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1.. SPDX-License-Identifier: GPL-2.0
2
3======================================
4EROFS - Enhanced Read-Only File System
5======================================
6
7Overview
8========
9
10EROFS filesystem stands for Enhanced Read-Only File System. It aims to form a
11generic read-only filesystem solution for various read-only use cases instead
12of just focusing on storage space saving without considering any side effects
13of runtime performance.
14
15It is designed to meet the needs of flexibility, feature extendability and user
16payload friendly, etc. Apart from those, it is still kept as a simple
17random-access friendly high-performance filesystem to get rid of unneeded I/O
18amplification and memory-resident overhead compared to similar approaches.
19
20It is implemented to be a better choice for the following scenarios:
21
22 - read-only storage media or
23
24 - part of a fully trusted read-only solution, which means it needs to be
25 immutable and bit-for-bit identical to the official golden image for
26 their releases due to security or other considerations and
27
28 - hope to minimize extra storage space with guaranteed end-to-end performance
29 by using compact layout, transparent file compression and direct access,
30 especially for those embedded devices with limited memory and high-density
31 hosts with numerous containers.
32
33Here are the main features of EROFS:
34
35 - Little endian on-disk design;
36
37 - Block-based distribution and file-based distribution over fscache are
38 supported;
39
40 - Support multiple devices to refer to external blobs, which can be used
41 for container images;
42
43 - 32-bit block addresses for each device, therefore 16TiB address space at
44 most with 4KiB block size for now;
45
46 - Two inode layouts for different requirements:
47
48 ===================== ============ ======================================
49 compact (v1) extended (v2)
50 ===================== ============ ======================================
51 Inode metadata size 32 bytes 64 bytes
52 Max file size 4 GiB 16 EiB (also limited by max. vol size)
53 Max uids/gids 65536 4294967296
54 Per-inode timestamp no yes (64 + 32-bit timestamp)
55 Max hardlinks 65536 4294967296
56 Metadata reserved 8 bytes 18 bytes
57 ===================== ============ ======================================
58
59 - Support extended attributes as an option;
60
61 - Support a bloom filter that speeds up negative extended attribute lookups;
62
63 - Support POSIX.1e ACLs by using extended attributes;
64
65 - Support transparent data compression as an option:
66 LZ4, MicroLZMA and DEFLATE algorithms can be used on a per-file basis; In
67 addition, inplace decompression is also supported to avoid bounce compressed
68 buffers and unnecessary page cache thrashing.
69
70 - Support chunk-based data deduplication and rolling-hash compressed data
71 deduplication;
72
73 - Support tailpacking inline compared to byte-addressed unaligned metadata
74 or smaller block size alternatives;
75
76 - Support merging tail-end data into a special inode as fragments.
77
78 - Support large folios to make use of THPs (Transparent Hugepages);
79
80 - Support direct I/O on uncompressed files to avoid double caching for loop
81 devices;
82
83 - Support FSDAX on uncompressed images for secure containers and ramdisks in
84 order to get rid of unnecessary page cache.
85
86 - Support file-based on-demand loading with the Fscache infrastructure.
87
88The following git tree provides the file system user-space tools under
89development, such as a formatting tool (mkfs.erofs), an on-disk consistency &
90compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs):
91
92- git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
93
94For more information, please also refer to the documentation site:
95
96- https://erofs.docs.kernel.org
97
98Bugs and patches are welcome, please kindly help us and send to the following
99linux-erofs mailing list:
100
101- linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
102
103Mount options
104=============
105
106=================== =========================================================
107(no)user_xattr Setup Extended User Attributes. Note: xattr is enabled
108 by default if CONFIG_EROFS_FS_XATTR is selected.
109(no)acl Setup POSIX Access Control List. Note: acl is enabled
110 by default if CONFIG_EROFS_FS_POSIX_ACL is selected.
111cache_strategy=%s Select a strategy for cached decompression from now on:
112
113 ========== =============================================
114 disabled In-place I/O decompression only;
115 readahead Cache the last incomplete compressed physical
116 cluster for further reading. It still does
117 in-place I/O decompression for the rest
118 compressed physical clusters;
119 readaround Cache the both ends of incomplete compressed
120 physical clusters for further reading.
121 It still does in-place I/O decompression
122 for the rest compressed physical clusters.
123 ========== =============================================
124dax={always,never} Use direct access (no page cache). See
125 Documentation/filesystems/dax.rst.
126dax A legacy option which is an alias for ``dax=always``.
127device=%s Specify a path to an extra device to be used together.
