1.. SPDX-License-Identifier: GPL-2.0 2 3====================================== 4EROFS - Enhanced Read-Only File System 5====================================== 6 7Overview 8======== 9 10EROFS filesystem stands for Enhanced Read-Only File System. It aims to form a 11generic read-only filesystem solution for various read-only use cases instead 12of just focusing on storage space saving without considering any side effects 13of runtime performance. 14 15It is designed to meet the needs of flexibility, feature extendability and user 16payload friendly, etc. Apart from those, it is still kept as a simple 17random-access friendly high-performance filesystem to get rid of unneeded I/O 18amplification and memory-resident overhead compared to similar approaches. 19 20It is implemented to be a better choice for the following scenarios: 21 22 - read-only storage media or 23 24 - part of a fully trusted read-only solution, which means it needs to be 25 immutable and bit-for-bit identical to the official golden image for 26 their releases due to security or other considerations and 27 28 - hope to minimize extra storage space with guaranteed end-to-end performance 29 by using compact layout, transparent file compression and direct access, 30 especially for those embedded devices with limited memory and high-density 31 hosts with numerous containers. 32 33Here are the main features of EROFS: 34 35 - Little endian on-disk design; 36 37 - Block-based distribution and file-based distribution over fscache are 38 supported; 39 40 - Support multiple devices to refer to external blobs, which can be used 41 for container images; 42 43 - 4KiB block size and 32-bit block addresses for each device, therefore 44 16TiB address space at most for now; 45 46 - Two inode layouts for different requirements: 47 48 ===================== ============ ====================================== 49 compact (v1) extended (v2) 50 ===================== ============ ====================================== 51 Inode metadata size 32 bytes 64 bytes 52 Max file size 4 GiB 16 EiB (also limited by max. vol size) 53 Max uids/gids 65536 4294967296 54 Per-inode timestamp no yes (64 + 32-bit timestamp) 55 Max hardlinks 65536 4294967296 56 Metadata reserved 8 bytes 18 bytes 57 ===================== ============ ====================================== 58 59 - Support extended attributes as an option; 60 61 - Support POSIX.1e ACLs by using extended attributes; 62 63 - Support transparent data compression as an option: 64 LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition, 65 inplace decompression is also supported to avoid bounce compressed buffers 66 and page cache thrashing. 67 68 - Support chunk-based data deduplication and rolling-hash compressed data 69 deduplication; 70 71 - Support tailpacking inline compared to byte-addressed unaligned metadata 72 or smaller block size alternatives; 73 74 - Support merging tail-end data into a special inode as fragments. 75 76 - Support large folios for uncompressed files. 77 78 - Support direct I/O on uncompressed files to avoid double caching for loop 79 devices; 80 81 - Support FSDAX on uncompressed images for secure containers and ramdisks in 82 order to get rid of unnecessary page cache. 83 84 - Support file-based on-demand loading with the Fscache infrastructure. 85 86The following git tree provides the file system user-space tools under 87development, such as a formatting tool (mkfs.erofs), an on-disk consistency & 88compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs): 89 90- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git 91 92Bugs and patches are welcome, please kindly help us and send to the following 93linux-erofs mailing list: 94 95- linux-erofs mailing list <linux-erofs@lists.ozlabs.org> 96 97Mount options 98============= 99 100=================== ========================================================= 101(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled 102 by default if CONFIG_EROFS_FS_XATTR is selected. 103(no)acl Setup POSIX Access Control List. Note: acl is enabled 104 by default if CONFIG_EROFS_FS_POSIX_ACL is selected. 105cache_strategy=%s Select a strategy for cached decompression from now on: 106 107 ========== ============================================= 108 disabled In-place I/O decompression only; 109 readahead Cache the last incomplete compressed physical 110 cluster for further reading. It still does 111 in-place I/O decompression for the rest 112 compressed physical clusters; 113 readaround Cache the both ends of incomplete compressed 114 physical clusters for further reading. 115 It still does in-place I/O decompression 116 for the rest compressed physical clusters. 