1========================================== 2Using the RAM disk block device with Linux 3========================================== 4 5.. Contents: 6 7 1) Overview 8 2) Kernel Command Line Parameters 9 3) Using "rdev -r" 10 4) An Example of Creating a Compressed RAM Disk 11 12 131) Overview 14----------- 15 16The RAM disk driver is a way to use main system memory as a block device. It 17is required for initrd, an initial filesystem used if you need to load modules 18in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can 19also be used for a temporary filesystem for crypto work, since the contents 20are erased on reboot. 21 22The RAM disk dynamically grows as more space is required. It does this by using 23RAM from the buffer cache. The driver marks the buffers it is using as dirty 24so that the VM subsystem does not try to reclaim them later. 25 26The RAM disk supports up to 16 RAM disks by default, and can be reconfigured 27to support an unlimited number of RAM disks (at your own risk). Just change 28the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu 29and (re)build the kernel. 30 31To use RAM disk support with your system, run './MAKEDEV ram' from the /dev 32directory. RAM disks are all major number 1, and start with minor number 0 33for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd. 34 35The new RAM disk also has the ability to load compressed RAM disk images, 36allowing one to squeeze more programs onto an average installation or 37rescue floppy disk. 38 39 402) Parameters 41--------------------------------- 42 432a) Kernel Command Line Parameters 44 45 ramdisk_size=N 46 Size of the ramdisk. 47 48This parameter tells the RAM disk driver to set up RAM disks of N k size. The 49default is 4096 (4 MB). 50 512b) Module parameters 52 53 rd_nr 54 /dev/ramX devices created. 55 56 max_part 57 Maximum partition number. 58 59 rd_size 60 See ramdisk_size. 61 623) Using "rdev -r" 63------------------ 64 65The usage of the word (two bytes) that "rdev -r" sets in the kernel image is 66as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up 67to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit 6814 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a 69prompt/wait sequence is to be given before trying to read the RAM disk. Since 70the RAM disk dynamically grows as data is being written into it, a size field 71is not required. Bits 11 to 13 are not currently used and may as well be zero. 72These numbers are no magical secrets, as seen below:: 73 74 ./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF 75 ./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000 76 ./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000 77 78Consider a typical two floppy disk setup, where you will have the 79kernel on disk one, and have already put a RAM disk image onto disk #2. 80 81Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk 82starts at an offset of 0 kB from the beginning of the floppy. 83The command line equivalent is: "ramdisk_start=0" 84 85You want bit 14 as one, indicating that a RAM disk is to be loaded. 86The command line equivalent is: "load_ramdisk=1" 87 88You want bit 15 as one, indicating that you want a prompt/keypress 89sequence so that you have a chance to switch floppy disks. 90The command line equivalent is: "prompt_ramdisk=1" 91 92Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word. 93So to create disk one of the set, you would do:: 94 95 /usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0 96 /usr/src/linux# rdev /dev/fd0 /dev/fd0 97 /usr/src/linux# rdev -r /dev/fd0 49152 98 99If you make a boot disk that has LILO, then for the above, you would use:: 100 101 append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1" 102 103Since the default start = 0 and the default prompt = 1, you could use:: 104 105 append = "load_ramdisk=1" 106 107 1084) An Example of Creating a Compressed RAM Disk 109----------------------------------------------- 110 111To create a RAM disk image, you will need a spare block device to 112construct it on. This can be the RAM disk device itself, or an 113unused disk partition (such as an unmounted swap partition). For this 114example, we will use the RAM disk device, "/dev/ram0". 115 116Note: This technique should not be done on a machine with less than 8 MB 117of RAM. If using a spare disk partition instead of /dev/ram0, then this 118restriction does not apply. 119 120a) Decide on the RAM disk size that you want. Say 2 MB for this example. 121 Create it by writing to the RAM disk device. (This step is not currently 122 required, but may be in the future.) It is wise to zero out the 123 area (esp. for disks) so that maximal compression is achieved for 124 the unused blocks of the image that you are about to create:: 125 126 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048 127 128b) Make a filesystem on it. Say ext2fs for this example:: 129 130 mke2fs -vm0 /dev/ram0 2048 131 132c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...) 133 and unmount it again. 134 135d) Compress the contents of the RAM disk. The level of compression 136 will be approximately 50% of the space used by the files. Unused 137 space on the RAM disk will compress to almost nothing:: 138 139 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz 140 141e) Put the kernel onto the floppy:: 142 143 dd if=zImage of=/dev/fd0 bs=1k 144 145f) Put the RAM disk image onto the floppy, after the kernel. Use an offset 146 that is slightly larger than the kernel, so that you can put another 147 (possibly larger) kernel onto the same floppy later without overlapping 148 the RAM disk image. An offset of 400 kB for kernels about 350 kB in 149 size would be reasonable. Make sure offset+size of ram_image.gz is 150 not larger than the total space on your floppy (usually 1440 kB):: 151 152 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400 153 154g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc. 155 For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would 156 have 2^15 + 2^14 + 400 = 49552:: 157 158 rdev /dev/fd0 /dev/fd0 159 rdev -r /dev/fd0 49552 160 161That is it. You now have your boot/root compressed RAM disk floppy. Some 162users may wish to combine steps (d) and (f) by using a pipe. 163 164 165 Paul Gortmaker 12/95 166 167Changelog: 168---------- 169 17010-22-04 : 171 Updated to reflect changes in command line options, remove 172 obsolete references, general cleanup. 173 James Nelson (james4765@gmail.com) 174 175 17612-95 : 177 Original Document 178