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