| blob | e2b07cc9120a24564e8a0946f9d84b44cc18a32a |
1 ROMFS - ROM FILE SYSTEM
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.
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.
19 To create such a file system, you'll need a user program named
20 genromfs. It is available on http://romfs.sourceforge.net/
22 As the name suggests, romfs could be also used (space-efficiently) on
23 various read-only media, like (E)EPROM disks if someone will have the
24 motivation.. :)
26 However, the main purpose of romfs is to have a very small kernel,
27 which has only this filesystem linked in, and then can load any module
28 later, with the current module utilities. It can also be used to run
29 some program to decide if you need SCSI devices, and even IDE or
30 floppy drives can be loaded later if you use the "initrd"--initial
31 RAM disk--feature of the kernel. This would not be really news
32 flash, but with romfs, you can even spare off your ext2 or minix or
33 maybe even affs filesystem until you really know that you need it.
35 For example, a distribution boot disk can contain only the cd disk
36 drivers (and possibly the SCSI drivers), and the ISO 9660 filesystem
37 module. The kernel can be small enough, since it doesn't have other
38 filesystems, like the quite large ext2fs module, which can then be
39 loaded off the CD at a later stage of the installation. Another use
40 would be for a recovery disk, when you are reinstalling a workstation
41 from the network, and you will have all the tools/modules available
42 from a nearby server, so you don't want to carry two disks for this
43 purpose, just because it won't fit into ext2.
45 romfs operates on block devices as you can expect, and the underlying
46 structure is very simple. Every accessible structure begins on 16
47 byte boundaries for fast access. The minimum space a file will take
48 is 32 bytes (this is an empty file, with a less than 16 character
49 name). The maximum overhead for any non-empty file is the header, and
50 the 16 byte padding for the name and the contents, also 16+14+15 = 45
51 bytes. This is quite rare however, since most file names are longer
52 than 3 bytes, and shorter than 15 bytes.
54 The layout of the filesystem is the following:
56 offset content
58 +---+---+---+---+
59 0 | - | r | o | m | \
60 +---+---+---+---+ The ASCII representation of those bytes
61 4 | 1 | f | s | - | / (i.e. "-rom1fs-")
62 +---+---+---+---+
63 8 | full size | The number of accessible bytes in this fs.
64 +---+---+---+---+
65 12 | checksum | The checksum of the FIRST 512 BYTES.
66 +---+---+---+---+
67 16 | volume name | The zero terminated name of the volume,
68 : : padded to 16 byte boundary.
69 +---+---+---+---+
70 xx | file |
71 : headers :
73 Every multi byte value (32 bit words, I'll use the longwords term from
74 now on) must be in big endian order.
76 The first eight bytes identify the filesystem, even for the casual
77 inspector. After that, in the 3rd longword, it contains the number of
78 bytes accessible from the start of this filesystem. The 4th longword
79 is the checksum of the first 512 bytes (or the number of bytes
80 accessible, whichever is smaller). The applied algorithm is the same
81 as in the AFFS filesystem, namely a simple sum of the longwords
82 (assuming bigendian quantities again). For details, please consult
83 the source. This algorithm was chosen because although it's not quite
84 reliable, it does not require any tables, and it is very simple.
86 The following bytes are now part of the file system; each file header
87 must begin on a 16 byte boundary.
89 offset content
91 +---+---+---+---+
92 0 | next filehdr|X| The offset of the next file header
93 +---+---+---+---+ (zero if no more files)
94 4 | spec.info | Info for directories/hard links/devices
95 +---+---+---+---+
96 8 | size | The size of this file in bytes
97 +---+---+---+---+
98 12 | checksum | Covering the meta data, including the file
99 +---+---+---+---+ name, and padding
100 16 | file name | The zero terminated name of the file,
101 : : padded to 16 byte boundary
102 +---+---+---+---+
103 xx | file data |
104 : :
106 Since the file headers begin always at a 16 byte boundary, the lowest
107 4 bits would be always zero in the next filehdr pointer. These four
108 bits are used for the mode information. Bits 0..2 specify the type of
109 the file; while bit 4 shows if the file is executable or not. The
110 permissions are assumed to be world readable, if this bit is not set,
111 and world executable if it is; except the character and block devices,
112 they are never accessible for other than owner. The owner of every
113 file is user and group 0, this should never be a problem for the
114 intended use. The mapping of the 8 possible values to file types is
115 the following:
117 mapping spec.info means
118 0 hard link link destination [file header]
119 1 directory first file's header
120 2 regular file unused, must be zero [MBZ]
121 3 symbolic link unused, MBZ (file data is the link content)
122 4 block device 16/16 bits major/minor number
123 5 char device - " -
124 6 socket unused, MBZ
125 7 fifo unused, MBZ
127 Note that hard links are specifically marked in this filesystem, but
128 they will behave as you can expect (i.e. share the inode number).
129 Note also that it is your responsibility to not create hard link
130 loops, and creating all the . and .. links for directories. This is
131 normally done correctly by the genromfs program. Please refrain from
132 using the executable bits for special purposes on the socket and fifo
133 special files, they may have other uses in the future. Additionally,
134 please remember that only regular files, and symlinks are supposed to
135 have a nonzero size field; they contain the number of bytes available
136 directly after the (padded) file name.
138 Another thing to note is that romfs works on file headers and data
139 aligned to 16 byte boundaries, but most hardware devices and the block
140 device drivers are unable to cope with smaller than block-sized data.
141 To overcome this limitation, the whole size of the file system must be
142 padded to an 1024 byte boundary.
144 If you have any problems or suggestions concerning this file system,
145 please contact me. However, think twice before wanting me to add
146 features and code, because the primary and most important advantage of
147 this file system is the small code. On the other hand, don't be
148 alarmed, I'm not getting that much romfs related mail. Now I can
149 understand why Avery wrote poems in the ARCnet docs to get some more
150 feedback. :)
152 romfs has also a mailing list, and to date, it hasn't received any
153 traffic, so you are welcome to join it to discuss your ideas. :)
155 It's run by ezmlm, so you can subscribe to it by sending a message
156 to romfs-subscribe@shadow.banki.hu, the content is irrelevant.
158 Pending issues:
160 - Permissions and owner information are pretty essential features of a
161 Un*x like system, but romfs does not provide the full possibilities.
162 I have never found this limiting, but others might.
164 - The file system is read only, so it can be very small, but in case
165 one would want to write _anything_ to a file system, he still needs
166 a writable file system, thus negating the size advantages. Possible
167 solutions: implement write access as a compile-time option, or a new,
168 similarly small writable filesystem for RAM disks.
170 - Since the files are only required to have alignment on a 16 byte
171 boundary, it is currently possibly suboptimal to read or execute files
172 from the filesystem. It might be resolved by reordering file data to
173 have most of it (i.e. except the start and the end) laying at "natural"
174 boundaries, thus it would be possible to directly map a big portion of
175 the file contents to the mm subsystem.
177 - Compression might be an useful feature, but memory is quite a
178 limiting factor in my eyes.
180 - Where it is used?
182 - Does it work on other architectures than intel and motorola?
185 Have fun,
186 Janos Farkas <chexum@shadow.banki.hu>