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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | ====================================== | |
6e95d0a0 | 4 | EROFS - Enhanced Read-Only File System |
e66d8631 MCC |
5 | ====================================== |
6 | ||
fdb05364 GX |
7 | Overview |
8 | ======== | |
9 | ||
6e95d0a0 GX |
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. | |
fdb05364 | 14 | |
6e95d0a0 GX |
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: | |
e66d8631 | 21 | |
fdb05364 GX |
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 | |
6e95d0a0 | 26 | their releases due to security or other considerations and |
fdb05364 | 27 | |
dfeab2e9 GX |
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 | |
6e95d0a0 | 31 | hosts with numerous containers. |
fdb05364 | 32 | |
2109901d | 33 | Here are the main features of EROFS: |
e66d8631 | 34 | |
fdb05364 GX |
35 | - Little endian on-disk design; |
36 | ||
2109901d GX |
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 | - 4KiB block size and 32-bit block addresses for each device, therefore | |
44 | 16TiB address space at most for now; | |
fdb05364 | 45 | |
6e95d0a0 | 46 | - Two inode layouts for different requirements: |
e66d8631 | 47 | |
6e95d0a0 | 48 | ===================== ============ ====================================== |
ffafde47 | 49 | compact (v1) extended (v2) |
6e95d0a0 | 50 | ===================== ============ ====================================== |
e66d8631 | 51 | Inode metadata size 32 bytes 64 bytes |
6e95d0a0 | 52 | Max file size 4 GiB 16 EiB (also limited by max. vol size) |
e66d8631 | 53 | Max uids/gids 65536 4294967296 |
a1108dcd | 54 | Per-inode timestamp no yes (64 + 32-bit timestamp) |
e66d8631 | 55 | Max hardlinks 65536 4294967296 |
6e95d0a0 GX |
56 | Metadata reserved 8 bytes 18 bytes |
57 | ===================== ============ ====================================== | |
58 | ||
2109901d | 59 | - Support extended attributes as an option; |
fdb05364 | 60 | |
2109901d | 61 | - Support POSIX.1e ACLs by using extended attributes; |
516c115c | 62 | |
46f2e044 | 63 | - Support transparent data compression as an option: |
6e95d0a0 GX |
64 | LZ4 and MicroLZMA algorithms can be used on a per-file basis; In addition, |
65 | inplace decompression is also supported to avoid bounce compressed buffers | |
66 | and page cache thrashing. | |
67 | ||
2109901d GX |
68 | - Support chunk-based data deduplication and rolling-hash compressed data |
69 | deduplication; | |
70 | ||
71 | - Support tailpacking inline compared to byte-addressed unaligned metadata | |
72 | or smaller block size alternatives; | |
73 | ||
74 | - Support merging tail-end data into a special inode as fragments. | |
75 | ||
e6687b89 JX |
76 | - Support large folios for uncompressed files. |
77 | ||
6e95d0a0 GX |
78 | - Support direct I/O on uncompressed files to avoid double caching for loop |
79 | devices; | |
dfeab2e9 | 80 | |
6e95d0a0 GX |
81 | - Support FSDAX on uncompressed images for secure containers and ramdisks in |
82 | order to get rid of unnecessary page cache. | |
83 | ||
6e95d0a0 | 84 | - Support file-based on-demand loading with the Fscache infrastructure. |
fdb05364 GX |
85 | |
86 | The following git tree provides the file system user-space tools under | |
6e95d0a0 GX |
87 | development, such as a formatting tool (mkfs.erofs), an on-disk consistency & |
88 | compatibility checking tool (fsck.erofs), and a debugging tool (dump.erofs): | |
e66d8631 MCC |
89 | |
90 | - git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git | |
fdb05364 GX |
91 | |
92 | Bugs and patches are welcome, please kindly help us and send to the following | |
93 | linux-erofs mailing list: | |
e66d8631 MCC |
94 | |
95 | - linux-erofs mailing list <linux-erofs@lists.ozlabs.org> | |
fdb05364 | 96 | |
fdb05364 GX |
97 | Mount options |
98 | ============= | |
99 | ||
e66d8631 | 100 | =================== ========================================================= |
fdb05364 GX |
101 | (no)user_xattr Setup Extended User Attributes. Note: xattr is enabled |
