README
1erofs-utils
2===========
3
4erofs-utils includes user-space tools for erofs filesystem.
5Currently mkfs.erofs and erofsfuse (experimental) are available.
6
7Dependencies & build
8--------------------
9
10 lz4 1.8.0+ for lz4 enabled [2], lz4 1.9.3+ highly recommended [4][5].
11
12 libfuse 2.6+ for erofsfuse enabled as a plus.
13
14How to build for lz4-1.9.0 or above
15~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
16
17To build, you can run the following commands in order:
18
19::
20
21 $ ./autogen.sh
22 $ ./configure
23 $ make
24
25mkfs.erofs binary will be generated under mkfs folder.
26
27* For lz4 < 1.9.2, there are some stability issues about
28 LZ4_compress_destSize(). (lz4hc isn't impacted) [3].
29
30** For lz4 = 1.9.2, there is a noticeable regression about
31 LZ4_decompress_safe_partial() [5], which impacts erofsfuse
32 functionality for legacy images (without 0PADDING).
33
34How to build for lz4-1.8.0~1.8.3
35~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
36
37For these old lz4 versions, lz4hc algorithm cannot be supported
38without lz4-static installed due to LZ4_compress_HC_destSize()
39unstable api usage, which means lz4 will only be available if
40lz4-static isn't found.
41
42On Fedora, lz4-static can be installed by using:
43
44 yum install lz4-static.x86_64
45
46However, it's still not recommended using those versions directly
47since there are serious bugs in these compressors, see [2] [3] [4]
48as well.
49
50mkfs.erofs
51----------
52
53two main kinds of erofs images can be generated: (un)compressed.
54
55 - For uncompressed images, there will be none of compression
56 files in these images. However, it can decide whether the tail
57 block of a file should be inlined or not properly [1].
58
59 - For compressed images, it will try to use lz4(hc) algorithm
60 first for each regular file and see if storage space can be
61 saved with compression. If not, fallback to an uncompressed
62 file.
63
64How to generate erofs images
65~~~~~~~~~~~~~~~~~~~~~~~~~~~~
66
67Currently lz4 and lz4hc are available for compression, e.g.
68 $ mkfs.erofs -zlz4hc foo.erofs.img foo/
69
70Or leave all files uncompressed as an option:
71 $ mkfs.erofs foo.erofs.img foo/
72
73How to generate legacy erofs images
74~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
75
76Decompression inplace and compacted indexes have been introduced in
77linux-5.3, which are not forward-compatible with older kernels.
78
79In order to generate _legacy_ erofs images for old kernels,
80consider adding "-E legacy-compress" to the command line, e.g.
81
82 $ mkfs.erofs -E legacy-compress -zlz4hc foo.erofs.img foo/
83
84Obsoleted erofs.mkfs
85~~~~~~~~~~~~~~~~~~~~
86
87There is an original erofs.mkfs version developed by Li Guifu,
88which was replaced by the new erofs-utils implementation.
89
90git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git -b obsoleted_mkfs
91
92PLEASE NOTE: This version is highly _NOT recommended_ now.
93
94erofsfuse (experimental, unstable)
95----------------------------------
96
97erofsfuse is introduced to support erofs format for various platforms
98(including older linux kernels) and new on-disk features iteration.
99It can also be used as an unpacking tool for unprivileged users.
100
101It supports fixed-sized output decompression *without* any in-place
102I/O or in-place decompression optimization. Also like the other FUSE
103implementations, it suffers from most common performance issues (e.g.
104significant I/O overhead, double caching, etc.)
105
106Therefore, NEVER use it if performance is the top concern.
107
108Note that xattr & ACL aren't implemented yet due to the current Android
109use-case vs limited time. If you have some interest, contribution is,
110as always, welcome.
