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 file compression as an option:
  54    LZ4 algorithm with 4 KB 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 ===================    =========================================================
  88
  89 On-disk details
  90 ===============
  91
  92 Summary
  93 -------
  94 Different from other read-only file systems, an EROFS volume is designed
  95 to be as simple as possible::
  96
  97                                 |-> aligned with the block size
  98    ____________________________________________________________
  99   | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
 100   |_|__|_|_____|__________|_____|______|__________|_____|______|
 101   0 +1K
 102
 103 All data areas should be aligned with the block size, but metadata areas
 104 may not. All metadatas can be now observed in two different spaces (views):
 105
 106  1. Inode metadata space
 107
 108     Each valid inode should be aligned with an inode slot, which is a fixed
 109     value (32 bytes) and designed to be kept in line with compact inode size.
 110
 111     Each inode can be directly found with the following formula:
 112          inode offset = meta_blkaddr * block_size + 32 * nid
 113
 114     ::
 115
 116                                     |-> aligned with 8B
 117                                             |-> followed closely
 118         + meta_blkaddr blocks                                      |-> another slot
 119         _____________________________________________________________________
 120         |  ...   | inode |  xattrs  | extents  | data inline | ... | inode ...
 121         |________|_______|(optional)|(optional)|__(optional)_|_____|__________
 122                 |-> aligned with the inode slot size
 123                     .                   .
 124                     .                         .
 125                 .                              .
 126                 .                                    .
 127             .                                         .
 128             .                                              .
 129         .____________________________________________________|-> aligned with 4B
 130         | xattr_ibody_header | shared xattrs | inline xattrs |
 131         |____________________|_______________|_______________|
 132         |->    12 bytes    <-|->x * 4 bytes<-|               .
 133                             .                .                 .
 134                         .                      .                   .
 135                 .                           .                     .
 136             ._______________________________.______________________.
 137             | id | id | id | id |  ... | id | ent | ... | ent| ... |
 138             |____|____|____|____|______|____|_____|_____|____|_____|
 139                                             |-> aligned with 4B
 140                                                         |-> aligned with 4B
 141
 142     Inode could be 32 or 64 bytes, which can be distinguished from a common
 143     field which all inode versions have -- i_format::
 144
 145         __________________               __________________
 146        |     i_format     |             |     i_format     |
 147        |__________________|             |__________________|
 148        |        ...       |             |        ...       |
 149        |                  |             |                  |
 150        |__________________| 32 bytes    |                  |
 151                                         |                  |
 152                                         |__________________| 64 bytes
 153
 154     Xattrs, extents, data inline are followed by the corresponding inode with
 155     proper alignment, and they could be optional for different data mappings.
 156     _currently_ total 4 valid data mappings are supported:
 157
 158     ==  ====================================================================
 159      0  flat file data without data inline (no extent);
 160      1  fixed-sized output data compression (with non-compacted indexes);
 161      2  flat file data with tail packing data inline (no extent);
 162      3  fixed-sized output data compression (with compacted indexes, v5.3+).
 163     ==  ====================================================================
 164
 165     The size of the optional xattrs is indicated by i_xattr_count in inode
 166     header. Large xattrs or xattrs shared by many different files can be
 167     stored in shared xattrs metadata rather than inlined right after inode.
 168
 169  2. Shared xattrs metadata space
 170
 171     Shared xattrs space is similar to the above inode space, started with
 172     a specific block indicated by xattr_blkaddr, organized one by one with
 173     proper align.
 174
 175     Each share xattr can also be directly found by the following formula:
 176          xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
 177
 178     ::
 179
 180                             |-> aligned by  4 bytes
 181         + xattr_blkaddr blocks                     |-> aligned with 4 bytes
 182         _________________________________________________________________________
 183         |  ...   | xattr_entry |  xattr data | ... |  xattr_entry | xattr data  ...
 184         |________|_____________|_____________|_____|______________|_______________
 185
 186 Directories
 187 -----------
 188 All directories are now organized in a compact on-disk format. Note that
 189 each directory block is divided into index and name areas in order to support
 190 random file lookup, and all directory entries are _strictly_ recorded in
 191 alphabetical order in order to support improved prefix binary search
 192 algorithm (could refer to the related source code).
 193
 194 ::
 195
 196                     ___________________________
 197                     /                           |
 198                 /              ______________|________________
 199                 /              /              | nameoff1       | nameoffN-1
 200     ____________.______________._______________v________________v__________
 201     | dirent | dirent | ... | dirent | filename | filename | ... | filename |
 202     |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
 203         \                           ^
 204         \                          |                           * could have
 205         \                         |                             trailing '\0'
 206             \________________________| nameoff0
 207
 208                                 Directory block
 209
 210 Note that apart from the offset of the first filename, nameoff0 also indicates
 211 the total number of directory entries in this block since it is no need to
 212 introduce another on-disk field at all.
 213
 214 Compression
 215 -----------
 216 Currently, EROFS supports 4KB fixed-sized output transparent file compression,
 217 as illustrated below::
 218
 219             |---- Variant-Length Extent ----|-------- VLE --------|----- VLE -----
 220             clusterofs                      clusterofs            clusterofs
 221             |                               |                     |   logical data
 222     _________v_______________________________v_____________________v_______________
 223     ... |    .        |             |        .    |             |  .          | ...
 224     ____|____.________|_____________|________.____|_____________|__.__________|____
 225         |-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|
 226             size          size          size          size          size
 227             .                             .                .                   .
 228             .                       .               .                  .
 229                 .                  .              .                .
 230         _______._____________._____________._____________._____________________
 231             ... |             |             |             | ... physical data
 232         _______|_____________|_____________|_____________|_____________________
 233                 |-> cluster <-|-> cluster <-|-> cluster <-|
 234                     size          size          size
 235
 236 Currently each on-disk physical cluster can contain 4KB (un)compressed data
 237 at most. For each logical cluster, there is a corresponding on-disk index to
 238 describe its cluster type, physical cluster address, etc.
 239
 240 See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.