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Mauro Carvalho Chehabe66d8632020-02-17 17:12:01 +0100[diff] [blame]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 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
26Here 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
56The following git tree provides the file system user-space tools under
57development (ex, formatting tool mkfs.erofs):
58
59- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
60
61Bugs and patches are welcome, please kindly help us and send to the following
62linux-erofs mailing list:
63
64- linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
65
66Mount 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.
74cache_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
89On-disk details
90===============
91
92Summary
93-------
94Different from other read-only file systems, an EROFS volume is designed
95to be as simple as possible::
96
97 |-> aligned with the block size
98 ____________________________________________________________
99 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
100 |_|__|_|_____|__________|_____|______|__________|_____|______|
101 0 +1K
102
103All data areas should be aligned with the block size, but metadata areas
104may 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
186Directories
187-----------
188All directories are now organized in a compact on-disk format. Note that
189each directory block is divided into index and name areas in order to support
190random file lookup, and all directory entries are _strictly_ recorded in
191alphabetical order in order to support improved prefix binary search
192algorithm (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
210Note that apart from the offset of the first filename, nameoff0 also indicates
211the total number of directory entries in this block since it is no need to
212introduce another on-disk field at all.
213
214Compression
215-----------
216Currently, EROFS supports 4KB fixed-sized output transparent file compression,
217as 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
236Currently each on-disk physical cluster can contain 4KB (un)compressed data
237at most. For each logical cluster, there is a corresponding on-disk index to
238describe its cluster type, physical cluster address, etc.
239
240See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.