4 EROFS file-system stands for Enhanced Read-Only File System. Different
5 from other read-only file systems, it aims to be designed for flexibility,
6 scalability, but be kept simple and high performance.
8 It is designed as a better filesystem solution for the following scenarios:
9 - read-only storage media or
11 - part of a fully trusted read-only solution, which means it needs to be
12 immutable and bit-for-bit identical to the official golden image for
13 their releases due to security and other considerations and
15 - hope to save some extra storage space with guaranteed end-to-end performance
16 by using reduced metadata and transparent file compression, especially
17 for those embedded devices with limited memory (ex, smartphone);
19 Here is the main features of EROFS:
20 - Little endian on-disk design;
22 - Currently 4KB block size (nobh) and therefore maximum 16TB address space;
24 - Metadata & data could be mixed by design;
26 - 2 inode versions for different requirements:
28 Inode metadata size: 32 bytes 64 bytes
29 Max file size: 4 GB 16 EB (also limited by max. vol size)
30 Max uids/gids: 65536 4294967296
31 File creation time: no yes (64 + 32-bit timestamp)
32 Max hardlinks: 65536 4294967296
33 Metadata reserved: 4 bytes 14 bytes
35 - Support extended attributes (xattrs) as an option;
37 - Support xattr inline and tail-end data inline for all files;
39 - Support POSIX.1e ACLs by using xattrs;
41 - Support transparent file compression as an option:
42 LZ4 algorithm with 4 KB fixed-output compression for high performance;
44 The following git tree provides the file system user-space tools under
45 development (ex, formatting tool mkfs.erofs):
46 >> git://git.kernel.org/pub/scm/linux/kernel/git/xiang/erofs-utils.git
48 Bugs and patches are welcome, please kindly help us and send to the following
49 linux-erofs mailing list:
50 >> linux-erofs mailing list <linux-erofs@lists.ozlabs.org>
55 (no)user_xattr Setup Extended User Attributes. Note: xattr is enabled
56 by default if CONFIG_EROFS_FS_XATTR is selected.
57 (no)acl Setup POSIX Access Control List. Note: acl is enabled
58 by default if CONFIG_EROFS_FS_POSIX_ACL is selected.
59 cache_strategy=%s Select a strategy for cached decompression from now on:
60 disabled: In-place I/O decompression only;
61 readahead: Cache the last incomplete compressed physical
62 cluster for further reading. It still does
63 in-place I/O decompression for the rest
64 compressed physical clusters;
65 readaround: Cache the both ends of incomplete compressed
66 physical clusters for further reading.
67 It still does in-place I/O decompression
68 for the rest compressed physical clusters.
75 Different from other read-only file systems, an EROFS volume is designed
76 to be as simple as possible:
78 |-> aligned with the block size
79 ____________________________________________________________
80 | |SB| | ... | Metadata | ... | Data | Metadata | ... | Data |
81 |_|__|_|_____|__________|_____|______|__________|_____|______|
84 All data areas should be aligned with the block size, but metadata areas
85 may not. All metadatas can be now observed in two different spaces (views):
86 1. Inode metadata space
87 Each valid inode should be aligned with an inode slot, which is a fixed
88 value (32 bytes) and designed to be kept in line with v1 inode size.
90 Each inode can be directly found with the following formula:
91 inode offset = meta_blkaddr * block_size + 32 * nid
95 + meta_blkaddr blocks |-> another slot
96 _____________________________________________________________________
97 | ... | inode | xattrs | extents | data inline | ... | inode ...
98 |________|_______|(optional)|(optional)|__(optional)_|_____|__________
99 |-> aligned with the inode slot size
106 .____________________________________________________|-> aligned with 4B
107 | xattr_ibody_header | shared xattrs | inline xattrs |
108 |____________________|_______________|_______________|
109 |-> 12 bytes <-|->x * 4 bytes<-| .
113 ._______________________________.______________________.
114 | id | id | id | id | ... | id | ent | ... | ent| ... |
115 |____|____|____|____|______|____|_____|_____|____|_____|
119 Inode could be 32 or 64 bytes, which can be distinguished from a common
120 field which all inode versions have -- i_advise:
122 __________________ __________________
123 | i_advise | | i_advise |
124 |__________________| |__________________|
127 |__________________| 32 bytes | |
129 |__________________| 64 bytes
131 Xattrs, extents, data inline are followed by the corresponding inode with
132 proper alignes, and they could be optional for different data mappings,
133 _currently_ there are totally 3 valid data mappings supported:
135 1) flat file data without data inline (no extent);
136 2) fixed-output size data compression (must have extents);
137 3) flat file data with tail-end data inline (no extent);
139 The size of the optional xattrs is indicated by i_xattr_count in inode
140 header. Large xattrs or xattrs shared by many different files can be
141 stored in shared xattrs metadata rather than inlined right after inode.
143 2. Shared xattrs metadata space
144 Shared xattrs space is similar to the above inode space, started with
145 a specific block indicated by xattr_blkaddr, organized one by one with
148 Each share xattr can also be directly found by the following formula:
149 xattr offset = xattr_blkaddr * block_size + 4 * xattr_id
151 |-> aligned by 4 bytes
152 + xattr_blkaddr blocks |-> aligned with 4 bytes
153 _________________________________________________________________________
154 | ... | xattr_entry | xattr data | ... | xattr_entry | xattr data ...
155 |________|_____________|_____________|_____|______________|_______________
159 All directories are now organized in a compact on-disk format. Note that
160 each directory block is divided into index and name areas in order to support
161 random file lookup, and all directory entries are _strictly_ recorded in
162 alphabetical order in order to support improved prefix binary search
163 algorithm (could refer to the related source code).
165 ___________________________
167 / ______________|________________
168 / / | nameoff1 | nameoffN-1
169 ____________.______________._______________v________________v__________
170 | dirent | dirent | ... | dirent | filename | filename | ... | filename |
171 |___.0___|____1___|_____|___N-1__|____0_____|____1_____|_____|___N-1____|
175 \________________________| nameoff0
179 Note that apart from the offset of the first filename, nameoff0 also indicates
180 the total number of directory entries in this block since it is no need to
181 introduce another on-disk field at all.
185 Currently, EROFS supports 4KB fixed-output clustersize transparent file
186 compression, as illustrated below:
188 |---- Variant-Length Extent ----|-------- VLE --------|----- VLE -----
189 clusterofs clusterofs clusterofs
191 _________v_______________________________v_____________________v_______________
192 ... | . | | . | | . | ...
193 ____|____.________|_____________|________.____|_____________|__.__________|____
194 |-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|-> cluster <-|
195 size size size size size
199 _______._____________._____________._____________._____________________
200 ... | | | | ... physical data
201 _______|_____________|_____________|_____________|_____________________
202 |-> cluster <-|-> cluster <-|-> cluster <-|
205 Currently each on-disk physical cluster can contain 4KB (un)compressed data
206 at most. For each logical cluster, there is a corresponding on-disk index to
207 describe its cluster type, physical cluster address, etc.
209 See "struct z_erofs_vle_decompressed_index" in erofs_fs.h for more details.