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Using the RAM disk block device with Linux

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Contents:

        1) Overview

        2) Kernel Command Line Parameters

        3) Using "rdev -r" With New Kernels

        4) An Example of Creating a Compressed ramdisk

1) Overview

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As of kernel v1.3.48, the ramdisk driver was substantially changed.

The older versions would grab a chunk of memory off the top before

handing the remainder to the kernel at boot time. Thus a size parameter

had to be specified via "ramdisk=1440" or "rdev -r /dev/fd0 1440" so

that the driver knew how much memory to grab.

Now the ramdisk dynamically grows as more space is required. It does

this by using RAM from the buffer cache. The driver marks the buffers

it is using with a new "BH_Protected" flag so that the kernel does

not try to reuse them later. This means that the old size parameter

is no longer used, new command line parameters exist, and the behavior

of the "rdev -r" or "ramsize" (usually a symbolic link to "rdev")

command has changed.

Also, the new ramdisk supports up to 16 ramdisks out of the box, and can

be reconfigured in rd.c to support up to 255 ramdisks.  To use multiple

ramdisk support with your system, run 'mknod /dev/ramX b 1 X' and chmod

(to change it's permissions) it to your liking.  The default /dev/ram(disk)

uses minor #1, so start with ram2 and go from there.

The old "ramdisk=" has been changed to "ramdisk_size="

to make it clearer.  The original "ramdisk=" has been kept around

for compatibility reasons, but it will probably be removed in 2.1.x.

The new ramdisk also has the ability to load compressed ramdisk images,

allowing one to squeeze more programs onto an average installation or

rescue floppy disk.

Notes: You may have "dev/ram" or "/dev/ramdisk" or both. They are

equivalent from the standpoint of this document. Also, the new ramdisk

is a config option. When running "make config", make sure you enable

ramdisk support for the kernel you intend to use the ramdisk with.

2) Kernel Command Line Parameters

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        ramdisk_start=NNN

        =================

To allow a kernel image to reside on a floppy disk along with a compressed

ramdisk image, the "ramdisk_start=" command was added. The kernel

can't be included into the compressed ramdisk filesystem image, because

it needs to be stored starting at block zero so that the BIOS can load the

bootsector and then the kernel can bootstrap itself to get going.

Note: If you are using an uncompressed ramdisk image, then the kernel can

be a part of the filesystem image that is being loaded into the ramdisk,

and the floppy can be booted with LILO, or the two can be separate as

is done for the compressed images.

If you are using a two-disk boot/root setup (kernel on #1, ramdisk image

on #2) then the ramdisk would start at block zero, and an offset of

zero would be used. Since this is the default value, you would not need

to actually use the command at all.

If instead, you have a "zImage" of about 350k, and a "fs_image.gz" of

say about 1MB, and you want them both on the same disk, then you

would use an offset. If you stored the "fs_image.gz" onto the floppy

starting at an offset of 400kB, you would use "ramdisk_start=400".

        load_ramdisk=N

        ==============

This parameter tells the kernel whether it is to try to load a

ramdisk image or not. Specifying "load_ramdisk=1" will tell the

kernel to load a floppy into the ramdisk. The default value is

zero, meaning that the kernel should not try to load a ramdisk.

        prompt_ramdisk=N

        ================

This parameter tells the kernel whether or not to give you a prompt

asking you to insert the floppy containing the ramdisk image. In

a single floppy configuration the ramdisk image is on the same floppy

as the kernel that just finished loading/booting and so a prompt

is not needed. In this case one can use "prompt_ramdisk=0". In a

two floppy configuration, you will need the chance to switch disks,

and thus "prompt_ramdisk=1" can be used. Since this is the default

value, it doesn't really need to be specified.

        ramdisk_size=N

        ==============

This parameter tells the ramdisk driver to set up ramdisks of Nk size.  The

default is 4096 (4MB).

