erofs.rst (15528B)
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 is the main features of EROFS: 34 35 - Little endian on-disk design; 36 37 - 4KiB block size and 32-bit block addresses, therefore 16TiB address space 38 at most for now; 39 40 - Two inode layouts for different requirements: 41 42 ===================== ============ ====================================== 43 compact (v1) extended (v2) 44 ===================== ============ ====================================== 45 Inode metadata size 32 bytes 64 bytes 46 Max file size 4 GiB 16 EiB (also limited by max. vol size) 47 Max uids/gids 65536 4294967296 48 Per-inode timestamp no yes (64 + 32-bit timestamp) 49 Max hardlinks 65536 4294967296 50 Metadata reserved 8 bytes 18 bytes 51 ===================== ============ ====================================== 52 53 - Metadata and data could be mixed as an option; 54 55 - Support extended attributes (xattrs) as an option; 56 57 - Support tailpacking data and xattr inline compared to byte-addressed 58 unaligned metadata or smaller block size alternatives; 59 60 - Support POSIX.1e ACLs by using xattrs; 61 62 - Support transparent data compression as an option: 63 LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition, 64 inplace decompression is also supported to avoid bounce compressed buffers 65 and page cache thrashing. 66 67 - Support direct I/O on uncompressed files to avoid double caching for loop 68 devices; 69 70 - Support FSDAX on uncompressed images for secure containers and ramdisks in 71 order to get rid of unnecessary page cache. 72 73 - Support multiple devices for multi blob container images; 74 75 - Support file-based on-demand loading with the Fscache infrastructure. 76 77The following git tree provides the file system user-space tools under 78development, such as a formatting tool (mkfs.erofs), an on-disk consistency & 79compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs): 80 81- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git 82 83Bugs and patches are welcome, please kindly help us and send to the following 84linux-erofs mailing list: 85 86- linux-erofs mailing list <linux-erofs@lists.ozlabs.org> 87 88Mount options 89============= 90 91=================== ========================================================= 92(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled 93 by default if CONFIG_EROFS_FS_XATTR is selected. 94(no)acl Setup POSIX Access Control List. Note: acl is enabled 95 by default if CONFIG_EROFS_FS_POSIX_ACL is selected. 96cache_strategy=%s Select a strategy for cached decompression from now on: 97 98 ========== ============================================= 99 disabled In-place I/O decompression only; 100 readahead Cache the last incomplete compressed physical 101 cluster for further reading. It still does 102 in-place I/O decompression for the rest 103 compressed physical clusters; 104 readaround Cache the both ends of incomplete compressed 105 physical clusters for further reading. 106 It still does in-place I/O decompression 107 for the rest compressed physical clusters. 108 ========== ============================================= 109dax={always,never} Use direct access (no page cache). See 110 Documentation/filesystems/dax.rst. 111dax A legacy option which is an alias for ``dax=always``. 112device=%s Specify a path to an extra device to be used together. 113fsid=%s Specify a filesystem image ID for Fscache back-end. 114=================== ========================================================= 115 116Sysfs Entries 117============= 118 119Information about mounted erofs file systems can be found in /sys/fs/erofs. 120Each mounted filesystem will have a directory in /sys/fs/erofs based on its 121device name (i.e., /sys/fs/erofs/sda). 122(see also Documentation/ABI/testing/sysfs-fs-erofs) 123 124On-disk details 125=============== 126 127Summary 128------- 129Different from other read-only file systems, an EROFS volume is designed 130to be as simple as possible:: 131 132 |-> aligned with the block size 133 ____________________________________________________________ 134 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | 135 |_|__|_|_____|__________|_____|______|__________|_____|______| 136 0 +1K 137 138All data areas should be aligned with the block size, but metadata areas 139may not. All metadatas can be now observed in two different spaces (views): 140 141 1. Inode metadata space 142 143 Each valid inode should be aligned with an inode slot, which is a fixed 144 value (32 bytes) and designed to be kept in line with compact inode size. 145 146 Each inode can be directly found with the following formula: 147 inode offset = meta_blkaddr * block_size + 32 * nid 148 149 :: 150 151 |-> aligned with 8B 152 |-> followed closely 153 + meta_blkaddr blocks |-> another slot 154 _____________________________________________________________________ 155 | ... | inode | xattrs | extents | data inline | ... | inode ... 156 |________|_______|(optional)|(optional)|__(optional)_|_____|__________ 157 |-> aligned with the inode slot size 158 . . 159 . . 160 . . 161 . . 162 . . 163 . . 164 .____________________________________________________|-> aligned with 4B 165 | xattr_ibody_header | shared xattrs | inline xattrs | 166 |____________________|_______________|_______________| 167 |-> 12 bytes <-|->x * 4 bytes<-| . 168 . . . 169 . . . 170 . . . 171 ._______________________________.______________________. 172 | id | id | id | id | ... | id | ent | ... | ent| ... | 173 |____|____|____|____|______|____|_____|_____|____|_____| 174 |-> aligned with 4B 175 |-> aligned with 4B 176 177 Inode could be 32 or 64 bytes, which can be distinguished from a common 178 field which all inode versions have -- i_format:: 179 180 __________________ __________________ 181 | i_format | | i_format | 182 |__________________| |__________________| 183 | ... | | ... | 184 | | | | 185 |__________________| 32 bytes | | 186 | | 187 |__________________| 64 bytes 188 189 Xattrs, extents, data inline are followed by the corresponding inode with 190 proper alignment, and they could be optional for different data mappings. 