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