117 ========== ============================================= 118dax={always,never} Use direct access (no page cache). See 119 Documentation/filesystems/dax.rst. 120dax A legacy option which is an alias for ``dax=always``. 121device=%s Specify a path to an extra device to be used together. 122fsid=%s Specify a filesystem image ID for Fscache back-end. 123domain_id=%s Specify a domain ID in fscache mode so that different images 124 with the same blobs under a given domain ID can share storage. 125=================== ========================================================= 126 127Sysfs Entries 128============= 129 130Information about mounted erofs file systems can be found in /sys/fs/erofs. 131Each mounted filesystem will have a directory in /sys/fs/erofs based on its 132device name (i.e., /sys/fs/erofs/sda). 133(see also Documentation/ABI/testing/sysfs-fs-erofs) 134 135On-disk details 136=============== 137 138Summary 139------- 140Different from other read-only file systems, an EROFS volume is designed 141to be as simple as possible:: 142 143 |-> aligned with the block size 144 ____________________________________________________________ 145 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | 146 |_|__|_|_____|__________|_____|______|__________|_____|______| 147 0 +1K 148 149All data areas should be aligned with the block size, but metadata areas 150may not. All metadatas can be now observed in two different spaces (views): 151 152 1. Inode metadata space 153 154 Each valid inode should be aligned with an inode slot, which is a fixed 155 value (32 bytes) and designed to be kept in line with compact inode size. 156 157 Each inode can be directly found with the following formula: 158 inode offset = meta_blkaddr * block_size + 32 * nid 159 160 :: 161 162 |-> aligned with 8B 163 |-> followed closely 164 + meta_blkaddr blocks |-> another slot 165 _____________________________________________________________________ 166 | ... | inode | xattrs | extents | data inline | ... | inode ... 167 |________|_______|(optional)|(optional)|__(optional)_|_____|__________ 168 |-> aligned with the inode slot size 169 . . 170 . . 171 . . 172 . . 173 . . 174 . . 175 .____________________________________________________|-> aligned with 4B 176 | xattr_ibody_header | shared xattrs | inline xattrs | 177 |____________________|_______________|_______________| 178 |-> 12 bytes <-|->x * 4 bytes<-| . 179 . . . 180 . . . 181 . . . 182 ._______________________________.______________________. 183 | id | id | id | id | ... | id | ent | ... | ent| ... | 184 |____|____|____|____|______|____|_____|_____|____|_____| 185 |-> aligned with 4B 186 |-> aligned with 4B 187 188 Inode could be 32 or 64 bytes, which can be distinguished from a common 189 field which all inode versions have -- i_format:: 190 191 __________________ __________________ 192 | i_format | | i_format | 193 |__________________| |__________________| 194 | ... | | ... | 195 | | | | 196 |__________________| 32 bytes | | 197 | | 198 |__________________| 64 bytes 199 200 Xattrs, extents, data inline are followed by the corresponding inode with 201 proper alignment, and they could be optional for different data mappings. 202 _currently_ total 5 data layouts are supported: 203 204 == ==================================================================== 205 0 flat file data without data inline (no extent); 206 1 fixed-sized output data compression (with non-compacted indexes); 207 2 flat file data with tail packing data inline (no extent); 208 3 fixed-sized output data compression (with compacted indexes, v5.3+); 209 4 chunk-based file (v5.15+). 210 == ==================================================================== 211 212 The size of the optional xattrs is indicated by i_xattr_count in inode 213 header. Large xattrs or xattrs shared by many different files can be 214 stored in shared xattrs metadata rather than inlined right after inode. 215 216 2. Shared xattrs metadata space 217 218 Shared xattrs space is similar to the above inode space, started with 219 a specific block indicated by xattr_blkaddr, organized one by one with 220 proper align. 221 222 Each share xattr can also be directly found by the following formula: 223 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id 224 225:: 226 227 |-> aligned by 4 bytes 228 + xattr_blkaddr blocks |-> aligned with 4 bytes 229 _________________________________________________________________________ 230 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... 