111
112How to build erofsfuse
113~~~~~~~~~~~~~~~~~~~~~~
114
115It's disabled by default as an experimental feature for now, to enable
116and build it manually:
117
118 $ ./configure --enable-fuse
119 $ make
120
121erofsfuse binary will be generated under fuse folder.
122
123How to mount an erofs image with erofsfuse
124~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
125
126As the other FUSE implementations, it's quite simple to mount with
127erofsfuse, e.g.:
128 $ erofsfuse foo.erofs.img foo/
129
130Alternatively, to make it run in foreground (with debugging level 3):
131 $ erofsfuse -f --dbglevel=3 foo.erofs.img foo/
132
133To debug erofsfuse (also automatically run in foreground):
134 $ erofsfuse -d foo.erofs.img foo/
135
136To unmount an erofsfuse mountpoint as a non-root user:
137 $ fusermount -u foo/
138
139Contribution
140------------
141
142erofs-utils is under GPLv2+ as a part of erofs project,
143feel free to send patches or feedback to us.
144
145To:
146 linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
147 Li Guifu <bluce.liguifu@huawei.com>
148 Miao Xie <miaoxie@huawei.com>
149 Fang Wei <fangwei1@huawei.com>
150
151Cc:
152 Gao Xiang <xiang@kernel.org>
153 Chao Yu <yuchao0@huawei.com>
154
155Comments
156--------
157
158[1] According to the erofs on-disk format, the tail block of files
159 could be inlined aggressively with its metadata in order to reduce
160 the I/O overhead and save the storage space (called tail-packing).
161
162[2] There was a bug until lz4-1.8.3, which can crash erofs-utils
163 randomly. Fortunately bugfix by our colleague Qiuyang Sun was
164 merged in lz4-1.9.0.
165
166 For more details, please refer to
167 https://github.com/lz4/lz4/commit/660d21272e4c8a0f49db5fc1e6853f08713dff82
168
169[3] There were many bugfixes merged into lz4-1.9.2 for
170 LZ4_compress_destSize(), and I once ran into some crashs due to
171 those issues. * Again lz4hc is not affected. *
172
173 [LZ4_compress_destSize] Allow 2 more bytes of match length
174 https://github.com/lz4/lz4/commit/690009e2c2f9e5dcb0d40e7c0c40610ce6006eda
175
176 [LZ4_compress_destSize] Fix rare data corruption bug
177 https://github.com/lz4/lz4/commit/6bc6f836a18d1f8fd05c8fc2b42f1d800bc25de1
178
179 [LZ4_compress_destSize] Fix overflow condition
180 https://github.com/lz4/lz4/commit/13a2d9e34ffc4170720ce417c73e396d0ac1471a
181
182 [LZ4_compress_destSize] Fix off-by-one error in fix
183 https://github.com/lz4/lz4/commit/7c32101c655d93b61fc212dcd512b87119dd7333
184
185 [LZ4_compress_destSize] Fix off-by-one error
186 https://github.com/lz4/lz4/commit/d7cad81093cd805110291f84d64d385557d0ffba
187
188 since upstream lz4 doesn't have stable branch for old versions, it's
189 preferred to use latest upstream lz4 library (although some regressions
190 could happen since new features are also introduced to latest upstream
191 version as well) or backport all stable bugfixes to old stable versions,
192 e.g. our unofficial lz4 fork: https://github.com/erofs/lz4
193
194[4] LZ4HC didn't compress long zeroed buffer properly with
195 LZ4_compress_HC_destSize()
196 https://github.com/lz4/lz4/issues/784
197
198 which has been resolved in
199 https://github.com/lz4/lz4/commit/e7fe105ac6ed02019d34731d2ba3aceb11b51bb1
200
201 and already included in lz4-1.9.3, see:
202 https://github.com/lz4/lz4/releases/tag/v1.9.3
203
204[5] LZ4_decompress_safe_partial is broken in 1.9.2
205 https://github.com/lz4/lz4/issues/783
206
207 which is also resolved in lz4-1.9.3.
208
209