1.. SPDX-License-Identifier: GPL-2.0 2 3====================================== 4Enhanced Read-Only File System - EROFS 5====================================== 6 7Overview 8======== 9 10EROFS file-system stands for Enhanced Read-Only File System. Different 11from other read-only file systems, it aims to be designed for flexibility, 12scalability, but be kept simple and high performance. 13 14It is designed as a better filesystem solution for the following scenarios: 15 16 - read-only storage media or 17 18 - part of a fully trusted read-only solution, which means it needs to be 19 immutable and bit-for-bit identical to the official golden image for 20 their releases due to security and other considerations and 21 22 - hope to minimize extra storage space with guaranteed end-to-end performance 23 by using compact layout, transparent file compression and direct access, 24 especially for those embedded devices with limited memory and high-density 25 hosts with numerous containers; 26 27Here is the main features of EROFS: 28 29 - Little endian on-disk design; 30 31 - Currently 4KB block size (nobh) and therefore maximum 16TB address space; 32 33 - Metadata & data could be mixed by design; 34 35 - 2 inode versions for different requirements: 36 37 ===================== ============ ===================================== 38 compact (v1) extended (v2) 39 ===================== ============ ===================================== 40 Inode metadata size 32 bytes 64 bytes 41 Max file size 4 GB 16 EB (also limited by max. vol size) 42 Max uids/gids 65536 4294967296 43 Per-inode timestamp no yes (64 + 32-bit timestamp) 44 Max hardlinks 65536 4294967296 45 Metadata reserved 4 bytes 14 bytes 46 ===================== ============ ===================================== 47 48 - Support extended attributes (xattrs) as an option; 49 50 - Support xattr inline and tail-end data inline for all files; 51 52 - Support POSIX.1e ACLs by using xattrs; 53 54 - Support transparent data compression as an option: 55 LZ4 algorithm with the fixed-sized output compression for high performance; 56 57 - Multiple device support for multi-layer container images. 58 59The following git tree provides the file system user-space tools under 60development (ex, formatting tool mkfs.erofs): 61 62- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git 63 64Bugs and patches are welcome, please kindly help us and send to the following 65linux-erofs mailing list: 66 67- linux-erofs mailing list <linux-erofs@lists.ozlabs.org> 68 69Mount options 70============= 71 72=================== ========================================================= 73(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled 74 by default if CONFIG_EROFS_FS_XATTR is selected. 75(no)acl Setup POSIX Access Control List. Note: acl is enabled 76 by default if CONFIG_EROFS_FS_POSIX_ACL is selected. 77cache_strategy=%s Select a strategy for cached decompression from now on: 78 79 ========== ============================================= 80 disabled In-place I/O decompression only; 81 readahead Cache the last incomplete compressed physical 82 cluster for further reading. It still does 83 in-place I/O decompression for the rest 84 compressed physical clusters; 85 readaround Cache the both ends of incomplete compressed 86 physical clusters for further reading. 87 It still does in-place I/O decompression 88 for the rest compressed physical clusters. 89 ========== ============================================= 90dax={always,never} Use direct access (no page cache). See 91 Documentation/filesystems/dax.rst. 92dax A legacy option which is an alias for ``dax=always``. 93device=%s Specify a path to an extra device to be used together. 94=================== ========================================================= 95 96Sysfs Entries 97============= 98 99Information about mounted erofs file systems can be found in /sys/fs/erofs. 100Each mounted filesystem will have a directory in /sys/fs/erofs based on its 101device name (i.e., /sys/fs/erofs/sda). 102(see also Documentation/ABI/testing/sysfs-fs-erofs) 103 104On-disk details 105=============== 106 107Summary 108------- 109Different from other read-only file systems, an EROFS volume is designed 110to be as simple as possible:: 111 112 |-> aligned with the block size 113 ____________________________________________________________ 114 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | 115 |_|__|_|_____|__________|_____|______|__________|_____|______| 116 0 +1K 117 118All data areas should be aligned with the block size, but metadata areas 119may not. All metadatas can be now observed in two different spaces (views): 120 121 1. Inode metadata space 122 123 Each valid inode should be aligned with an inode slot, which is a fixed 124 value (32 bytes) and designed to be kept in line with compact inode size. 125 126 Each inode can be directly found with the following formula: 127 inode offset = meta_blkaddr * block_size + 32 * nid 128 129 :: 130 131 |-> aligned with 8B 132 |-> followed closely 133 + meta_blkaddr blocks |-> another slot 134 _____________________________________________________________________ 135 | ... | inode | xattrs | extents | data inline | ... | inode ... 136 |________|_______|(optional)|(optional)|__(optional)_|_____|__________ 137 |-> aligned with the inode slot size 138 . . 139 . . 140 . . 141 . . 142 . . 143 . . 144 .____________________________________________________|-> aligned with 4B 145 | xattr_ibody_header | shared xattrs | inline xattrs | 146 |____________________|_______________|_______________| 147 |-> 12 bytes <-|->x * 4 bytes<-| . 148 . . . 149 . . . 150 . . . 151 ._______________________________.______________________. 152 | id | id | id | id | ... | id | ent | ... | ent| ... | 153 |____|____|____|____|______|____|_____|_____|____|_____| 154 |-> aligned with 4B 155 |-> aligned with 4B 156 157 Inode could be 32 or 64 bytes, which can be distinguished from a common 158 field which all inode versions have -- i_format:: 159 160 __________________ __________________ 161 | i_format | | i_format | 162 |__________________| |__________________| 163 | ... | | ... | 164 | | | | 165 |__________________| 32 bytes | | 166 | | 167 |__________________| 64 bytes 168 169 Xattrs, extents, data inline are followed by the corresponding inode with 170 proper alignment, and they could be optional for different data mappings. 