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 - 32-bit block addresses for each device, therefore 16TiB address space at 44 most with 4KiB block size 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 a bloom filter that speeds up negative extended attribute lookups; 62 63 - Support POSIX.1e ACLs by using extended attributes; 64 65 - Support transparent data compression as an option: 66 LZ4, MicroLZMA and DEFLATE algorithms can be used on a per-file basis; In 67 addition, inplace decompression is also supported to avoid bounce compressed 68 buffers and unnecessary page cache thrashing. 69 70 - Support chunk-based data deduplication and rolling-hash compressed data 71 deduplication; 72 73 - Support tailpacking inline compared to byte-addressed unaligned metadata 74 or smaller block size alternatives; 75 76 - Support merging tail-end data into a special inode as fragments. 77 78 - Support large folios to make use of THPs (Transparent Hugepages); 79 80 - Support direct I/O on uncompressed files to avoid double caching for loop 81 devices; 82 83 - Support FSDAX on uncompressed images for secure containers and ramdisks in 84 order to get rid of unnecessary page cache. 85 86 - Support file-based on-demand loading with the Fscache infrastructure. 87 88The following git tree provides the file system user-space tools under 89development, such as a formatting tool (mkfs.erofs), an on-disk consistency & 90compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs): 91 92- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git 93 94For more information, please also refer to the documentation site: 95 96- https://erofs.docs.kernel.org 97 98Bugs and patches are welcome, please kindly help us and send to the following 99linux-erofs mailing list: 100 101- linux-erofs mailing list <linux-erofs@lists.ozlabs.org> 102 103Mount options 104============= 105 106=================== ========================================================= 107(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled 108 by default if CONFIG_EROFS_FS_XATTR is selected. 109(no)acl Setup POSIX Access Control List. Note: acl is enabled 110 by default if CONFIG_EROFS_FS_POSIX_ACL is selected. 111cache_strategy=%s Select a strategy for cached decompression from now on: 112 113 ========== ============================================= 114 disabled In-place I/O decompression only; 115 readahead Cache the last incomplete compressed physical 116 cluster for further reading. It still does 117 in-place I/O decompression for the rest 118 compressed physical clusters; 119 readaround Cache the both ends of incomplete compressed 120 physical clusters for further reading. 121 It still does in-place I/O decompression 122 for the rest compressed physical clusters. 123 ========== ============================================= 124dax={always,never} Use direct access (no page cache). See 125 Documentation/filesystems/dax.rst. 126dax A legacy option which is an alias for ``dax=always``. 127device=%s Specify a path to an extra device to be used together. 128fsid=%s Specify a filesystem image ID for Fscache back-end. 129domain_id=%s Specify a domain ID in fscache mode so that different images 130 with the same blobs under a given domain ID can share storage. 131=================== ========================================================= 132 133Sysfs Entries 134============= 135 136Information about mounted erofs file systems can be found in /sys/fs/erofs. 137Each mounted filesystem will have a directory in /sys/fs/erofs based on its 138device name (i.e., /sys/fs/erofs/sda). 139(see also Documentation/ABI/testing/sysfs-fs-erofs) 140 141On-disk details 142=============== 143 144Summary 145------- 146Different from other read-only file systems, an EROFS volume is designed 147to be as simple as possible:: 148 149 |-> aligned with the block size 150 ____________________________________________________________ 151 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | 152 |_|__|_|_____|__________|_____|______|__________|_____|______| 153 0 +1K 154 155All data areas should be aligned with the block size, but metadata areas 156may not. All metadatas can be now observed in two different spaces (views): 157 158 1. Inode metadata space 159 160 Each valid inode should be aligned with an inode slot, which is a fixed 161 value (32 bytes) and designed to be kept in line with compact inode size. 162 163 Each inode can be directly found with the following formula: 164 inode offset = meta_blkaddr * block_size + 32 * nid 165 166 :: 167 168 |-> aligned with 8B 169 |-> followed closely 170 + meta_blkaddr blocks |-> another slot 171 _____________________________________________________________________ 172 | ... | inode | xattrs | extents | data inline | ... | inode ... 173 |________|_______|(optional)|(optional)|__(optional)_|_____|__________ 174 |-> aligned with the inode slot size 175 . . 176 . . 177 . . 178 . . 179 . . 180 . . 181 .____________________________________________________|-> aligned with 4B 182 | xattr_ibody_header | shared xattrs | inline xattrs | 183 |____________________|_______________|_______________| 184 |-> 12 bytes <-|->x * 4 bytes<-| . 185 . . . 186 . . . 187 . . . 188 ._______________________________.______________________. 189 | id | id | id | id | ... | id | ent | ... | ent| ... | 190 |____|____|____|____|______|____|_____|_____|____|_____| 191 |-> aligned with 4B 192 |-> aligned with 4B 193 194 Inode could be 32 or 64 bytes, which can be distinguished from a common 195 field which all inode versions have -- i_format:: 196 197 __________________ __________________ 198 | i_format | | i_format | 199 |__________________| |__________________| 200 | ... | | ... | 201 | | | | 202 |__________________| 32 bytes | | 203 | | 204 |__________________| 64 bytes 205 206 Xattrs, extents, data inline are placed after the corresponding inode with 207 proper alignment, and they could be optional for different data mappings. 208 _currently_ total 5 data layouts are supported: 209 210 == ==================================================================== 211 0 flat file data without data inline (no extent); 212 1 fixed-sized output data compression (with non-compacted indexes); 213 2 flat file data with tail packing data inline (no extent); 214 3 fixed-sized output data compression (with compacted indexes, v5.3+); 215 4 chunk-based file (v5.15+). 216 == ==================================================================== 217 218 The size of the optional xattrs is indicated by i_xattr_count in inode 219 header. Large xattrs or xattrs shared by many different files can be 220 stored in shared xattrs metadata rather than inlined right after inode. 221 222 2. Shared xattrs metadata space 223 224 Shared xattrs space is similar to the above inode space, started with 225 a specific block indicated by xattr_blkaddr, organized one by one with 226 proper align. 