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 save some extra storage space with guaranteed end-to-end performance 23 by using reduced metadata and transparent file compression, especially 24 for those embedded devices with limited memory (ex, smartphone); 25 26Here is the main features of EROFS: 27 28 - Little endian on-disk design; 29 30 - Currently 4KB block size (nobh) and therefore maximum 16TB address space; 31 32 - Metadata & data could be mixed by design; 33 34 - 2 inode versions for different requirements: 35 36 ===================== ============ ===================================== 37 compact (v1) extended (v2) 38 ===================== ============ ===================================== 39 Inode metadata size 32 bytes 64 bytes 40 Max file size 4 GB 16 EB (also limited by max. vol size) 41 Max uids/gids 65536 4294967296 42 File change time no yes (64 + 32-bit timestamp) 43 Max hardlinks 65536 4294967296 44 Metadata reserved 4 bytes 14 bytes 45 ===================== ============ ===================================== 46 47 - Support extended attributes (xattrs) as an option; 48 49 - Support xattr inline and tail-end data inline for all files; 50 51 - Support POSIX.1e ACLs by using xattrs; 52 53 - Support transparent file compression as an option: 54 LZ4 algorithm with 4 KB fixed-sized output compression for high performance. 55 56The following git tree provides the file system user-space tools under 57development (ex, formatting tool mkfs.erofs): 58 59- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git 60 61Bugs and patches are welcome, please kindly help us and send to the following 62linux-erofs mailing list: 63 64- linux-erofs mailing list <linux-erofs@lists.ozlabs.org> 65 66Mount options 67============= 68 69=================== ========================================================= 70(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled 71 by default if CONFIG_EROFS_FS_XATTR is selected. 72(no)acl Setup POSIX Access Control List. Note: acl is enabled 73 by default if CONFIG_EROFS_FS_POSIX_ACL is selected. 74cache_strategy=%s Select a strategy for cached decompression from now on: 75 76 ========== ============================================= 77 disabled In-place I/O decompression only; 78 readahead Cache the last incomplete compressed physical 79 cluster for further reading. It still does 80 in-place I/O decompression for the rest 81 compressed physical clusters; 82 readaround Cache the both ends of incomplete compressed 83 physical clusters for further reading. 84 It still does in-place I/O decompression 85 for the rest compressed physical clusters. 86 ========== ============================================= 87dax={always,never} Use direct access (no page cache). See 88 Documentation/filesystems/dax.rst. 89dax A legacy option which is an alias for ``dax=always``. 90=================== ========================================================= 91 92On-disk details 93=============== 94 95Summary 96------- 97Different from other read-only file systems, an EROFS volume is designed 98to be as simple as possible:: 99 100 |-> aligned with the block size 101 ____________________________________________________________ 102 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | 103 |_|__|_|_____|__________|_____|______|__________|_____|______| 104 0 +1K 105 106All data areas should be aligned with the block size, but metadata areas 107may not. All metadatas can be now observed in two different spaces (views): 108 109 1. Inode metadata space 110 111 Each valid inode should be aligned with an inode slot, which is a fixed 112 value (32 bytes) and designed to be kept in line with compact inode size. 113 114 Each inode can be directly found with the following formula: 115 inode offset = meta_blkaddr * block_size + 32 * nid 116 117 :: 118 119 |-> aligned with 8B 120 |-> followed closely 121 + meta_blkaddr blocks |-> another slot 122 _____________________________________________________________________ 123 | ... | inode | xattrs | extents | data inline | ... | inode ... 124 |________|_______|(optional)|(optional)|__(optional)_|_____|__________ 125 |-> aligned with the inode slot size 126 . . 127 . . 128 . . 129 . . 130 . . 131 . . 132 .____________________________________________________|-> aligned with 4B 133 | xattr_ibody_header | shared xattrs | inline xattrs | 134 |____________________|_______________|_______________| 135 |-> 12 bytes <-|->x * 4 bytes<-| . 136 . . . 137 . . . 138 . . . 139 ._______________________________.______________________. 140 | id | id | id | id | ... | id | ent | ... | ent| ... | 141 |____|____|____|____|______|____|_____|_____|____|_____| 142 |-> aligned with 4B 143 |-> aligned with 4B 144 145 Inode could be 32 or 64 bytes, which can be distinguished from a common 146 field which all inode versions have -- i_format:: 147 148 __________________ __________________ 149 | i_format | | i_format | 150 |__________________| |__________________| 151 | ... | | ... | 152 | | | | 153 |__________________| 32 bytes | | 154 | | 155 |__________________| 64 bytes 156 157 Xattrs, extents, data inline are followed by the corresponding inode with 158 proper alignment, and they could be optional for different data mappings. 159 _currently_ total 4 valid data mappings are supported: 160 161 == ==================================================================== 162 0 flat file data without data inline (no extent); 163 1 fixed-sized output data compression (with non-compacted indexes); 164 2 flat file data with tail packing data inline (no extent); 165 3 fixed-sized output data compression (with compacted indexes, v5.3+). 166 == ==================================================================== 167 168 The size of the optional xattrs is indicated by i_xattr_count in inode 169 header. Large xattrs or xattrs shared by many different files can be 170 stored in shared xattrs metadata rather than inlined right after inode. 171 172 2. Shared xattrs metadata space 173 174 Shared xattrs space is similar to the above inode space, started with 175 a specific block indicated by xattr_blkaddr, organized one by one with 176 proper align. 177 178 Each share xattr can also be directly found by the following formula: 179 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id 180 181 :: 182 183 |-> aligned by 4 bytes 184 + xattr_blkaddr blocks |-> aligned with 4 bytes 185 _________________________________________________________________________ 186 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... 187 |________|_____________|_____________|_____|______________|_______________ 188 189Directories 190----------- 191All directories are now organized in a compact on-disk format. Note that 192each directory block is divided into index and name areas in order to support 193random file lookup, and all directory entries are _strictly_ recorded in 194alphabetical order in order to support improved prefix binary search 195algorithm (could refer to the related source code). 196 197:: 198 199 ___________________________ 200 / | 201 / ______________|________________ 202 / / | nameoff1 | nameoffN-1 203 ____________.______________._______________v________________v__________ 204 | dirent | dirent | ... | dirent | filename | filename | ... | filename | 205 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| 206 \ ^ 207 \ | * could have 208 \ | trailing '\0' 209 \________________________| nameoff0 210 211 Directory block 212 213Note that apart from the offset of the first filename, nameoff0 also indicates 214the total number of directory entries in this block since it is no need to 215introduce another on-disk field at all. 216 217Compression 218----------- 219Currently, EROFS supports 4KB fixed-sized output transparent file compression, 220as illustrated below:: 221 222 |---- Variant-Length Extent ----|-------- VLE --------|----- VLE ----- 223 clusterofs clusterofs clusterofs 224 | | | logical data 225 _________v_______________________________v_____________________v_______________ 226 ... | . | | . | | . | ... 227 ____|____.________|_____________|________.____|_____________|__.__________|____ 228 |-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-| 229 size size size size size 230 . . . . 231 . . . . 232 . . . . 233 _______._____________._____________._____________._____________________ 234 ... | | | | ... physical data 235 _______|_____________|_____________|_____________|_____________________ 236 |-> cluster <-|-> cluster <-|-> cluster <-| 237 size size size 238 239Currently each on-disk physical cluster can contain 4KB (un)compressed data 240at most. For each logical cluster, there is a corresponding on-disk index to 241describe its cluster type, physical cluster address, etc. 242 243See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 244