102 | by default if CONFIG_EROFS_FS_XATTR is selected. | |
103 | (no)acl Setup POSIX Access Control List. Note: acl is enabled | |
104 | by default if CONFIG_EROFS_FS_POSIX_ACL is selected. | |
4279f3f9 | 105 | cache_strategy=%s Select a strategy for cached decompression from now on: |
e66d8631 MCC |
106 | |
107 | ========== ============================================= | |
108 | disabled In-place I/O decompression only; | |
109 | readahead Cache the last incomplete compressed physical | |
4279f3f9 GX |
110 | cluster for further reading. It still does |
111 | in-place I/O decompression for the rest | |
112 | compressed physical clusters; | |
e66d8631 | 113 | readaround Cache the both ends of incomplete compressed |
4279f3f9 GX |
114 | physical clusters for further reading. |
115 | It still does in-place I/O decompression | |
116 | for the rest compressed physical clusters. | |
e66d8631 | 117 | ========== ============================================= |
06252e9c GX |
118 | dax={always,never} Use direct access (no page cache). See |
119 | Documentation/filesystems/dax.rst. | |
120 | dax A legacy option which is an alias for ``dax=always``. | |
dfeab2e9 | 121 | device=%s Specify a path to an extra device to be used together. |
6e95d0a0 | 122 | fsid=%s Specify a filesystem image ID for Fscache back-end. |
b22c7b97 JX |
123 | domain_id=%s Specify a domain ID in fscache mode so that different images |
124 | with the same blobs under a given domain ID can share storage. | |
e66d8631 | 125 | =================== ========================================================= |
fdb05364 | 126 | |
168e9a76 HJ |
127 | Sysfs Entries |
128 | ============= | |
129 | ||
130 | Information about mounted erofs file systems can be found in /sys/fs/erofs. | |
131 | Each mounted filesystem will have a directory in /sys/fs/erofs based on its | |
132 | device name (i.e., /sys/fs/erofs/sda). | |
133 | (see also Documentation/ABI/testing/sysfs-fs-erofs) | |
134 | ||
fdb05364 GX |
135 | On-disk details |
136 | =============== | |
137 | ||
138 | Summary | |
139 | ------- | |
140 | Different from other read-only file systems, an EROFS volume is designed | |
e66d8631 | 141 | to be as simple as possible:: |
fdb05364 GX |
142 | |
143 | |-> aligned with the block size | |
144 | ____________________________________________________________ | |
145 | | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data | | |
146 | |_|__|_|_____|__________|_____|______|__________|_____|______| | |
147 | 0 +1K | |
148 | ||
149 | All data areas should be aligned with the block size, but metadata areas | |
150 | may not. All metadatas can be now observed in two different spaces (views): | |
e66d8631 | 151 | |
fdb05364 | 152 | 1. Inode metadata space |
e66d8631 | 153 | |
fdb05364 | 154 | Each valid inode should be aligned with an inode slot, which is a fixed |
ffafde47 | 155 | value (32 bytes) and designed to be kept in line with compact inode size. |
fdb05364 GX |
156 | |
157 | Each inode can be directly found with the following formula: | |
158 | inode offset = meta_blkaddr * block_size + 32 * nid | |
159 | ||
e66d8631 MCC |
160 | :: |
161 | ||
1b55767d GX |
162 | |-> aligned with 8B |
163 | |-> followed closely | |
164 | + meta_blkaddr blocks |-> another slot | |
165 | _____________________________________________________________________ | |
166 | | ... | inode | xattrs | extents | data inline | ... | inode ... | |
167 | |________|_______|(optional)|(optional)|__(optional)_|_____|__________ | |
168 | |-> aligned with the inode slot size | |
169 | . . | |
170 | . . | |
171 | . . | |
172 | . . | |
173 | . . | |
174 | . . | |
175 | .____________________________________________________|-> aligned with 4B | |
176 | | xattr_ibody_header | shared xattrs | inline xattrs | | |
177 | |____________________|_______________|_______________| | |
178 | |-> 12 bytes <-|->x * 4 bytes<-| . | |
179 | . . . | |
180 | . . . | |
181 | . . . | |
182 | ._______________________________.______________________. | |
183 | | id | id | id | id | ... | id | ent | ... | ent| ... | | |