171 _currently_ total 5 data layouts are supported: 172 173 == ==================================================================== 174 0 flat file data without data inline (no extent); 175 1 fixed-sized output data compression (with non-compacted indexes); 176 2 flat file data with tail packing data inline (no extent); 177 3 fixed-sized output data compression (with compacted indexes, v5.3+); 178 4 chunk-based file (v5.15+). 179 == ==================================================================== 180 181 The size of the optional xattrs is indicated by i_xattr_count in inode 182 header. Large xattrs or xattrs shared by many different files can be 183 stored in shared xattrs metadata rather than inlined right after inode. 184 185 2. Shared xattrs metadata space 186 187 Shared xattrs space is similar to the above inode space, started with 188 a specific block indicated by xattr_blkaddr, organized one by one with 189 proper align. 190 191 Each share xattr can also be directly found by the following formula: 192 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id 193 194:: 195 196 |-> aligned by 4 bytes 197 + xattr_blkaddr blocks |-> aligned with 4 bytes 198 _________________________________________________________________________ 199 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... 200 |________|_____________|_____________|_____|______________|_______________ 201 202Directories 203----------- 204All directories are now organized in a compact on-disk format. Note that 205each directory block is divided into index and name areas in order to support 206random file lookup, and all directory entries are _strictly_ recorded in 207alphabetical order in order to support improved prefix binary search 208algorithm (could refer to the related source code). 209 210:: 211 212 ___________________________ 213 / | 214 / ______________|________________ 215 / / | nameoff1 | nameoffN-1 216 ____________.______________._______________v________________v__________ 217 | dirent | dirent | ... | dirent | filename | filename | ... | filename | 218 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| 219 \ ^ 220 \ | * could have 221 \ | trailing '\0' 222 \________________________| nameoff0 223 Directory block 224 225Note that apart from the offset of the first filename, nameoff0 also indicates 226the total number of directory entries in this block since it is no need to 227introduce another on-disk field at all. 228 229Chunk-based file 230---------------- 231In order to support chunk-based data deduplication, a new inode data layout has 232been supported since Linux v5.15: Files are split in equal-sized data chunks 233with ``extents`` area of the inode metadata indicating how to get the chunk 234data: these can be simply as a 4-byte block address array or in the 8-byte 235chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more 236details.) 237 238By the way, chunk-based files are all uncompressed for now. 239 240Data compression 241---------------- 242EROFS implements LZ4 fixed-sized output compression which generates fixed-sized 243compressed data blocks from variable-sized input in contrast to other existing 244fixed-sized input solutions. Relatively higher compression ratios can be gotten 245by using fixed-sized output compression since nowadays popular data compression 246algorithms are mostly LZ77-based and such fixed-sized output approach can be 247benefited from the historical dictionary (aka. sliding window). 248 249In details, original (uncompressed) data is turned into several variable-sized 250extents and in the meanwhile, compressed into physical clusters (pclusters). 251In order to record each variable-sized extent, logical clusters (lclusters) are 252introduced as the basic unit of compress indexes to indicate whether a new 253extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now 254fixed in block size, as illustrated below:: 255 256 |<- variable-sized extent ->|<- VLE ->| 257 clusterofs clusterofs clusterofs 258 | | | 259 _________v_________________________________v_______________________v________ 260 ... | . | | . | | . ... 261 ____|____._________|______________|________.___ _|______________|__.________ 262 |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| 263 (HEAD) (NONHEAD) (HEAD) (NONHEAD) . 264 . CBLKCNT . . 265 . . . 266 . . . 267 _______._____________________________.______________._________________ 268 ... | | | | ... 269 _______|______________|______________|______________|_________________ 270 |-> big pcluster <-|-> pcluster <-| 271 272A physical cluster can be seen as a container of physical compressed blocks 273which contains compressed data. Previously, only lcluster-sized (4KB) pclusters 274were supported. After big pcluster feature is introduced (available since 275Linux v5.13), pcluster can be a multiple of lcluster size. 276 277For each HEAD lcluster, clusterofs is recorded to indicate where a new extent 278starts and blkaddr is used to seek the compressed data. For each NONHEAD 279lcluster, delta0 and delta1 are available instead of blkaddr to indicate the 280distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is 281also a HEAD lcluster except that its data is uncompressed. See the comments 282around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 283 284If big pcluster is enabled, pcluster size in lclusters needs to be recorded as 285well. Let the delta0 of the first NONHEAD lcluster store the compressed block 286count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy 287to understand its delta0 is constantly 1, as illustrated below:: 288 289 __________________________________________________________ 290 | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | 291 |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| 292 |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| 293 a lcluster-sized pcluster (without CBLKCNT) ^ 294 295If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, 296but it's easy to know the size of such pcluster is 1 lcluster as well. 297