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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | ======================= | |
4 | ROMFS - ROM File System | |
5 | ======================= | |
1da177e4 LT |
6 | |
7 | This is a quite dumb, read only filesystem, mainly for initial RAM | |
8 | disks of installation disks. It has grown up by the need of having | |
9 | modules linked at boot time. Using this filesystem, you get a very | |
10 | similar feature, and even the possibility of a small kernel, with a | |
11 | file system which doesn't take up useful memory from the router | |
12 | functions in the basement of your office. | |
13 | ||
14 | For comparison, both the older minix and xiafs (the latter is now | |
15 | defunct) filesystems, compiled as module need more than 20000 bytes, | |
16 | while romfs is less than a page, about 4000 bytes (assuming i586 | |
17 | code). Under the same conditions, the msdos filesystem would need | |
18 | about 30K (and does not support device nodes or symlinks), while the | |
19 | nfs module with nfsroot is about 57K. Furthermore, as a bit unfair | |
20 | comparison, an actual rescue disk used up 3202 blocks with ext2, while | |
21 | with romfs, it needed 3079 blocks. | |
22 | ||
23 | To create such a file system, you'll need a user program named | |
ddf12286 | 24 | genromfs. It is available on http://romfs.sourceforge.net/ |
1da177e4 LT |
25 | |
26 | As the name suggests, romfs could be also used (space-efficiently) on | |
27 | various read-only media, like (E)EPROM disks if someone will have the | |
28 | motivation.. :) | |
29 | ||
30 | However, the main purpose of romfs is to have a very small kernel, | |
31 | which has only this filesystem linked in, and then can load any module | |
32 | later, with the current module utilities. It can also be used to run | |
33 | some program to decide if you need SCSI devices, and even IDE or | |
34 | floppy drives can be loaded later if you use the "initrd"--initial | |
35 | RAM disk--feature of the kernel. This would not be really news | |
36 | flash, but with romfs, you can even spare off your ext2 or minix or | |
37 | maybe even affs filesystem until you really know that you need it. | |
38 | ||
39 | For example, a distribution boot disk can contain only the cd disk | |
40 | drivers (and possibly the SCSI drivers), and the ISO 9660 filesystem | |
41 | module. The kernel can be small enough, since it doesn't have other | |
42 | filesystems, like the quite large ext2fs module, which can then be | |
43 | loaded off the CD at a later stage of the installation. Another use | |
44 | would be for a recovery disk, when you are reinstalling a workstation | |
45 | from the network, and you will have all the tools/modules available | |
46 | from a nearby server, so you don't want to carry two disks for this | |
47 | purpose, just because it won't fit into ext2. | |
48 | ||
49 | romfs operates on block devices as you can expect, and the underlying | |
50 | structure is very simple. Every accessible structure begins on 16 | |
51 | byte boundaries for fast access. The minimum space a file will take | |
52 | is 32 bytes (this is an empty file, with a less than 16 character | |
53 | name). The maximum overhead for any non-empty file is the header, and | |
54 | the 16 byte padding for the name and the contents, also 16+14+15 = 45 | |
55 | bytes. This is quite rare however, since most file names are longer | |
56 | than 3 bytes, and shorter than 15 bytes. | |
57 | ||
6db0a480 | 58 | The layout of the filesystem is the following:: |
1da177e4 | 59 | |
6db0a480 | 60 | offset content |
1da177e4 LT |
61 | |
62 | +---+---+---+---+ | |
63 | 0 | - | r | o | m | \ | |
64 | +---+---+---+---+ The ASCII representation of those bytes | |
65 | 4 | 1 | f | s | - | / (i.e. "-rom1fs-") | |
66 | +---+---+---+---+ | |
67 | 8 | full size | The number of accessible bytes in this fs. | |
68 | +---+---+---+---+ | |
69 | 12 | checksum | The checksum of the FIRST 512 BYTES. | |
70 | +---+---+---+---+ | |
71 | 16 | volume name | The zero terminated name of the volume, | |
72 | : : padded to 16 byte boundary. | |
73 | +---+---+---+---+ | |
74 | xx | file | | |
75 | : headers : | |
76 | ||
77 | Every multi byte value (32 bit words, I'll use the longwords term from | |
78 | now on) must be in big endian order. | |
79 | ||
80 | The first eight bytes identify the filesystem, even for the casual | |
81 | inspector. After that, in the 3rd longword, it contains the number of | |
82 | bytes accessible from the start of this filesystem. The 4th longword | |
83 | is the checksum of the first 512 bytes (or the number of bytes | |
84 | accessible, whichever is smaller). The applied algorithm is the same | |
85 | as in the AFFS filesystem, namely a simple sum of the longwords | |
86 | (assuming bigendian quantities again). For details, please consult | |
87 | the source. This algorithm was chosen because although it's not quite | |
88 | reliable, it does not require any tables, and it is very simple. | |
89 | ||
90 | The following bytes are now part of the file system; each file header | |
6db0a480 | 91 | must begin on a 16 byte boundary:: |
1da177e4 | 92 | |
6db0a480 | 93 | offset content |
1da177e4 LT |
94 | |
95 | +---+---+---+---+ | |
96 | 0 | next filehdr|X| The offset of the next file header | |
97 | +---+---+---+---+ (zero if no more files) | |