3) Using "rdev -r" With New Kernels

-----------------------------------

The usage of the word (two bytes) that "rdev -r" sets in the kernel image

has changed. The low 11 bits (0 -> 10) specify an offset (in 1k blocks)

of up to 2MB (2^11) of where to find the ramdisk (this used to be the

size). Bit 14 indicates that a ramdisk is to be loaded, and bit 15

indicates whether a prompt/wait sequence is to be given before trying

to read the ramdisk. Since the ramdisk dynamically grows as data is

being written into it, a size field is no longer required. Bits 11

to 13 are not presently used and may as well be zero. These numbers

are no magical secrets, as seen below:

./arch/i386/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK     0x07FF

./arch/i386/kernel/setup.c:#define RAMDISK_PROMPT_FLAG          0x8000

./arch/i386/kernel/setup.c:#define RAMDISK_LOAD_FLAG            0x4000

Consider a typical two floppy disk setup, where you will have the

kernel on disk one, and have already put a ramdisk image onto disk #2.

Hence you want to set bits 0 to 13 as zero, meaning that your ramdisk

starts at an offset of zero kB from the beginning of the floppy.

The command line equivalent is: "ramdisk_start=0"

You want bit 14 as one, indicating that a ramdisk is to be loaded.

The command line equivalent is: "load_ramdisk=1"

You want bit 15 as one, indicating that you want a prompt/keypress

sequence so that you have a chance to switch floppy disks.

The command line equivalent is: "prompt_ramdisk=1"

Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.

So to create disk one of the set, you would do:

        /usr/src/linux# cat arch/i386/boot/zImage > /dev/fd0

        /usr/src/linux# rdev /dev/fd0 /dev/fd0

        /usr/src/linux# rdev -r /dev/fd0 49152

If you make a boot disk that has LILO, then for the above, you would use:

        append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"

Since the default start = 0 and the default prompt = 1, you could use:

        append = "load_ramdisk=1"

4) An Example of Creating a Compressed ramdisk

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To create a ramdisk image, you will need a spare block device to

construct it on. This can be the ramdisk device itself, or an

unused disk partition (such as an unmounted swap partition). For this

example, we will use the ramdisk device, "/dev/ram".

Note: This technique should not be done on a machine with less than 8MB

of RAM. If using a spare disk partition instead of /dev/ram, then this

restriction does not apply.

a) Decide on the ramdisk size that you want. Say 2MB for this example.

   Create it by writing to the ramdisk device. (This step is not presently

   required, but may be in the future.) It is wise to zero out the

   area (esp. for disks) so that maximal compression is achieved for

   the unused blocks of the image that you are about to create.

        dd if=/dev/zero of=/dev/ram bs=1k count=2048

b) Make a filesystem on it. Say ext2fs for this example.

        mke2fs -vm0 /dev/ram 2048

c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)

   and unmount it again.

d) Compress the contents of the ramdisk. The level of compression

   will be approximately 50% of the space used by the files. Unused

   space on the ramdisk will compress to almost nothing.

        dd if=/dev/ram bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz

e) Put the kernel onto the floppy

        dd if=zImage of=/dev/fd0 bs=1k

f) Put the ramdisk image onto the floppy, after the kernel. Use an offset

   that is slightly larger than the kernel, so that you can put another

   (possibly larger) kernel onto the same floppy later without overlapping

   the ramdisk image. An offset of 400kB for kernels about 350kB in

   size would be reasonable. Make sure offset+size of ram_image.gz is

   not larger than the total space on your floppy (usually 1440kB).

        dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400

g) Use "rdev" to set the boot device, ramdisk offset, prompt flag, etc.

   For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would

   have 2^15 + 2^14 + 400 = 49552.

        rdev /dev/fd0 /dev/fd0

        rdev -r /dev/fd0 49552

That is it. You now have your boot/root compressed ramdisk floppy. Some

users may wish to combine steps (d) and (f) by using a pipe.

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                                                Paul Gortmaker 12/95