Documentation/filesystems/erofs.rst

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