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, DEFLATE and Zstandard algorithms can be used on a per-file 67 basis; In addition, inplace decompression is also supported to avoid bounce 68 compressed 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 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. 128directio (For file-backed mounts) Use direct I/O to access backing 129 files, and asynchronous I/O will be enabled if supported. 130fsid=%s Specify a filesystem image ID for Fscache back-end. 131domain_id=%s Specify a trusted domain ID for fscache mode so that 132 different images with the same blobs, identified by blob IDs, 133 can share storage within the same trusted domain. 134 Also used for different filesystems with inode page sharing 135 enabled to share page cache within the trusted domain. 136fsoffset=%llu Specify block-aligned filesystem offset for the primary device. 137inode_share Enable inode page sharing for this filesystem. Inodes with 138 identical content within the same domain ID can share the 139 page cache. 140=================== ========================================================= 141 142Sysfs Entries 143============= 144 145Information about mounted erofs file systems can be found in /sys/fs/erofs. 146Each mounted filesystem will have a directory in /sys/fs/erofs based on its 147device name (i.e., /sys/fs/erofs/sda). 148(see also Documentation/ABI/testing/sysfs-fs-erofs) 149 150On-disk details 151=============== 152 153Summary 154------- 155Different from other read-only file systems, an EROFS volume is designed 156to be as simple as possible:: 157 158 |-> aligned with the block size 159 ____________________________________________________________ 160 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | 161 |_|__|_|_____|__________|_____|______|__________|_____|______| 162 0 +1K 163 164All data areas should be aligned with the block size, but metadata areas 165may not. All metadata can be now observed in two different spaces (views): 166 167 1. Inode metadata space 168 169 Each valid inode should be aligned with an inode slot, which is a fixed 170 value (32 bytes) and designed to be kept in line with compact inode size. 171 172 Each inode can be directly found with the following formula: 173 inode offset = meta_blkaddr * block_size + 32 * nid 174 175 :: 176 177 |-> aligned with 8B 178 |-> followed closely 179 + meta_blkaddr blocks |-> another slot 180 _____________________________________________________________________ 181 | ... | inode | xattrs | extents | data inline | ... | inode ... 182 |________|_______|(optional)|(optional)|__(optional)_|_____|__________ 183 |-> aligned with the inode slot size 184 . . 185 . . 186 . . 187 . . 188 . . 189 . . 190 .____________________________________________________|-> aligned with 4B 191 | xattr_ibody_header | shared xattrs | inline xattrs | 192 |____________________|_______________|_______________| 193 |-> 12 bytes <-|->x * 4 bytes<-| . 194 . . . 195 . . . 196 . . . 197 ._______________________________.______________________. 198 | id | id | id | id | ... | id | ent | ... | ent| ... | 199 |____|____|____|____|______|____|_____|_____|____|_____| 200 |-> aligned with 4B 201 |-> aligned with 4B 202 203 Inode could be 32 or 64 bytes, which can be distinguished from a common 204 field which all inode versions have -- i_format:: 205 206 __________________ __________________ 207 | i_format | | i_format | 208 |__________________| |__________________| 209 | ... | | ... | 210 | | | | 211 |__________________| 32 bytes | | 212 | | 213 |__________________| 64 bytes 214 215 Xattrs, extents, data inline are placed after the corresponding inode with 216 proper alignment, and they could be optional for different data mappings. 217 _currently_ total 5 data layouts are supported: 218 219 == ==================================================================== 220 0 flat file data without data inline (no extent); 221 1 fixed-sized output data compression (with non-compacted indexes); 222 2 flat file data with tail packing data inline (no extent); 223 3 fixed-sized output data compression (with compacted indexes, v5.3+); 224 4 chunk-based file (v5.15+). 225 == ==================================================================== 226 227 The size of the optional xattrs is indicated by i_xattr_count in inode 228 header. Large xattrs or xattrs shared by many different files can be 229 stored in shared xattrs metadata rather than inlined right after inode. 230 231 2. Shared xattrs metadata space 232 233 Shared xattrs space is similar to the above inode space, started with 234 a specific block indicated by xattr_blkaddr, organized one by one with 235 proper align. 236 237 Each share xattr can also be directly found by the following formula: 238 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id 239 240:: 241 242 |-> aligned by 4 bytes 243 + xattr_blkaddr blocks |-> aligned with 4 bytes 244 _________________________________________________________________________ 245 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... 246 |________|_____________|_____________|_____|______________|_______________ 247 248Directories 249----------- 250All directories are now organized in a compact on-disk format. Note that 251each directory block is divided into index and name areas in order to support 252random file lookup, and all directory entries are _strictly_ recorded in 253alphabetical order in order to support improved prefix binary search 254algorithm (could refer to the related source code). 255 256:: 257 258 ___________________________ 259 / | 260 / ______________|________________ 261 / / | nameoff1 | nameoffN-1 262 ____________.