184 | |____|____|____|____|______|____|_____|_____|____|_____| | |
185 | |-> aligned with 4B | |
186 | |-> aligned with 4B | |
fdb05364 GX |
187 | |
188 | Inode could be 32 or 64 bytes, which can be distinguished from a common | |
e66d8631 | 189 | field which all inode versions have -- i_format:: |
fdb05364 GX |
190 | |
191 | __________________ __________________ | |
ffafde47 | 192 | | i_format | | i_format | |
fdb05364 GX |
193 | |__________________| |__________________| |
194 | | ... | | ... | | |
195 | | | | | | |
196 | |__________________| 32 bytes | | | |
197 | | | | |
198 | |__________________| 64 bytes | |
199 | ||
200 | Xattrs, extents, data inline are followed by the corresponding inode with | |
ffafde47 | 201 | proper alignment, and they could be optional for different data mappings. |
2a9dc7a8 | 202 | _currently_ total 5 data layouts are supported: |
fdb05364 | 203 | |
e66d8631 | 204 | == ==================================================================== |
ffafde47 GX |
205 | 0 flat file data without data inline (no extent); |
206 | 1 fixed-sized output data compression (with non-compacted indexes); | |
207 | 2 flat file data with tail packing data inline (no extent); | |
2a9dc7a8 GX |
208 | 3 fixed-sized output data compression (with compacted indexes, v5.3+); |
209 | 4 chunk-based file (v5.15+). | |
e66d8631 | 210 | == ==================================================================== |
fdb05364 GX |
211 | |
212 | The size of the optional xattrs is indicated by i_xattr_count in inode | |
213 | header. Large xattrs or xattrs shared by many different files can be | |
214 | stored in shared xattrs metadata rather than inlined right after inode. | |
215 | ||
216 | 2. Shared xattrs metadata space | |
e66d8631 | 217 | |
fdb05364 GX |
218 | Shared xattrs space is similar to the above inode space, started with |
219 | a specific block indicated by xattr_blkaddr, organized one by one with | |
220 | proper align. | |
221 | ||
222 | Each share xattr can also be directly found by the following formula: | |
223 | xattr offset = xattr_blkaddr * block_size + 4 * xattr_id | |
224 | ||
1b55767d | 225 | :: |
e66d8631 | 226 | |
1b55767d GX |
227 | |-> aligned by 4 bytes |
228 | + xattr_blkaddr blocks |-> aligned with 4 bytes | |
229 | _________________________________________________________________________ | |
230 | | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ... | |
231 | |________|_____________|_____________|_____|______________|_______________ | |
fdb05364 GX |
232 | |
233 | Directories | |
234 | ----------- | |
235 | All directories are now organized in a compact on-disk format. Note that | |
236 | each directory block is divided into index and name areas in order to support | |
237 | random file lookup, and all directory entries are _strictly_ recorded in | |
238 | alphabetical order in order to support improved prefix binary search | |
239 | algorithm (could refer to the related source code). | |
240 | ||
e66d8631 MCC |
241 | :: |
242 | ||
1b55767d GX |
243 | ___________________________ |
244 | / | | |
245 | / ______________|________________ | |
246 | / / | nameoff1 | nameoffN-1 | |
247 | ____________.______________._______________v________________v__________ | |
248 | | dirent | dirent | ... | dirent | filename | filename | ... | filename | | |
249 | |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____| | |
250 | \ ^ | |
251 | \ | * could have | |
252 | \ | trailing '\0' | |
253 | \________________________| nameoff0 | |
254 | Directory block | |
fdb05364 GX |
255 | |
256 | Note that apart from the offset of the first filename, nameoff0 also indicates | |
257 | the total number of directory entries in this block since it is no need to | |
258 | introduce another on-disk field at all. | |
259 | ||
6e95d0a0 GX |
260 | Chunk-based files |
261 | ----------------- | |
2a9dc7a8 GX |
262 | In order to support chunk-based data deduplication, a new inode data layout has |
263 | been supported since Linux v5.15: Files are split in equal-sized data chunks | |
264 | with ``extents`` area of the inode metadata indicating how to get the chunk | |