98 | 4 | spec.info | Info for directories/hard links/devices | |
99 | +---+---+---+---+ | |
100 | 8 | size | The size of this file in bytes | |
101 | +---+---+---+---+ | |
102 | 12 | checksum | Covering the meta data, including the file | |
103 | +---+---+---+---+ name, and padding | |
104 | 16 | file name | The zero terminated name of the file, | |
105 | : : padded to 16 byte boundary | |
106 | +---+---+---+---+ | |
107 | xx | file data | | |
108 | : : | |
109 | ||
110 | Since the file headers begin always at a 16 byte boundary, the lowest | |
111 | 4 bits would be always zero in the next filehdr pointer. These four | |
112 | bits are used for the mode information. Bits 0..2 specify the type of | |
113 | the file; while bit 4 shows if the file is executable or not. The | |
114 | permissions are assumed to be world readable, if this bit is not set, | |
115 | and world executable if it is; except the character and block devices, | |
116 | they are never accessible for other than owner. The owner of every | |
117 | file is user and group 0, this should never be a problem for the | |
118 | intended use. The mapping of the 8 possible values to file types is | |
119 | the following: | |
120 | ||
6db0a480 | 121 | == =============== ============================================ |
1da177e4 | 122 | mapping spec.info means |
6db0a480 | 123 | == =============== ============================================ |
1da177e4 LT |
124 | 0 hard link link destination [file header] |
125 | 1 directory first file's header | |
126 | 2 regular file unused, must be zero [MBZ] | |
127 | 3 symbolic link unused, MBZ (file data is the link content) | |
128 | 4 block device 16/16 bits major/minor number | |
129 | 5 char device - " - | |
130 | 6 socket unused, MBZ | |
131 | 7 fifo unused, MBZ | |
6db0a480 | 132 | == =============== ============================================ |
1da177e4 LT |
133 | |
134 | Note that hard links are specifically marked in this filesystem, but | |
135 | they will behave as you can expect (i.e. share the inode number). | |
136 | Note also that it is your responsibility to not create hard link | |
137 | loops, and creating all the . and .. links for directories. This is | |
138 | normally done correctly by the genromfs program. Please refrain from | |
139 | using the executable bits for special purposes on the socket and fifo | |
140 | special files, they may have other uses in the future. Additionally, | |
141 | please remember that only regular files, and symlinks are supposed to | |
142 | have a nonzero size field; they contain the number of bytes available | |
143 | directly after the (padded) file name. | |
144 | ||
145 | Another thing to note is that romfs works on file headers and data | |
146 | aligned to 16 byte boundaries, but most hardware devices and the block | |
147 | device drivers are unable to cope with smaller than block-sized data. | |
148 | To overcome this limitation, the whole size of the file system must be | |
149 | padded to an 1024 byte boundary. | |
150 | ||
151 | If you have any problems or suggestions concerning this file system, | |
152 | please contact me. However, think twice before wanting me to add | |
153 | features and code, because the primary and most important advantage of | |
154 | this file system is the small code. On the other hand, don't be | |
155 | alarmed, I'm not getting that much romfs related mail. Now I can | |
156 | understand why Avery wrote poems in the ARCnet docs to get some more | |
157 | feedback. :) | |
158 | ||
159 | romfs has also a mailing list, and to date, it hasn't received any | |
160 | traffic, so you are welcome to join it to discuss your ideas. :) | |
161 | ||
162 | It's run by ezmlm, so you can subscribe to it by sending a message | |
163 | to romfs-subscribe@shadow.banki.hu, the content is irrelevant. | |
164 | ||
165 | Pending issues: | |
166 | ||
167 | - Permissions and owner information are pretty essential features of a | |
6db0a480 MCC |
168 | Un*x like system, but romfs does not provide the full possibilities. |
169 | I have never found this limiting, but others might. | |
1da177e4 LT |
170 | |
171 | - The file system is read only, so it can be very small, but in case | |
6db0a480 MCC |
172 | one would want to write _anything_ to a file system, he still needs |
173 | a writable file system, thus negating the size advantages. Possible | |
174 | solutions: implement write access as a compile-time option, or a new, | |
175 | similarly small writable filesystem for RAM disks. | |
1da177e4 LT |
176 | |
177 | - Since the files are only required to have alignment on a 16 byte | |
6db0a480 MCC |
178 | boundary, it is currently possibly suboptimal to read or execute files |
179 | from the filesystem. It might be resolved by reordering file data to | |
180 | have most of it (i.e. except the start and the end) laying at "natural" | |
181 | boundaries, thus it would be possible to directly map a big portion of | |
182 | the file contents to the mm subsystem. | |
1da177e4 LT |
183 | |
184 | - Compression might be an useful feature, but memory is quite a | |
6db0a480 | 185 | limiting factor in my eyes. |
1da177e4 LT |
186 | |
187 | - Where it is used? | |
188 | ||
189 | - Does it work on other architectures than intel and motorola? | |
190 | ||
191 | ||
192 | Have fun, | |
6db0a480 | 193 | |
1da177e4 | 194 | Janos Farkas <chexum@shadow.banki.hu> |