1 .. SPDX-License-Identifier: GPL-2.0 2 3 ======================= 4 ROMFS - ROM File System 5 ======================= 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 24 genromfs. It is available on http://romfs.sourceforge.net/ 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 58 The layout of the filesystem is the following:: 59 60 offset content 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 91 must begin on a 16 byte boundary:: 92 93 offset content 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: 120 121 == =============== ============================================ 122 mapping spec.info means 123 == =============== ============================================ 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 132 == =============== ============================================ 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 168 Un*x like system, but romfs does not provide the full possibilities. 169 I have never found this limiting, but others might. 170 171 - The file system is read only, so it can be very small, but in case 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. 176 177 - Since the files are only required to have alignment on a 16 byte 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. 183 184 - Compression might be an useful feature, but memory is quite a 185 limiting factor in my eyes. 186 187 - Where it is used? 188 189 - Does it work on other architectures than intel and motorola? 190 191 192 Have fun, 193 194 Janos Farkas <chexum@shadow.banki.hu>
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