128fsid=%s Specify a filesystem image ID for Fscache back-end.
129domain_id=%s Specify a domain ID in fscache mode so that different images
130 with the same blobs under a given domain ID can share storage.
131=================== =========================================================
132
133Sysfs Entries
134=============
135
136Information about mounted erofs file systems can be found in /sys/fs/erofs.
137Each mounted filesystem will have a directory in /sys/fs/erofs based on its
138device name (i.e., /sys/fs/erofs/sda).
139(see also Documentation/ABI/testing/sysfs-fs-erofs)
140
141On-disk details
142===============
143
144Summary
145-------
146Different from other read-only file systems, an EROFS volume is designed
147to be as simple as possible::
148
149 |-> aligned with the block size
150 ____________________________________________________________
151 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
152 |_|__|_|_____|__________|_____|______|__________|_____|______|
153 0 +1K
154
155All data areas should be aligned with the block size, but metadata areas
156may not. All metadatas can be now observed in two different spaces (views):
157
158 1. Inode metadata space
159
160 Each valid inode should be aligned with an inode slot, which is a fixed
161 value (32 bytes) and designed to be kept in line with compact inode size.
162
163 Each inode can be directly found with the following formula:
164 inode offset = meta_blkaddr * block_size + 32 * nid
165
166 ::
167
168 |-> aligned with 8B
169 |-> followed closely
170 + meta_blkaddr blocks |-> another slot
171 _____________________________________________________________________
172 | ... | inode | xattrs | extents | data inline | ... | inode ...
173 |________|_______|(optional)|(optional)|__(optional)_|_____|__________
174 |-> aligned with the inode slot size
175 . .
176 . .
177 . .
178 . .
179 . .
180 . .
181 .____________________________________________________|-> aligned with 4B
182 | xattr_ibody_header | shared xattrs | inline xattrs |
183 |____________________|_______________|_______________|
184 |-> 12 bytes <-|->x * 4 bytes<-| .
185 . . .
186 . . .
187 . . .
188 ._______________________________.______________________.
189 | id | id | id | id | ... | id | ent | ... | ent| ... |
190 |____|____|____|____|______|____|_____|_____|____|_____|
191 |-> aligned with 4B
192 |-> aligned with 4B
193
194 Inode could be 32 or 64 bytes, which can be distinguished from a common
195 field which all inode versions have -- i_format::
196
197 __________________ __________________
198 | i_format | | i_format |
199 |__________________| |__________________|
200 | ... | | ... |
201 | | | |
202 |__________________| 32 bytes | |
203 | |
204 |__________________| 64 bytes
205
206 Xattrs, extents, data inline are placed after the corresponding inode with
207 proper alignment, and they could be optional for different data mappings.
208 _currently_ total 5 data layouts are supported:
209
210 == ====================================================================
211 0 flat file data without data inline (no extent);
212 1 fixed-sized output data compression (with non-compacted indexes);
213 2 flat file data with tail packing data inline (no extent);
214 3 fixed-sized output data compression (with compacted indexes, v5.3+);
215 4 chunk-based file (v5.15+).
216 == ====================================================================
217
218 The size of the optional xattrs is indicated by i_xattr_count in inode
219 header. Large xattrs or xattrs shared by many different files can be
220 stored in shared xattrs metadata rather than inlined right after inode.
221
222 2. Shared xattrs metadata space
223
224 Shared xattrs space is similar to the above inode space, started with
225 a specific block indicated by xattr_blkaddr, organized one by one with
226 proper align.
227
228 Each share xattr can also be directly found by the following formula:
229 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
230
231::
232
233 |-> aligned by 4 bytes
234 + xattr_blkaddr blocks |-> aligned with 4 bytes
235 _________________________________________________________________________
236 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ...
237 |________|_____________|_____________|_____|______________|_______________
238
239Directories
240-----------
241All directories are now organized in a compact on-disk format. Note that
242each directory block is divided into index and name areas in order to support
243random file lookup, and all directory entries are _strictly_ recorded in
244alphabetical order in order to support improved prefix binary search
245algorithm (could refer to the related source code).
246
247::
248
249 ___________________________
250 / |
251 / ______________|________________
252 / / | nameoff1 | nameoffN-1
253 ____________.______________._______________v________________v__________
254 | dirent | dirent | ... | dirent | filename | filename | ... | filename |
255 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
256 \ ^
257 \ | * could have
258 \ | trailing '\0'
259 \________________________| nameoff0
260 Directory block
261
262Note that apart from the offset of the first filename, nameoff0 also indicates
263the total number of directory entries in this block since it is no need to
264introduce another on-disk field at all.
