linux-imx

ROMFS - ROM FILE SYSTEM

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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.

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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.

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To create such a file system, you'll need a user program named

genromfs. It is available on http://romfs.sourceforge.net/

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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.. :)

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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.

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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.

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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.

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The layout of the filesystem is the following:

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offset      content

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    +---+---+---+---+

  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.