227 228 Each share xattr can also be directly found by the following formula: 229 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id 230 231:: 232 233 |-> aligned by 4 bytes 234 + xattr_blkaddr blocks |-> aligned with 4 bytes 235 _________________________________________________________________________ 236 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... 237 |________|_____________|_____________|_____|______________|_______________ 238 239Directories 240----------- 241All directories are now organized in a compact on-disk format. Note that 242each directory block is divided into index and name areas in order to support 243random file lookup, and all directory entries are _strictly_ recorded in 244alphabetical order in order to support improved prefix binary search 245algorithm (could refer to the related source code). 246 247:: 248 249 ___________________________ 250 / | 251 / ______________|________________ 252 / / | nameoff1 | nameoffN-1 253 ____________.______________._______________v________________v__________ 254 | dirent | dirent | ... | dirent | filename | filename | ... | filename | 255 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| 256 \ ^ 257 \ | * could have 258 \ | trailing '\0' 259 \________________________| nameoff0 260 Directory block 261 262Note that apart from the offset of the first filename, nameoff0 also indicates 263the total number of directory entries in this block since it is no need to 264introduce another on-disk field at all. 265 266Chunk-based files 267----------------- 268In order to support chunk-based data deduplication, a new inode data layout has 269been supported since Linux v5.15: Files are split in equal-sized data chunks 270with ``extents`` area of the inode metadata indicating how to get the chunk 271data: these can be simply as a 4-byte block address array or in the 8-byte 272chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more 273details.) 274 275By the way, chunk-based files are all uncompressed for now. 276 277Long extended attribute name prefixes 278------------------------------------- 279There are use cases where extended attributes with different values can have 280only a few common prefixes (such as overlayfs xattrs). The predefined prefixes 281work inefficiently in both image size and runtime performance in such cases. 282 283The long xattr name prefixes feature is introduced to address this issue. The 284overall idea is that, apart from the existing predefined prefixes, the xattr 285entry could also refer to user-specified long xattr name prefixes, e.g. 286"trusted.overlay.". 287 288When referring to a long xattr name prefix, the highest bit (bit 7) of 289erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole 290represent the index of the referred long name prefix among all long name 291prefixes. Therefore, only the trailing part of the name apart from the long 292xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if 293the full xattr name matches exactly as its long xattr name prefix. 294 295All long xattr prefixes are stored one by one in the packed inode as long as 296the packed inode is valid, or in the meta inode otherwise. The 297xattr_prefix_count (of the on-disk superblock) indicates the total number of 298long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start 299offset of long name prefixes in the packed/meta inode. Note that, long extended 300attribute name prefixes are disabled if xattr_prefix_count is 0. 301 302Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4), 303where len represents the total size of the data part. The data part is actually 304represented by 'struct erofs_xattr_long_prefix', where base_index represents the 305index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for 306"trusted.overlay." long name prefix, while the infix string keeps the string 307after stripping the short prefix, e.g. "overlay." for the example above. 308 309Data compression 310---------------- 311EROFS implements fixed-sized output compression which generates fixed-sized 312compressed data blocks from variable-sized input in contrast to other existing 313fixed-sized input solutions. Relatively higher compression ratios can be gotten 314by using fixed-sized output compression since nowadays popular data compression 315algorithms are mostly LZ77-based and such fixed-sized output approach can be 316benefited from the historical dictionary (aka. sliding window). 317 318In details, original (uncompressed) data is turned into several variable-sized 319extents and in the meanwhile, compressed into physical clusters (pclusters). 320In order to record each variable-sized extent, logical clusters (lclusters) are 321introduced as the basic unit of compress indexes to indicate whether a new 322extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now 323fixed in block size, as illustrated below:: 324 325 |<- variable-sized extent ->|<- VLE ->| 326 clusterofs clusterofs clusterofs 327 | | | 328 _________v_________________________________v_______________________v________ 329 ... | . | | . | | . ... 330 ____|____._________|______________|________.___ _|______________|__.________ 331 |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| 332 (HEAD) (NONHEAD) (HEAD) (NONHEAD) . 333 . CBLKCNT . . 334 . . . 335 . . . 336 _______._____________________________.______________._________________ 337 ... | | | | ... 338 _______|______________|______________|______________|_________________ 339 |-> big pcluster <-|-> pcluster <-| 340 341A physical cluster can be seen as a container of physical compressed blocks 342which contains compressed data. Previously, only lcluster-sized (4KB) pclusters 343were supported. After big pcluster feature is introduced (available since 344Linux v5.13), pcluster can be a multiple of lcluster size. 345 346For each HEAD lcluster, clusterofs is recorded to indicate where a new extent 347starts and blkaddr is used to seek the compressed data. For each NONHEAD 348lcluster, delta0 and delta1 are available instead of blkaddr to indicate the 349distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is 350also a HEAD lcluster except that its data is uncompressed. See the comments 351around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 352 353If big pcluster is enabled, pcluster size in lclusters needs to be recorded as 354well. Let the delta0 of the first NONHEAD lcluster store the compressed block 355count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy 356to understand its delta0 is constantly 1, as illustrated below:: 357 358 __________________________________________________________ 359 | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | 360 |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| 361 |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| 362 a lcluster-sized pcluster (without CBLKCNT) ^ 363 364If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, 365but it's easy to know the size of such pcluster is 1 lcluster as well. 366 367Since Linux v6.1, each pcluster can be used for multiple variable-sized extents, 368therefore it can be used for compressed data deduplication. 369