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-Using the RAM disk block device with Linux
-------------------------------------------
-
-Contents:
-
- 1) Overview
- 2) Kernel Command Line Parameters
- 3) Using "rdev -r"
- 4) An Example of Creating a Compressed RAM Disk
-
-
-1) Overview
------------
-
-The RAM disk driver is a way to use main system memory as a block device. It
-is required for initrd, an initial filesystem used if you need to load modules
-in order to access the root filesystem (see Documentation/initrd.txt). It can
-also be used for a temporary filesystem for crypto work, since the contents
-are erased on reboot.
-
-The RAM disk 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 as dirty
-so that the VM subsystem does not try to reclaim them later.
-
-The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
-to support an unlimited number of RAM disks (at your own risk). Just change
-the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
-and (re)build the kernel.
-
-To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
-directory. RAM disks are all major number 1, and start with minor number 0
-for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
-
-The new RAM disk also has the ability to load compressed RAM disk images,
-allowing one to squeeze more programs onto an average installation or
-rescue floppy disk.
-
-
-2) Kernel Command Line Parameters
----------------------------------
-
- ramdisk_size=N
- ==============
-
-This parameter tells the RAM disk driver to set up RAM disks of N k size. The
-default is 4096 (4 MB) (8192 (8 MB) on S390).
-
- ramdisk_blocksize=N
- ===================
-
-This parameter tells the RAM disk driver how many bytes to use per block. The
-default is 1024 (BLOCK_SIZE).
-
-
-3) Using "rdev -r"
-------------------
-
-The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
-as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
-to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
-14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
-prompt/wait sequence is to be given before trying to read the RAM disk. Since
-the RAM disk dynamically grows as data is being written into it, a size field
-is not required. Bits 11 to 13 are not currently used and may as well be zero.
-These numbers are no magical secrets, as seen below:
-
-./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
-./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
-./arch/x86/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 RAM disk image onto disk #2.
-
-Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
-starts at an offset of 0 kB from the beginning of the floppy.
-The command line equivalent is: "ramdisk_start=0"
-
-You want bit 14 as one, indicating that a RAM disk 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/x86/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 RAM Disk
-----------------------------------------------
-
-To create a RAM disk image, you will need a spare block device to
-construct it on. This can be the RAM disk device itself, or an
-unused disk partition (such as an unmounted swap partition). For this
-example, we will use the RAM disk device, "/dev/ram0".
-
-Note: This technique should not be done on a machine with less than 8 MB
-of RAM. If using a spare disk partition instead of /dev/ram0, then this
-restriction does not apply.
-
-a) Decide on the RAM disk size that you want. Say 2 MB for this example.
- Create it by writing to the RAM disk device. (This step is not currently
- 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/ram0 bs=1k count=2048
-
-b) Make a filesystem on it. Say ext2fs for this example.
-
- mke2fs -vm0 /dev/ram0 2048
-
-c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
- and unmount it again.
-
-d) Compress the contents of the RAM disk. The level of compression
- will be approximately 50% of the space used by the files. Unused
- space on the RAM disk will compress to almost nothing.
-
- dd if=/dev/ram0 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 RAM disk 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 RAM disk image. An offset of 400 kB for kernels about 350 kB in
- size would be reasonable. Make sure offset+size of ram_image.gz is
- not larger than the total space on your floppy (usually 1440 kB).
-
- dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
-
-g) Use "rdev" to set the boot device, RAM disk 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 RAM disk floppy. Some
-users may wish to combine steps (d) and (f) by using a pipe.
-
---------------------------------------------------------------------------
- Paul Gortmaker 12/95
-
-Changelog:
-----------
-
-10-22-04 : Updated to reflect changes in command line options, remove
- obsolete references, general cleanup.
- James Nelson (james4765@gmail.com)
-
-
-12-95 : Original Document