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Diffstat (limited to 'Documentation/filesystems/erofs.rst')
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diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst index 7119aa213be7..08194f194b94 100644 --- a/Documentation/filesystems/erofs.rst +++ b/Documentation/filesystems/erofs.rst @@ -1,66 +1,100 @@ .. SPDX-License-Identifier: GPL-2.0 ====================================== -Enhanced Read-Only File System - EROFS +EROFS - Enhanced Read-Only File System ====================================== Overview ======== -EROFS file-system stands for Enhanced Read-Only File System. Different -from other read-only file systems, it aims to be designed for flexibility, -scalability, but be kept simple and high performance. +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. -It is designed as a better filesystem solution for the following scenarios: +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. + +It is implemented to be a better choice for the following scenarios: - read-only storage media or - part of a fully trusted read-only solution, which means it needs to be immutable and bit-for-bit identical to the official golden image for - their releases due to security and other considerations and + their releases due to security or other considerations and - 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; + hosts with numerous containers. -Here is the main features of EROFS: +Here are the main features of EROFS: - Little endian on-disk design; - - Currently 4KB block size (nobh) and therefore maximum 16TB address space; + - Block-based distribution and file-based distribution over fscache are + supported; + + - Support multiple devices to refer to external blobs, which can be used + for container images; - - Metadata & data could be mixed by design; + - 32-bit block addresses for each device, therefore 16TiB address space at + most with 4KiB block size for now; - - 2 inode versions for different requirements: + - Two inode layouts for different requirements: - ===================== ============ ===================================== + ===================== ============ ====================================== compact (v1) extended (v2) - ===================== ============ ===================================== + ===================== ============ ====================================== Inode metadata size 32 bytes 64 bytes - Max file size 4 GB 16 EB (also limited by max. vol size) + Max file size 4 GiB 16 EiB (also limited by max. vol size) Max uids/gids 65536 4294967296 - File change time no yes (64 + 32-bit timestamp) + Per-inode timestamp no yes (64 + 32-bit timestamp) Max hardlinks 65536 4294967296 - Metadata reserved 4 bytes 14 bytes - ===================== ============ ===================================== + Metadata reserved 8 bytes 18 bytes + ===================== ============ ====================================== - - Support extended attributes (xattrs) as an option; + - Support extended attributes as an option; - - Support xattr inline and tail-end data inline for all files; + - Support a bloom filter that speeds up negative extended attribute lookups; - - Support POSIX.1e ACLs by using xattrs; + - Support POSIX.1e ACLs by using extended attributes; - Support transparent data compression as an option: - LZ4 algorithm with the fixed-sized output compression for high performance; + LZ4, MicroLZMA and DEFLATE 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. + + - Support chunk-based data deduplication and rolling-hash compressed data + deduplication; + + - Support tailpacking inline compared to byte-addressed unaligned metadata + or smaller block size alternatives; + + - Support merging tail-end data into a special inode as fragments. + + - Support large folios to make use of THPs (Transparent Hugepages); + + - Support direct I/O on uncompressed files to avoid double caching for loop + devices; - - Multiple device support for multi-layer container images. + - 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. The following git tree provides the file system user-space tools under -development (ex, formatting tool mkfs.erofs): +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 +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: @@ -82,7 +116,7 @@ cache_strategy=%s Select a strategy for cached decompression from now on: cluster for further reading. It still does in-place I/O decompression for the rest compressed physical clusters; - readaround Cache the both ends of incomplete compressed + 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. @@ -91,6 +125,10 @@ 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. +fsid=%s Specify a filesystem image ID for Fscache back-end. +domain_id=%s Specify a domain ID in fscache mode so that different images + with the same blobs under a given domain ID can share storage. +fsoffset=%llu Specify block-aligned filesystem offset for the primary device. =================== ========================================================= Sysfs Entries @@ -166,7 +204,7 @@ may not. All metadatas can be now observed in two different spaces (views): | | |__________________| 64 bytes - Xattrs, extents, data inline are followed by the corresponding inode with + 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: @@ -226,8 +264,8 @@ 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 file ----------------- +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 @@ -237,9 +275,41 @@ 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 LZ4 fixed-sized output compression which generates fixed-sized +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 @@ -294,3 +364,6 @@ to understand its delta0 is constantly 1, as illustrated below:: 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. |