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diff --git a/Documentation/ramdisk.txt b/Documentation/ramdisk.txt new file mode 100644 index 000000000000..7c25584e082c --- /dev/null +++ b/Documentation/ramdisk.txt @@ -0,0 +1,167 @@ +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. + +Also, the RAM disk supports up to 16 RAM disks out of the box, and can +be reconfigured to support up to 255 RAM disks - change "#define NUM_RAMDISKS" +in drivers/block/rd.c. 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 old "ramdisk=<ram_size>" has been changed to "ramdisk_size=<ram_size>" to +make it clearer. The original "ramdisk=<ram_size>" has been kept around for +compatibility reasons, but it may be removed in the future. + +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 512. + + +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/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 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/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 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 |
