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Diffstat (limited to 'Documentation/filesystems/romfs.txt')
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diff --git a/Documentation/filesystems/romfs.txt b/Documentation/filesystems/romfs.txt deleted file mode 100644 index e2b07cc9120a..000000000000 --- a/Documentation/filesystems/romfs.txt +++ /dev/null @@ -1,186 +0,0 @@ -ROMFS - ROM FILE SYSTEM - -This is a quite dumb, read only filesystem, mainly for initial RAM -disks of installation disks. It has grown up by the need of having -modules linked at boot time. Using this filesystem, you get a very -similar feature, and even the possibility of a small kernel, with a -file system which doesn't take up useful memory from the router -functions in the basement of your office. - -For comparison, both the older minix and xiafs (the latter is now -defunct) filesystems, compiled as module need more than 20000 bytes, -while romfs is less than a page, about 4000 bytes (assuming i586 -code). Under the same conditions, the msdos filesystem would need -about 30K (and does not support device nodes or symlinks), while the -nfs module with nfsroot is about 57K. Furthermore, as a bit unfair -comparison, an actual rescue disk used up 3202 blocks with ext2, while -with romfs, it needed 3079 blocks. - -To create such a file system, you'll need a user program named -genromfs. It is available on http://romfs.sourceforge.net/ - -As the name suggests, romfs could be also used (space-efficiently) on -various read-only media, like (E)EPROM disks if someone will have the -motivation.. :) - -However, the main purpose of romfs is to have a very small kernel, -which has only this filesystem linked in, and then can load any module -later, with the current module utilities. It can also be used to run -some program to decide if you need SCSI devices, and even IDE or -floppy drives can be loaded later if you use the "initrd"--initial -RAM disk--feature of the kernel. This would not be really news -flash, but with romfs, you can even spare off your ext2 or minix or -maybe even affs filesystem until you really know that you need it. - -For example, a distribution boot disk can contain only the cd disk -drivers (and possibly the SCSI drivers), and the ISO 9660 filesystem -module. The kernel can be small enough, since it doesn't have other -filesystems, like the quite large ext2fs module, which can then be -loaded off the CD at a later stage of the installation. Another use -would be for a recovery disk, when you are reinstalling a workstation -from the network, and you will have all the tools/modules available -from a nearby server, so you don't want to carry two disks for this -purpose, just because it won't fit into ext2. - -romfs operates on block devices as you can expect, and the underlying -structure is very simple. Every accessible structure begins on 16 -byte boundaries for fast access. The minimum space a file will take -is 32 bytes (this is an empty file, with a less than 16 character -name). The maximum overhead for any non-empty file is the header, and -the 16 byte padding for the name and the contents, also 16+14+15 = 45 -bytes. This is quite rare however, since most file names are longer -than 3 bytes, and shorter than 15 bytes. - -The layout of the filesystem is the following: - -offset content - - +---+---+---+---+ - 0 | - | r | o | m | \ - +---+---+---+---+ The ASCII representation of those bytes - 4 | 1 | f | s | - | / (i.e. "-rom1fs-") - +---+---+---+---+ - 8 | full size | The number of accessible bytes in this fs. - +---+---+---+---+ - 12 | checksum | The checksum of the FIRST 512 BYTES. - +---+---+---+---+ - 16 | volume name | The zero terminated name of the volume, - : : padded to 16 byte boundary. - +---+---+---+---+ - xx | file | - : headers : - -Every multi byte value (32 bit words, I'll use the longwords term from -now on) must be in big endian order. - -The first eight bytes identify the filesystem, even for the casual -inspector. After that, in the 3rd longword, it contains the number of -bytes accessible from the start of this filesystem. The 4th longword -is the checksum of the first 512 bytes (or the number of bytes -accessible, whichever is smaller). The applied algorithm is the same -as in the AFFS filesystem, namely a simple sum of the longwords -(assuming bigendian quantities again). For details, please consult -the source. This algorithm was chosen because although it's not quite -reliable, it does not