1.. SPDX-License-Identifier: GPL-2.0
2
3======================================
4Enhanced Read-Only File System - EROFS
5======================================
6
7Overview
8========
9
10EROFS file-system stands for Enhanced Read-Only File System. Different
11from other read-only file systems, it aims to be designed for flexibility,
12scalability, but be kept simple and high performance.
13
14It 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 minimize extra storage space with guaranteed end-to-end performance
23   by using compact layout, transparent file compression and direct access,
24   especially for those embedded devices with limited memory and high-density
25   hosts with numerous containers;
26
27Here is the main features of EROFS:
28
29 - Little endian on-disk design;
30
31 - Currently 4KB block size (nobh) and therefore maximum 16TB address space;
32
33 - Metadata & data could be mixed by design;
34
35 - 2 inode versions for different requirements:
36
37   =====================  ============  =====================================
38                          compact (v1)  extended (v2)
39   =====================  ============  =====================================
40   Inode metadata size    32 bytes      64 bytes
41   Max file size          4 GB          16 EB (also limited by max. vol size)
42   Max uids/gids          65536         4294967296
43   File change time       no            yes (64 + 32-bit timestamp)
44   Max hardlinks          65536         4294967296
45   Metadata reserved      4 bytes       14 bytes
46   =====================  ============  =====================================
47
48 - Support extended attributes (xattrs) as an option;
49
50 - Support xattr inline and tail-end data inline for all files;
51
52 - Support POSIX.1e ACLs by using xattrs;
53
54 - Support transparent data compression as an option:
55   LZ4 algorithm with the fixed-sized output compression for high performance;
56
57 - Multiple device support for multi-layer container images.
58
59The following git tree provides the file system user-space tools under
60development (ex, formatting tool mkfs.erofs):
61
62- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
63
64Bugs and patches are welcome, please kindly help us and send to the following
65linux-erofs mailing list:
66
67- linux-erofs mailing list   <linux-erofs@lists.ozlabs.org>
68
69Mount options
70=============
71
72===================    =========================================================
73(no)user_xattr         Setup Extended User Attributes. Note: xattr is enabled
74                       by default if CONFIG_EROFS_FS_XATTR is selected.
75(no)acl                Setup POSIX Access Control List. Note: acl is enabled
76                       by default if CONFIG_EROFS_FS_POSIX_ACL is selected.
77cache_strategy=%s      Select a strategy for cached decompression from now on:
78
79		       ==========  =============================================
80                         disabled  In-place I/O decompression only;
81                        readahead  Cache the last incomplete compressed physical
82                                   cluster for further reading. It still does
83                                   in-place I/O decompression for the rest
84                                   compressed physical clusters;
85                       readaround  Cache the both ends of incomplete compressed
86                                   physical clusters for further reading.
87                                   It still does in-place I/O decompression
88                                   for the rest compressed physical clusters.
89		       ==========  =============================================
90dax={always,never}     Use direct access (no page cache).  See
91                       Documentation/filesystems/dax.rst.
92dax                    A legacy option which is an alias for ``dax=always``.
93device=%s              Specify a path to an extra device to be used together.
94===================    =========================================================
95
96Sysfs Entries
97=============
98
99Information about mounted erofs file systems can be found in /sys/fs/erofs.
100Each mounted filesystem will have a directory in /sys/fs/erofs based on its
101device name (i.e., /sys/fs/erofs/sda).
102(see also Documentation/ABI/testing/sysfs-fs-erofs)
103
104On-disk details
105===============
106
107Summary
108-------
109Different from other read-only file systems, an EROFS volume is designed
110to be as simple as possible::
111
112                                |-> aligned with the block size
113   ____________________________________________________________
114  | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
115  |_|__|_|_____|__________|_____|______|__________|_____|______|
116  0 +1K
117
118All data areas should be aligned with the block size, but metadata areas
119may not. All metadatas can be now observed in two different spaces (views):
120
121 1. Inode metadata space
122
123    Each valid inode should be aligned with an inode slot, which is a fixed
124    value (32 bytes) and designed to be kept in line with compact inode size.
125
126    Each inode can be directly found with the following formula:
127         inode offset = meta_blkaddr * block_size + 32 * nid
128
129    ::
130
131                                 |-> aligned with 8B
132                                            |-> followed closely
133     + meta_blkaddr blocks                                      |-> another slot
134       _____________________________________________________________________
135     |  ...   | inode |  xattrs  | extents  | data inline | ... | inode ...
136     |________|_______|(optional)|(optional)|__(optional)_|_____|__________
137              |-> aligned with the inode slot size
138                   .                   .
139                 .                         .
140               .                              .
141             .                                    .
142           .                                         .
143         .                                              .
144       .____________________________________________________|-> aligned with 4B
145       | xattr_ibody_header | shared xattrs | inline xattrs |
146       |____________________|_______________|_______________|
147       |->    12 bytes    <-|->x * 4 bytes<-|               .
148                           .                .                 .
149                     .                      .                   .
150                .                           .                     .
151            ._______________________________.______________________.
152            | id | id | id | id |  ... | id | ent | ... | ent| ... |
153            |____|____|____|____|______|____|_____|_____|____|_____|
154                                            |-> aligned with 4B
155                                                        |-> aligned with 4B
156
157    Inode could be 32 or 64 bytes, which can be distinguished from a common
158    field which all inode versions have -- i_format::
159
160        __________________               __________________
161       |     i_format     |             |     i_format     |
162       |__________________|             |__________________|
163       |        ...       |             |        ...       |
164       |                  |             |                  |
165       |__________________| 32 bytes    |                  |
166                                        |                  |
167                                        |__________________| 64 bytes
168
169    Xattrs, extents, data inline are followed by the corresponding inode with
170    proper alignment, and they could be optional for different data mappings.
