ4 (Linux 5.3+); - LZMA (Linux 5.16+); - DEFLATE (Linux 6.6+); - Zstandard (Linux 6.10+). Alternatively, generate an uncompressed EROFS from a local directory: $ mkfs.erofs foo.erofs.img foo/ Additionally, you can specify a higher compression level to get a (slightly) smaller image than the default level: $ mkfs.erofs -zlz4hc,12 foo.erofs.img foo/ Multi-threaded support can be explicitly enabled with the ./configure option `--enable-multithreading`; otherwise, single-threaded compression will be used for now. It may take more time on multiprocessor platforms if multi-threaded support is not enabled. Currently, both `-Efragments` (not `-Eall-fragments`) and `-Ededupe` don't support multi-threading due to time limitations. Reproducible builds ~~~~~~~~~~~~~~~~~~~ Reproducible builds are typically used for verification and security, ensuring the same binaries/distributions to be reproduced in a deterministic way. Images generated by the same version of `mkfs.erofs` will be identical to previous runs if the same input is specified, and the same options are used. Specifically, variable timestamps and filesystem UUIDs can result in unreproducible EROFS images. `-T` and `-U` can be used to fix them. How to generate EROFS big pcluster images (Linux 5.13+) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ By default, EROFS formatter compresses data into separate one-block (e.g. 4KiB) filesystem physical clusters for outstanding random read performance. In other words, each EROFS filesystem block can be independently decompressed. However, other similar filesystems typically compress data into "blocks" of 128KiB or more for much smaller images. Users may prefer smaller images for archiving purposes, even if random performance is compromised with those configurations, and even worse when using 4KiB blocks. In order to fulfill users' needs, big plusters has been introduced since Linux 5.13, in which each physical clusters will be more than one blocks. Specifically, `-C` is used to specify the maximum size of each pcluster in bytes: $ mkfs.erofs -zlz4hc -C65536 foo.erofs.img foo/ Thus, in this case, pcluster sizes can be up to 64KiB. Note that large pcluster size can degrade random performance (though it may improve sequential read performance for typical storage devices), so please evaluate carefully in advance. Alternatively, you can make per-(sub)file compression strategies according to file access patterns if needed. How to generate EROFS images with multiple algorithms ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It's possible to generate an EROFS image with files in different algorithms due to various purposes. For example, LZMA for archival purposes and LZ4 for runtime purposes. In order to use alternative algorithms, just specify two or more compressing configurations together separated by ':' like below: -zlzma:lz4hc,12:lzma,9 -C32768 Although mkfs still choose the first one by default, you could try to write a compress-hints file like below: 4096 1 .*\.so$ 32768 2 .*\.txt$ 4096 sbin/.*$ 16384 0 .* and specify with `--compress-hints=` so that ".so" files will use "lz4hc,12" compression with 4k pclusters, ".txt" files will use "lzma,9" compression with 32k pclusters, files under "/sbin" will use the default "lzma" compression with 4k plusters and other files will use "lzma" compression with 16k pclusters. Note that the largest pcluster size should be specified with the "-C" option (here 32k pcluster size), otherwise all larger pclusters will be limited. How to generate well-compressed EROFS images ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Even if EROFS is not designed for such purposes in the beginning, it could still produce some smaller images (not always) compared to other approaches with better performance (see `docs/PERFORMANCE.md`). In order to build well-compressed EROFS images, try the following options: -C1048576 (5.13+) -Eztailpacking (5.16+) -Efragments / -Eall-fragments ( 6.1+); -Ededupe ( 6.1+). Also EROFS uses lz4hc level 9 by default, whereas some other approaches use lz4hc level 12 by default. So please explicitly specify `-zlz4hc,12 ` for comparison purposes. How to generate legacy EROFS images (Linux 4.19+) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Decompression inplace and compacted indexes have been introduced in Linux v5.3, which are not forward-compatible with older kernels. In order to generate _legacy_ EROFS images for old kernels, consider adding "-E legacy-compress" to the command line, e.g. $ mkfs.erofs -E legacy-compress -zlz4hc foo.erofs.img foo/ For Linux kernel >= 5.3, legacy EROFS images are _NOT recommended_ due to runtime performance loss compared with non-legacy images. Obsoleted erofs.mkfs ~~~~~~~~~~~~~~~~~~~~ There is an original erofs.mkfs version developed by Li Guifu, which was replaced by the new erofs-utils implementation. git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git -b obsoleted_mkfs PLEASE NOTE: This version is highly _NOT recommended_ now. erofsfuse --------- erofsfuse is introduced to support EROFS format for various platforms (including older linux kernels) and new on-disk features iteration. It can also be used as an unpacking tool for unprivileged users. It supports fixed-sized output decompression *without* any in-place I/O or in-place decompression optimization. Also like the other FUSE implementations, it suffers from most common performance issues (e.g. significant I/O overhead, double caching, etc.) Therefore, NEVER use it if performance is the top concern. How to mount an EROFS image with erofsfuse ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ As the other FUSE implementations, it's quite easy to mount by using erofsfuse, e.g.: $ erofsfuse foo.erofs.img foo/ Alternatively, to make it run in foreground (with debugging level 3): $ erofsfuse -f --dbglevel=3 foo.erofs.img foo/ To debug erofsfuse (also automatically run in foreground): $ erofsfuse -d foo.erofs.img foo/ To unmount an erofsfuse mountpoint as a non-root user: $ fusermount -u foo/ dump.erofs and fsck.erofs ------------------------- dump.erofs and fsck.erofs are used to analyze, check, and extract EROFS filesystems. Note that extended attributes and ACLs are still unsupported when extracting images with fsck.erofs. Note that fragment extraction with fsck.erofs could be slow now and it needs to be optimized later. If you are interested, contribution is, as always, welcome. Contribution ------------ erofs-utils is a part of EROFS filesystem project, which is completely community-driven open source software. If you have interest in EROFS, feel free to send feedback and/or patches to: linux-erofs mailing list <[email protected]> Comments -------- [1] According to the EROFS on-disk format, the tail blocks of files could be inlined aggressively with their metadata (called tail-packing) in order to minimize the extra I/Os and the storage space.
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