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 data compression as an option: 54 LZ4 algorithm with the 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 Directory block 208 209Note that apart from the offset of the first filename, nameoff0 also indicates 210the total number of directory entries in this block since it is no need to 211introduce another on-disk field at all. 212 213Data compression 214---------------- 215EROFS implements LZ4 fixed-sized output compression which generates fixed-sized 216compressed data blocks from variable-sized input in contrast to other existing 217fixed-sized input solutions. Relatively higher compression ratios can be gotten 218by using fixed-sized output compression since nowadays popular data compression 219algorithms are mostly LZ77-based and such fixed-sized output approach can be 220benefited from the historical dictionary (aka. sliding window). 221 222In details, original (uncompressed) data is turned into several variable-sized 223extents and in the meanwhile, compressed into physical clusters (pclusters). 224In order to record each variable-sized extent, logical clusters (lclusters) are 225introduced as the basic unit of compress indexes to indicate whether a new 226extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now 227fixed in block size, as illustrated below:: 228 229 |<- variable-sized extent ->|<- VLE ->| 230 clusterofs clusterofs clusterofs 231 | | | 232 _________v_________________________________v_______________________v________ 233 ... | . | | . | | . ... 234 ____|____._________|______________|________.___ _|______________|__.________ 235 |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| 236 (HEAD) (NONHEAD) (HEAD) (NONHEAD) . 237 . CBLKCNT . . 238 . . . 239 . . . 240 _______._____________________________.______________._________________ 241 ... | | | | ... 242 _______|______________|______________|______________|_________________ 243 |-> big pcluster <-|-> pcluster <-| 244 245A physical cluster can be seen as a container of physical compressed blocks 246which contains compressed data. Previously, only lcluster-sized (4KB) pclusters 247were supported. After big pcluster feature is introduced (available since 248Linux v5.13), pcluster can be a multiple of lcluster size. 249 250For each HEAD lcluster, clusterofs is recorded to indicate where a new extent 251starts and blkaddr is used to seek the compressed data. For each NONHEAD 252lcluster, delta0 and delta1 are available instead of blkaddr to indicate the 253distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is 254also a HEAD lcluster except that its data is uncompressed. See the comments 255around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 256 257If big pcluster is enabled, pcluster size in lclusters needs to be recorded as 258well. Let the delta0 of the first NONHEAD lcluster store the compressed block 259count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy 260to understand its delta0 is constantly 1, as illustrated below:: 261 262 __________________________________________________________ 263 | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | 264 |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| 265 |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| 266 a lcluster-sized pcluster (without CBLKCNT) ^ 267 268If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, 269but it's easy to know the size of such pcluster is 1 lcluster as well. 270