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 ========== ============================================= 87=================== ========================================================= 88 89On-disk details 90=============== 91 92Summary 93------- 94Different from other read-only file systems, an EROFS volume is designed 95to be as simple as possible:: 96 97 |-> aligned with the block size 98 ____________________________________________________________ 99 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | 100 |_|__|_|_____|__________|_____|______|__________|_____|______| 101 0 +1K 102 103All data areas should be aligned with the block size, but metadata areas 104may not. All metadatas can be now observed in two different spaces (views): 105 106 1. Inode metadata space 107 108 Each valid inode should be aligned with an inode slot, which is a fixed 109 value (32 bytes) and designed to be kept in line with compact inode size. 110 111 Each inode can be directly found with the following formula: 112 inode offset = meta_blkaddr * block_size + 32 * nid 113 114 :: 115 116 |-> aligned with 8B 117 |-> followed closely 118 + meta_blkaddr blocks |-> another slot 119 _____________________________________________________________________ 120 | ... | inode | xattrs | extents | data inline | ... | inode ... 121 |________|_______|(optional)|(optional)|__(optional)_|_____|__________ 122 |-> aligned with the inode slot size 123 . . 124 . . 125 . . 126 . . 127 . . 128 . . 129 .____________________________________________________|-> aligned with 4B 130 | xattr_ibody_header | shared xattrs | inline xattrs | 131 |____________________|_______________|_______________| 132 |-> 12 bytes <-|->x * 4 bytes<-| . 133 . . . 134 . . . 135 . . . 136 ._______________________________.______________________. 137 | id | id | id | id | ... | id | ent | ... | ent| ... | 138 |____|____|____|____|______|____|_____|_____|____|_____| 139 |-> aligned with 4B 140 |-> aligned with 4B 141 142 Inode could be 32 or 64 bytes, which can be distinguished from a common 143 field which all inode versions have -- i_format:: 144 145 __________________ __________________ 146 | i_format | | i_format | 147 |__________________| |__________________| 148 | ... | | ... | 149 | | | | 150 |__________________| 32 bytes | | 151 | | 152 |__________________| 64 bytes 153 154 Xattrs, extents, data inline are followed by the corresponding inode with 155 proper alignment, and they could be optional for different data mappings. 156 _currently_ total 4 valid data mappings are supported: 157 158 == ==================================================================== 159 0 flat file data without data inline (no extent); 160 1 fixed-sized output data compression (with non-compacted indexes); 161 2 flat file data with tail packing data inline (no extent); 162 3 fixed-sized output data compression (with compacted indexes, v5.3+). 163 == ==================================================================== 164 165 The size of the optional xattrs is indicated by i_xattr_count in inode 166 header. Large xattrs or xattrs shared by many different files can be 167 stored in shared xattrs metadata rather than inlined right after inode. 168 169 2. Shared xattrs metadata space 170 171 Shared xattrs space is similar to the above inode space, started with 172 a specific block indicated by xattr_blkaddr, organized one by one with 173 proper align. 174 175 Each share xattr can also be directly found by the following formula: 176 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id 177 178 :: 179 180 |-> aligned by 4 bytes 181 + xattr_blkaddr blocks |-> aligned with 4 bytes 182 _________________________________________________________________________ 183 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... 184 |________|_____________|_____________|_____|______________|_______________ 185 186Directories 187----------- 188All directories are now organized in a compact on-disk format. Note that 189each directory block is divided into index and name areas in order to support 190random file lookup, and all directory entries are _strictly_ recorded in 191alphabetical order in order to support improved prefix binary search 192algorithm (could refer to the related source code). 193 194:: 195 196 ___________________________ 197 / | 198 / ______________|________________ 199 / / | nameoff1 | nameoffN-1 200 ____________.______________._______________v________________v__________ 201 | dirent | dirent | ... | dirent | filename | filename | ... | filename | 202 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| 203 \ ^ 204 \ | * could have 205 \ | trailing '\0' 206 \________________________| nameoff0 207 208 Directory block 209 210Note that apart from the offset of the first filename, nameoff0 also indicates 211the total number of directory entries in this block since it is no need to 212introduce another on-disk field at all. 213 214Compression 215----------- 216Currently, EROFS supports 4KB fixed-sized output transparent file compression, 217as illustrated below:: 218 219 |---- Variant-Length Extent ----|-------- VLE --------|----- VLE ----- 220 clusterofs clusterofs clusterofs 221 | | | logical data 222 _________v_______________________________v_____________________v_______________ 223 ... | . | | . | | . | ... 224 ____|____.________|_____________|________.____|_____________|__.__________|____ 225 |-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-| 226 size size size size size 227 . . . . 228 . . . . 229 . . . . 230 _______._____________._____________._____________._____________________ 231 ... | | | | ... physical data 232 _______|_____________|_____________|_____________|_____________________ 233 |-> cluster <-|-> cluster <-|-> cluster <-| 234 size size size 235 236Currently each on-disk physical cluster can contain 4KB (un)compressed data 237at most. For each logical cluster, there is a corresponding on-disk index to 238describe its cluster type, physical cluster address, etc. 239 240See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 241