1 .. SPDX-License-Identifier: GPL-2.0
2
3 ======================================
4 EROFS - Enhanced Read-Only File System
5 ======================================
6
7 Overview
8 ========
9
10 EROFS filesystem stands for Enhanced Read-Only File System. It aims to form a
11 generic read-only filesystem solution for various read-only use cases instead
12 of just focusing on storage space saving without considering any side effects
13 of runtime performance.
14
15 It is designed to meet the needs of flexibility, feature extendability and user
16 payload friendly, etc. Apart from those, it is still kept as a simple
17 random-access friendly high-performance filesystem to get rid of unneeded I/O
18 amplification and memory-resident overhead compared to similar approaches.
19
20 It 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
33 Here 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
88 The following git tree provides the file system user-space tools under
89 development, such as a formatting tool (mkfs.erofs), an on-disk consistency &
90 compatibility 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
94 For more information, please also refer to the documentation site:
95
96 - https://erofs.docs.kernel.org
97
98 Bugs and patches are welcome, please kindly help us and send to the following
99 linux-erofs mailing list:
100
101 - linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
102
103 Mount 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.
111 cache_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 ========== =============================================
124 dax={always,never} Use direct access (no page cache). See
125 Documentation/filesystems/dax.rst.
126 dax A legacy option which is an alias for ``dax=always``.
127 device=%s Specify a path to an extra device to be used together.
128 fsid=%s Specify a filesystem image ID for Fscache back-end.
129 domain_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
133 Sysfs Entries
134 =============
135
136 Information about mounted erofs file systems can be found in /sys/fs/erofs.
137 Each mounted filesystem will have a directory in /sys/fs/erofs based on its
138 device name (i.e., /sys/fs/erofs/sda).
139 (see also Documentation/ABI/testing/sysfs-fs-erofs)
140
141 On-disk details
142 ===============
143
144 Summary
145 -------
146 Different from other read-only file systems, an EROFS volume is designed
147 to be as simple as possible::
148
149 |-> aligned with the block size
150 ____________________________________________________________
151 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
152 |_|__|_|_____|__________|_____|______|__________|_____|______|
153 0 +1K
154
155 All data areas should be aligned with the block size, but metadata areas
156 may 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
239 Directories
240 -----------
241 All directories are now organized in a compact on-disk format. Note that
242 each directory block is divided into index and name areas in order to support
243 random file lookup, and all directory entries are _strictly_ recorded in
244 alphabetical order in order to support improved prefix binary search
245 algorithm (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
262 Note that apart from the offset of the first filename, nameoff0 also indicates
263 the total number of directory entries in this block since it is no need to
264 introduce another on-disk field at all.
265
266 Chunk-based files
267 -----------------
268 In order to support chunk-based data deduplication, a new inode data layout has
269 been supported since Linux v5.15: Files are split in equal-sized data chunks
270 with ``extents`` area of the inode metadata indicating how to get the chunk
271 data: these can be simply as a 4-byte block address array or in the 8-byte
272 chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more
273 details.)
274
275 By the way, chunk-based files are all uncompressed for now.
276
277 Long extended attribute name prefixes
278 -------------------------------------
279 There are use cases where extended attributes with different values can have
280 only a few common prefixes (such as overlayfs xattrs). The predefined prefixes
281 work inefficiently in both image size and runtime performance in such cases.
282
283 The long xattr name prefixes feature is introduced to address this issue. The
284 overall idea is that, apart from the existing predefined prefixes, the xattr
285 entry could also refer to user-specified long xattr name prefixes, e.g.
286 "trusted.overlay.".
287
288 When referring to a long xattr name prefix, the highest bit (bit 7) of
289 erofs_xattr_entry.e_name_index is set, while the lower bits (bit 0-6) as a whole
290 represent the index of the referred long name prefix among all long name
291 prefixes. Therefore, only the trailing part of the name apart from the long
292 xattr name prefix is stored in erofs_xattr_entry.e_name, which could be empty if
293 the full xattr name matches exactly as its long xattr name prefix.
294
295 All long xattr prefixes are stored one by one in the packed inode as long as
296 the packed inode is valid, or in the meta inode otherwise. The
297 xattr_prefix_count (of the on-disk superblock) indicates the total number of
298 long xattr name prefixes, while (xattr_prefix_start * 4) indicates the start
299 offset of long name prefixes in the packed/meta inode. Note that, long extended
300 attribute name prefixes are disabled if xattr_prefix_count is 0.
301
302 Each long name prefix is stored in the format: ALIGN({__le16 len, data}, 4),
303 where len represents the total size of the data part. The data part is actually
304 represented by 'struct erofs_xattr_long_prefix', where base_index represents the
305 index 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
307 after stripping the short prefix, e.g. "overlay." for the example above.
308
309 Data compression
310 ----------------
311 EROFS implements fixed-sized output compression which generates fixed-sized
312 compressed data blocks from variable-sized input in contrast to other existing
313 fixed-sized input solutions. Relatively higher compression ratios can be gotten
314 by using fixed-sized output compression since nowadays popular data compression
315 algorithms are mostly LZ77-based and such fixed-sized output approach can be
316 benefited from the historical dictionary (aka. sliding window).
317
318 In details, original (uncompressed) data is turned into several variable-sized
319 extents and in the meanwhile, compressed into physical clusters (pclusters).
320 In order to record each variable-sized extent, logical clusters (lclusters) are
321 introduced as the basic unit of compress indexes to indicate whether a new
322 extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now
323 fixed 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
341 A physical cluster can be seen as a container of physical compressed blocks
342 which contains compressed data. Previously, only lcluster-sized (4KB) pclusters
343 were supported. After big pcluster feature is introduced (available since
344 Linux v5.13), pcluster can be a multiple of lcluster size.
345
346 For each HEAD lcluster, clusterofs is recorded to indicate where a new extent
347 starts and blkaddr is used to seek the compressed data. For each NONHEAD
348 lcluster, delta0 and delta1 are available instead of blkaddr to indicate the
349 distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is
350 also a HEAD lcluster except that its data is uncompressed. See the comments
351 around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.
352
353 If big pcluster is enabled, pcluster size in lclusters needs to be recorded as
354 well. Let the delta0 of the first NONHEAD lcluster store the compressed block
355 count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy
356 to 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
364 If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT,
365 but it's easy to know the size of such pcluster is 1 lcluster as well.
366
367 Since Linux v6.1, each pcluster can be used for multiple variable-sized extents,
368 therefore it can be used for compressed data deduplication.
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