265
266Chunk-based files
267-----------------
268In order to support chunk-based data deduplication, a new inode data layout has
269been supported since Linux v5.15: Files are split in equal-sized data chunks
270with ``extents`` area of the inode metadata indicating how to get the chunk
271data: these can be simply as a 4-byte block address array or in the 8-byte
272chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more
273details.)
274
275By the way, chunk-based files are all uncompressed for now.
276
277Long extended attribute name prefixes
278-------------------------------------
279There are use cases where extended attributes with different values can have
280only a few common prefixes (such as overlayfs xattrs). The predefined prefixes
281work inefficiently in both image size and runtime performance in such cases.
282
283The long xattr name prefixes feature is introduced to address this issue. The
284overall idea is that, apart from the existing predefined prefixes, the xattr
285entry could also refer to user-specified long xattr name prefixes, e.g.
286"trusted.overlay.".
287
288When referring to a long xattr name prefix, the highest bit (bit 7) of
289erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole
290represent the index of the referred long name prefix among all long name
291prefixes. Therefore, only the trailing part of the name apart from the long
292xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if
293the full xattr name matches exactly as its long xattr name prefix.
294
295All long xattr prefixes are stored one by one in the packed inode as long as
296the packed inode is valid, or in the meta inode otherwise. The
297xattr_prefix_count (of the on-disk superblock) indicates the total number of
298long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start
299offset of long name prefixes in the packed/meta inode. Note that, long extended
300attribute name prefixes are disabled if xattr_prefix_count is 0.
301
302Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4),
303where len represents the total size of the data part. The data part is actually
304represented by 'struct erofs_xattr_long_prefix', where base_index represents the
305index of the predefined xattr name prefix, e.g. EROFS_XATTR_INDEX_TRUSTED for
306"trusted.overlay." long name prefix, while the infix string keeps the string
307after stripping the short prefix, e.g. "overlay." for the example above.
308
309Data compression
310----------------
311EROFS implements fixed-sized output compression which generates fixed-sized
312compressed data blocks from variable-sized input in contrast to other existing
313fixed-sized input solutions. Relatively higher compression ratios can be gotten
314by using fixed-sized output compression since nowadays popular data compression
315algorithms are mostly LZ77-based and such fixed-sized output approach can be
316benefited from the historical dictionary (aka. sliding window).
317
318In details, original (uncompressed) data is turned into several variable-sized
319extents and in the meanwhile, compressed into physical clusters (pclusters).
320In order to record each variable-sized extent, logical clusters (lclusters) are
321introduced as the basic unit of compress indexes to indicate whether a new
322extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now
323fixed in block size, as illustrated below::
324
325 |<- variable-sized extent ->|<- VLE ->|
326 clusterofs clusterofs clusterofs
327 | | |
328 _________v_________________________________v_______________________v________
329 ... | . | | . | | . ...
330 ____|____._________|______________|________.___ _|______________|__.________
331 |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-|
332 (HEAD) (NONHEAD) (HEAD) (NONHEAD) .
333 . CBLKCNT . .
334 . . .
335 . . .
336 _______._____________________________.______________._________________
337 ... | | | | ...
338 _______|______________|______________|______________|_________________
339 |-> big pcluster <-|-> pcluster <-|
340
341A physical cluster can be seen as a container of physical compressed blocks
342which contains compressed data. Previously, only lcluster-sized (4KB) pclusters
343were supported. After big pcluster feature is introduced (available since
344Linux v5.13), pcluster can be a multiple of lcluster size.
345
346For each HEAD lcluster, clusterofs is recorded to indicate where a new extent
347starts and blkaddr is used to seek the compressed data. For each NONHEAD
348lcluster, delta0 and delta1 are available instead of blkaddr to indicate the
349distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is
350also a HEAD lcluster except that its data is uncompressed. See the comments
351around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
352
353If big pcluster is enabled, pcluster size in lclusters needs to be recorded as
354well. Let the delta0 of the first NONHEAD lcluster store the compressed block
355count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy
356to understand its delta0 is constantly 1, as illustrated below::
357
358 __________________________________________________________
359 | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD |
360 |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_|
361 |<----- a big pcluster (with CBLKCNT) ------>|<-- -->|
362 a lcluster-sized pcluster (without CBLKCNT) ^
363
364If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
365but it's easy to know the size of such pcluster is 1 lcluster as well.
366
367Since Linux v6.1, each pcluster can be used for multiple variable-sized extents,
368therefore it can be used for compressed data deduplication.
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.