require any tables, and it is very simple. - -The following bytes are now part of the file system; each file header -must begin on a 16 byte boundary. - -offset content - - +---+---+---+---+ - 0 | next filehdr|X| The offset of the next file header - +---+---+---+---+ (zero if no more files) - 4 | spec.info | Info for directories/hard links/devices - +---+---+---+---+ - 8 | size | The size of this file in bytes - +---+---+---+---+ - 12 | checksum | Covering the meta data, including the file - +---+---+---+---+ name, and padding - 16 | file name | The zero terminated name of the file, - : : padded to 16 byte boundary - +---+---+---+---+ - xx | file data | - : : - -Since the file headers begin always at a 16 byte boundary, the lowest -4 bits would be always zero in the next filehdr pointer. These four -bits are used for the mode information. Bits 0..2 specify the type of -the file; while bit 4 shows if the file is executable or not. The -permissions are assumed to be world readable, if this bit is not set, -and world executable if it is; except the character and block devices, -they are never accessible for other than owner. The owner of every -file is user and group 0, this should never be a problem for the -intended use. The mapping of the 8 possible values to file types is -the following: - - mapping spec.info means - 0 hard link link destination [file header] - 1 directory first file's header - 2 regular file unused, must be zero [MBZ] - 3 symbolic link unused, MBZ (file data is the link content) - 4 block device 16/16 bits major/minor number - 5 char device - " - - 6 socket unused, MBZ - 7 fifo unused, MBZ - -Note that hard links are specifically marked in this filesystem, but -they will behave as you can expect (i.e. share the inode number). -Note also that it is your responsibility to not create hard link -loops, and creating all the . and .. links for directories. This is -normally done correctly by the genromfs program. Please refrain from -using the executable bits for special purposes on the socket and fifo -special files, they may have other uses in the future. Additionally, -please remember that only regular files, and symlinks are supposed to -have a nonzero size field; they contain the number of bytes available -directly after the (padded) file name. - -Another thing to note is that romfs works on file headers and data -aligned to 16 byte boundaries, but most hardware devices and the block -device drivers are unable to cope with smaller than block-sized data. -To overcome this limitation, the whole size of the file system must be -padded to an 1024 byte boundary. - -If you have any problems or suggestions concerning this file system, -please contact me. However, think twice before wanting me to add -features and code, because the primary and most important advantage of -this file system is the small code. On the other hand, don't be -alarmed, I'm not getting that much romfs related mail. Now I can -understand why Avery wrote poems in the ARCnet docs to get some more -feedback. :) - -romfs has also a mailing list, and to date, it hasn't received any -traffic, so you are welcome to join it to discuss your ideas. :) - -It's run by ezmlm, so you can subscribe to it by sending a message -to romfs-subscribe@shadow.banki.hu, the content is irrelevant. - -Pending issues: - -- Permissions and owner information are pretty essential features of a -Un*x like system, but romfs does not provide the full possibilities. -I have never found this limiting, but others might. - -- The file system is read only, so it can be very small, but in case -one would want to write _anything_ to a file system, he still needs -a writable file system, thus negating the size advantages. Possible -solutions: implement write access as a compile-time option, or a new, -similarly small writable filesystem for RAM disks. - -- Since the files are only required to have alignment on a 16 byte -boundary, it is currently possibly suboptimal to read or execute files -from the filesystem. It might be resolved by reordering file data to -have most of it (i.e. except the start and the end) laying at "natural" -boundaries, thus it would be possible to directly map a big portion of -the file contents to the mm subsystem. - -- Compression might be an useful feature, but memory is quite a -limiting factor in my eyes. - -- Where it is used? - -- Does it work on other architectures than intel and motorola? - - -Have fun, -Janos Farkas <chexum@shadow.banki.hu> |