171    _currently_ total 5 data layouts are supported:
172
173    ==  ====================================================================
174     0  flat file data without data inline (no extent);
175     1  fixed-sized output data compression (with non-compacted indexes);
176     2  flat file data with tail packing data inline (no extent);
177     3  fixed-sized output data compression (with compacted indexes, v5.3+);
178     4  chunk-based file (v5.15+).
179    ==  ====================================================================
180
181    The size of the optional xattrs is indicated by i_xattr_count in inode
182    header. Large xattrs or xattrs shared by many different files can be
183    stored in shared xattrs metadata rather than inlined right after inode.
184
185 2. Shared xattrs metadata space
186
187    Shared xattrs space is similar to the above inode space, started with
188    a specific block indicated by xattr_blkaddr, organized one by one with
189    proper align.
190
191    Each share xattr can also be directly found by the following formula:
192         xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
193
194::
195
196                           |-> aligned by  4 bytes
197    + xattr_blkaddr blocks                     |-> aligned with 4 bytes
198     _________________________________________________________________________
199    |  ...   | xattr_entry |  xattr data | ... |  xattr_entry | xattr data  ...
200    |________|_____________|_____________|_____|______________|_______________
201
202Directories
203-----------
204All directories are now organized in a compact on-disk format. Note that
205each directory block is divided into index and name areas in order to support
206random file lookup, and all directory entries are _strictly_ recorded in
207alphabetical order in order to support improved prefix binary search
208algorithm (could refer to the related source code).
209
210::
211
212                  ___________________________
213                 /                           |
214                /              ______________|________________
215               /              /              | nameoff1       | nameoffN-1
216  ____________.______________._______________v________________v__________
217 | dirent | dirent | ... | dirent | filename | filename | ... | filename |
218 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
219      \                           ^
220       \                          |                           * could have
221        \                         |                             trailing '\0'
222         \________________________| nameoff0
223                             Directory block
224
225Note that apart from the offset of the first filename, nameoff0 also indicates
226the total number of directory entries in this block since it is no need to
227introduce another on-disk field at all.
228
229Chunk-based file
230----------------
231In order to support chunk-based data deduplication, a new inode data layout has
232been supported since Linux v5.15: Files are split in equal-sized data chunks
233with ``extents`` area of the inode metadata indicating how to get the chunk
234data: these can be simply as a 4-byte block address array or in the 8-byte
235chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more
236details.)
237
238By the way, chunk-based files are all uncompressed for now.
239
240Data compression
241----------------
242EROFS implements LZ4 fixed-sized output compression which generates fixed-sized
243compressed data blocks from variable-sized input in contrast to other existing
244fixed-sized input solutions. Relatively higher compression ratios can be gotten
245by using fixed-sized output compression since nowadays popular data compression
246algorithms are mostly LZ77-based and such fixed-sized output approach can be
247benefited from the historical dictionary (aka. sliding window).
248
249In details, original (uncompressed) data is turned into several variable-sized
250extents and in the meanwhile, compressed into physical clusters (pclusters).
251In order to record each variable-sized extent, logical clusters (lclusters) are
252introduced as the basic unit of compress indexes to indicate whether a new
253extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now
254fixed in block size, as illustrated below::
255
256          |<-    variable-sized extent    ->|<-       VLE         ->|
257        clusterofs                        clusterofs              clusterofs
258          |                                 |                       |
259 _________v_________________________________v_______________________v________
260 ... |    .         |              |        .     |              |  .   ...
261 ____|____._________|______________|________.___ _|______________|__.________
262     |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-|
263          (HEAD)        (NONHEAD)       (HEAD)        (NONHEAD)    .
264           .             CBLKCNT            .                    .
265            .                               .                  .
266             .                              .                .
267       _______._____________________________.______________._________________
268          ... |              |              |              | ...
269       _______|______________|______________|______________|_________________
270              |->      big pcluster       <-|-> pcluster <-|
271
272A physical cluster can be seen as a container of physical compressed blocks
273which contains compressed data. Previously, only lcluster-sized (4KB) pclusters
274were supported. After big pcluster feature is introduced (available since
275Linux v5.13), pcluster can be a multiple of lcluster size.
276
277For each HEAD lcluster, clusterofs is recorded to indicate where a new extent
278starts and blkaddr is used to seek the compressed data. For each NONHEAD
279lcluster, delta0 and delta1 are available instead of blkaddr to indicate the
280distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is
281also a HEAD lcluster except that its data is uncompressed. See the comments
282around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
283
284If big pcluster is enabled, pcluster size in lclusters needs to be recorded as
285well. Let the delta0 of the first NONHEAD lcluster store the compressed block
286count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy
287to understand its delta0 is constantly 1, as illustrated below::
288
289   __________________________________________________________
290  | HEAD |  NONHEAD  | NONHEAD | ... | NONHEAD | HEAD | HEAD |
291  |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_|
292     |<----- a big pcluster (with CBLKCNT) ------>|<--  -->|
293           a lcluster-sized pcluster (without CBLKCNT) ^
294
295If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
296but it's easy to know the size of such pcluster is 1 lcluster as well.
297