191 _currently_ total 5 data layouts are supported: 192 193 == ==================================================================== 194 0 flat file data without data inline (no extent); 195 1 fixed-sized output data compression (with non-compacted indexes); 196 2 flat file data with tail packing data inline (no extent); 197 3 fixed-sized output data compression (with compacted indexes, v5.3+); 198 4 chunk-based file (v5.15+). 199 == ==================================================================== 200 201 The size of the optional xattrs is indicated by i_xattr_count in inode 202 header. Large xattrs or xattrs shared by many different files can be 203 stored in shared xattrs metadata rather than inlined right after inode. 204 205 2. Shared xattrs metadata space 206 207 Shared xattrs space is similar to the above inode space, started with 208 a specific block indicated by xattr_blkaddr, organized one by one with 209 proper align. 210 211 Each share xattr can also be directly found by the following formula: 212 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id 213 214:: 215 216 |-> aligned by 4 bytes 217 + xattr_blkaddr blocks |-> aligned with 4 bytes 218 _________________________________________________________________________ 219 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... 220 |________|_____________|_____________|_____|______________|_______________ 221 222Directories 223----------- 224All directories are now organized in a compact on-disk format. Note that 225each directory block is divided into index and name areas in order to support 226random file lookup, and all directory entries are _strictly_ recorded in 227alphabetical order in order to support improved prefix binary search 228algorithm (could refer to the related source code). 229 230:: 231 232 ___________________________ 233 / | 234 / ______________|________________ 235 / / | nameoff1 | nameoffN-1 236 ____________.______________._______________v________________v__________ 237 | dirent | dirent | ... | dirent | filename | filename | ... | filename | 238 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| 239 \ ^ 240 \ | * could have 241 \ | trailing '\0' 242 \________________________| nameoff0 243 Directory block 244 245Note that apart from the offset of the first filename, nameoff0 also indicates 246the total number of directory entries in this block since it is no need to 247introduce another on-disk field at all. 248 249Chunk-based files 250----------------- 251In order to support chunk-based data deduplication, a new inode data layout has 252been supported since Linux v5.15: Files are split in equal-sized data chunks 253with ``extents`` area of the inode metadata indicating how to get the chunk 254data: these can be simply as a 4-byte block address array or in the 8-byte 255chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more 256details.) 257 258By the way, chunk-based files are all uncompressed for now. 259 260Data compression 261---------------- 262EROFS implements LZ4 fixed-sized output compression which generates fixed-sized 263compressed data blocks from variable-sized input in contrast to other existing 264fixed-sized input solutions. Relatively higher compression ratios can be gotten 265by using fixed-sized output compression since nowadays popular data compression 266algorithms are mostly LZ77-based and such fixed-sized output approach can be 267benefited from the historical dictionary (aka. sliding window). 268 269In details, original (uncompressed) data is turned into several variable-sized 270extents and in the meanwhile, compressed into physical clusters (pclusters). 271In order to record each variable-sized extent, logical clusters (lclusters) are 272introduced as the basic unit of compress indexes to indicate whether a new 273extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now 274fixed in block size, as illustrated below:: 275 276 |<- variable-sized extent ->|<- VLE ->| 277 clusterofs clusterofs clusterofs 278 | | | 279 _________v_________________________________v_______________________v________ 280 ... | . | | . | | . ... 281 ____|____._________|______________|________.___ _|______________|__.________ 282 |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| 283 (HEAD) (NONHEAD) (HEAD) (NONHEAD) . 284 . CBLKCNT . . 285 . . . 286 . . . 287 _______._____________________________.______________._________________ 288 ... | | | | ... 289 _______|______________|______________|______________|_________________ 290 |-> big pcluster <-|-> pcluster <-| 291 292A physical cluster can be seen as a container of physical compressed blocks 293which contains compressed data. Previously, only lcluster-sized (4KB) pclusters 294were supported. After big pcluster feature is introduced (available since 295Linux v5.13), pcluster can be a multiple of lcluster size. 296 297For each HEAD lcluster, clusterofs is recorded to indicate where a new extent 298starts and blkaddr is used to seek the compressed data. For each NONHEAD 299lcluster, delta0 and delta1 are available instead of blkaddr to indicate the 300distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is 301also a HEAD lcluster except that its data is uncompressed. See the comments 302around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. 303 304If big pcluster is enabled, pcluster size in lclusters needs to be recorded as 305well. Let the delta0 of the first NONHEAD lcluster store the compressed block 306count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy 307to understand its delta0 is constantly 1, as illustrated below:: 308 309 __________________________________________________________ 310 | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | 311 |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| 312 |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| 313 a lcluster-sized pcluster (without CBLKCNT) ^ 314 315If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, 316but it's easy to know the size of such pcluster is 1 lcluster as well.