265 | data: these can be simply as a 4-byte block address array or in the 8-byte | |
266 | chunk index form (see struct erofs_inode_chunk_index in erofs_fs.h for more | |
267 | details.) | |
268 | ||
269 | By the way, chunk-based files are all uncompressed for now. | |
270 | ||
46f2e044 GX |
271 | Data compression |
272 | ---------------- | |
2109901d | 273 | EROFS implements fixed-sized output compression which generates fixed-sized |
46f2e044 GX |
274 | compressed data blocks from variable-sized input in contrast to other existing |
275 | fixed-sized input solutions. Relatively higher compression ratios can be gotten | |
276 | by using fixed-sized output compression since nowadays popular data compression | |
277 | algorithms are mostly LZ77-based and such fixed-sized output approach can be | |
278 | benefited from the historical dictionary (aka. sliding window). | |
279 | ||
280 | In details, original (uncompressed) data is turned into several variable-sized | |
281 | extents and in the meanwhile, compressed into physical clusters (pclusters). | |
282 | In order to record each variable-sized extent, logical clusters (lclusters) are | |
283 | introduced as the basic unit of compress indexes to indicate whether a new | |
284 | extent is generated within the range (HEAD) or not (NONHEAD). Lclusters are now | |
285 | fixed in block size, as illustrated below:: | |
e66d8631 | 286 | |
1b55767d GX |
287 | |<- variable-sized extent ->|<- VLE ->| |
288 | clusterofs clusterofs clusterofs | |
289 | | | | | |
290 | _________v_________________________________v_______________________v________ | |
291 | ... | . | | . | | . ... | |
292 | ____|____._________|______________|________.___ _|______________|__.________ | |
293 | |-> lcluster <-|-> lcluster <-|-> lcluster <-|-> lcluster <-| | |
46f2e044 GX |
294 | (HEAD) (NONHEAD) (HEAD) (NONHEAD) . |
295 | . CBLKCNT . . | |
296 | . . . | |
297 | . . . | |
298 | _______._____________________________.______________._________________ | |
1b55767d GX |
299 | ... | | | | ... |
300 | _______|______________|______________|______________|_________________ | |
46f2e044 GX |
301 | |-> big pcluster <-|-> pcluster <-| |
302 | ||
303 | A physical cluster can be seen as a container of physical compressed blocks | |
304 | which contains compressed data. Previously, only lcluster-sized (4KB) pclusters | |
305 | were supported. After big pcluster feature is introduced (available since | |
306 | Linux v5.13), pcluster can be a multiple of lcluster size. | |
307 | ||
308 | For each HEAD lcluster, clusterofs is recorded to indicate where a new extent | |
309 | starts and blkaddr is used to seek the compressed data. For each NONHEAD | |
310 | lcluster, delta0 and delta1 are available instead of blkaddr to indicate the | |
311 | distance to its HEAD lcluster and the next HEAD lcluster. A PLAIN lcluster is | |
312 | also a HEAD lcluster except that its data is uncompressed. See the comments | |
313 | around "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details. | |
314 | ||
315 | If big pcluster is enabled, pcluster size in lclusters needs to be recorded as | |
316 | well. Let the delta0 of the first NONHEAD lcluster store the compressed block | |
317 | count with a special flag as a new called CBLKCNT NONHEAD lcluster. It's easy | |
318 | to understand its delta0 is constantly 1, as illustrated below:: | |
319 | ||
320 | __________________________________________________________ | |
321 | | HEAD | NONHEAD | NONHEAD | ... | NONHEAD | HEAD | HEAD | | |
322 | |__:___|_(CBLKCNT)_|_________|_____|_________|__:___|____:_| | |
323 | |<----- a big pcluster (with CBLKCNT) ------>|<-- -->| | |
324 | a lcluster-sized pcluster (without CBLKCNT) ^ | |
325 | ||
326 | If another HEAD follows a HEAD lcluster, there is no room to record CBLKCNT, | |
327 | but it's easy to know the size of such pcluster is 1 lcluster as well. | |
2109901d GX |
328 | |
329 | Since Linux v6.1, each pcluster can be used for multiple variable-sized extents, | |
330 | therefore it can be used for compressed data deduplication. |