______________._______________v________________v__________ 263 | dirent | dirent | ... | dirent | filename | filename | ... | filename | 264 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| 265 \ ^ 266 \ | * could have 267 \ | trailing '\0' 268 \________________________| nameoff0 269 Directory block 270 271Note that apart from the offset of the first filename, nameoff0 also indicates 272the total number of directory entries in this block since it is no need to 273introduce another on-disk field at all. 274 275Chunk-based files 276----------------- 277In order to support chunk-based data deduplication, a new inode data layout has 278been supported since Linux v5.15: Files are split in equal-sized data chunks 279with ``extents`` area of the inode metadata indicating how to get the chunk 280data: these can be simply as a 4-byte block address array or in the 8-byte 281chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more 282details.) 283 284By the way, chunk-based files are all uncompressed for now. 285 286Long extended attribute name prefixes 287------------------------------------- 288There are use cases where extended attributes with different values can have 289only a few common prefixes (such as overlayfs xattrs). The predefined prefixes 290work inefficiently in both image size and runtime performance in such cases. 291 292The long xattr name prefixes feature is introduced to address this issue. The 293overall idea is that, apart from the existing predefined prefixes, the xattr 294entry could also refer to user-specified long xattr name prefixes, e.g. 295"trusted.overlay.". 296 297When referring to a long xattr name prefix, the highest bit (bit 7) of 298erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole 299represent the index of the referred long name prefix among all long name 300prefixes. Therefore, only the trailing part of the name apart from the long 301xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if 302the full xattr name matches exactly as its long xattr name prefix. 303 304All long xattr prefixes are stored one by one in the packed inode as long as 305the packed inode is valid, or in the meta inode otherwise. The 306xattr_prefix_count (of the on-disk superblock) indicates the total number of 307long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start 308offset of long name prefixes in the packed/meta inode. Note that, long extended 309attribute name prefixes are disabled if xattr_prefix_count is 0. 310 311Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4), 312where len represents the total size of the data part. The data part is actually 313represented by 'struct erofs_xattr_long_prefix', where base_index represents the 314index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for 315"trusted.overlay." long name prefix, while the infix string keeps the string 316after stripping the short prefix, e.g. "overlay." for the example above. 317 318Data compression 319---------------- 320EROFS implements fixed-sized output compression which generates fixed-sized 321compressed data blocks from variable-sized input in contrast to other existing 322fixed-sized input solutions. Relatively higher compression ratios can be gotten 323by using fixed-sized output compression since nowadays popular data compression 324algorithms are mostly LZ77-based and such fixed-sized output approach can be 325benefited from the historical dictionary (aka. sliding window). 326 327In details, original (uncompressed) data is turned into several variable-sized 328extents and in the meanwhile, compressed into physical clusters (pclusters). 329In order to record each variable-sized extent, logical clusters (lclusters) are 330introduced as the basic unit of compress indexes to indicate whether a new 331extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now 332fixed in block size, as illustrated below:: 333 334 |<- variable-sized extent ->|<- VLE ->| 335 clusterofs clusterofs clusterofs 336 | | | 337 _________v_________________________________v_______________________v________ 338 ... | . | | . | | . ... 339 ____|____._________|______________|________.___ _|______________|__.________ 340 |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| 341 (HEAD) (NONHEAD) (HEAD) (NONHEAD) . 342 . CBLKCNT . . 343 . . . 344 . . . 345 _______._____________________________.______________._________________ 346 ... | | | | ... 347 _______|______________|______________|______________|_________________ 348 |-> big pcluster <-|-> pcluster <-| 349 350A physical cluster can be seen as a container of physical compressed blocks 351which contains compressed data. Previously, only lcluster-sized (4KB) pclusters 352were supported. After big pcluster feature is introduced (available since 353Linux v5.13), pcluster can be a multiple of lcluster size. 354 355For each HEAD lcluster, clusterofs is recorded to indicate where a new extent 356starts and blkaddr is used to seek the compressed data. For each NONHEAD 357lcluster, delta0 and delta1 are available instead of blkaddr to indicate the 358distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is 359also a HEAD lcluster except that its data is uncompressed. See the comments 360around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 361 362If big pcluster is enabled, pcluster size in lclusters needs to be recorded as 363well. Let the delta0 of the first NONHEAD lcluster store the compressed block 364count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy 365to understand its delta0 is constantly 1, as illustrated below:: 366 367 __________________________________________________________ 368 | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | 369 |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| 370 |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| 371 a lcluster-sized pcluster (without CBLKCNT) ^ 372 373If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, 374but it's easy to know the size of such pcluster is 1 lcluster as well. 375 376Since Linux v6.1, each pcluster can be used for multiple variable-sized extents, 377therefore it can be used for compressed data deduplication. 378