231 |________|_____________|_____________|_____|______________|_______________ 232 233Directories 234----------- 235All directories are now organized in a compact on-disk format. Note that 236each directory block is divided into index and name areas in order to support 237random file lookup, and all directory entries are _strictly_ recorded in 238alphabetical order in order to support improved prefix binary search 239algorithm (could refer to the related source code). 240 241:: 242 243 ___________________________ 244 / | 245 / ______________|________________ 246 / / | nameoff1 | nameoffN-1 247 ____________.______________._______________v________________v__________ 248 | dirent | dirent | ... | dirent | filename | filename | ... | filename | 249 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| 250 \ ^ 251 \ | * could have 252 \ | trailing '\0' 253 \________________________| nameoff0 254 Directory block 255 256Note that apart from the offset of the first filename, nameoff0 also indicates 257the total number of directory entries in this block since it is no need to 258introduce another on-disk field at all. 259 260Chunk-based files 261----------------- 262In order to support chunk-based data deduplication, a new inode data layout has 263been supported since Linux v5.15: Files are split in equal-sized data chunks 264with ``extents`` area of the inode metadata indicating how to get the chunk 265data: these can be simply as a 4-byte block address array or in the 8-byte 266chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more 267details.) 268 269By the way, chunk-based files are all uncompressed for now. 270 271Data compression 272---------------- 273EROFS implements fixed-sized output compression which generates fixed-sized 274compressed data blocks from variable-sized input in contrast to other existing 275fixed-sized input solutions. Relatively higher compression ratios can be gotten 276by using fixed-sized output compression since nowadays popular data compression 277algorithms are mostly LZ77-based and such fixed-sized output approach can be 278benefited from the historical dictionary (aka. sliding window). 279 280In details, original (uncompressed) data is turned into several variable-sized 281extents and in the meanwhile, compressed into physical clusters (pclusters). 282In order to record each variable-sized extent, logical clusters (lclusters) are 283introduced as the basic unit of compress indexes to indicate whether a new 284extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now 285fixed in block size, as illustrated below:: 286 287 |<- variable-sized extent ->|<- VLE ->| 288 clusterofs clusterofs clusterofs 289 | | | 290 _________v_________________________________v_______________________v________ 291 ... | . | | . | | . ... 292 ____|____._________|______________|________.___ _|______________|__.________ 293 |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| 294 (HEAD) (NONHEAD) (HEAD) (NONHEAD) . 295 . CBLKCNT . . 296 . . . 297 . . . 298 _______._____________________________.______________._________________ 299 ... | | | | ... 300 _______|______________|______________|______________|_________________ 301 |-> big pcluster <-|-> pcluster <-| 302 303A physical cluster can be seen as a container of physical compressed blocks 304which contains compressed data. Previously, only lcluster-sized (4KB) pclusters 305were supported. After big pcluster feature is introduced (available since 306Linux v5.13), pcluster can be a multiple of lcluster size. 307 308For each HEAD lcluster, clusterofs is recorded to indicate where a new extent 309starts and blkaddr is used to seek the compressed data. For each NONHEAD 310lcluster, delta0 and delta1 are available instead of blkaddr to indicate the 311distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is 312also a HEAD lcluster except that its data is uncompressed. See the comments 313around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 314 315If big pcluster is enabled, pcluster size in lclusters needs to be recorded as 316well. Let the delta0 of the first NONHEAD lcluster store the compressed block 317count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy 318to understand its delta0 is constantly 1, as illustrated below:: 319 320 __________________________________________________________ 321 | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | 322 |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| 323 |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| 324 a lcluster-sized pcluster (without CBLKCNT) ^ 325 326If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, 327but it's easy to know the size of such pcluster is 1 lcluster as well. 328 329Since Linux v6.1, each pcluster can be used for multiple variable-sized extents, 330therefore it can be used for compressed data deduplication. 331