Based on kernel version 4.16.1. Page generated on 2018-04-09 11:52 EST.
1 Using the RAM disk block device with Linux 2 ------------------------------------------ 3 4 Contents: 5 6 1) Overview 7 2) Kernel Command Line Parameters 8 3) Using "rdev -r" 9 4) An Example of Creating a Compressed RAM Disk 10 11 12 1) Overview 13 ----------- 14 15 The RAM disk driver is a way to use main system memory as a block device. It 16 is required for initrd, an initial filesystem used if you need to load modules 17 in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can 18 also be used for a temporary filesystem for crypto work, since the contents 19 are erased on reboot. 20 21 The RAM disk dynamically grows as more space is required. It does this by using 22 RAM from the buffer cache. The driver marks the buffers it is using as dirty 23 so that the VM subsystem does not try to reclaim them later. 24 25 The RAM disk supports up to 16 RAM disks by default, and can be reconfigured 26 to support an unlimited number of RAM disks (at your own risk). Just change 27 the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu 28 and (re)build the kernel. 29 30 To use RAM disk support with your system, run './MAKEDEV ram' from the /dev 31 directory. RAM disks are all major number 1, and start with minor number 0 32 for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd. 33 34 The new RAM disk also has the ability to load compressed RAM disk images, 35 allowing one to squeeze more programs onto an average installation or 36 rescue floppy disk. 37 38 39 2) Parameters 40 --------------------------------- 41 42 2a) Kernel Command Line Parameters 43 44 ramdisk_size=N 45 ============== 46 47 This parameter tells the RAM disk driver to set up RAM disks of N k size. The 48 default is 4096 (4 MB). 49 50 2b) Module parameters 51 52 rd_nr 53 ===== 54 /dev/ramX devices created. 55 56 max_part 57 ======== 58 Maximum partition number. 59 60 rd_size 61 ======= 62 See ramdisk_size. 63 64 3) Using "rdev -r" 65 ------------------ 66 67 The usage of the word (two bytes) that "rdev -r" sets in the kernel image is 68 as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up 69 to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit 70 14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a 71 prompt/wait sequence is to be given before trying to read the RAM disk. Since 72 the RAM disk dynamically grows as data is being written into it, a size field 73 is not required. Bits 11 to 13 are not currently used and may as well be zero. 74 These numbers are no magical secrets, as seen below: 75 76 ./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF 77 ./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000 78 ./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000 79 80 Consider a typical two floppy disk setup, where you will have the 81 kernel on disk one, and have already put a RAM disk image onto disk #2. 82 83 Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk 84 starts at an offset of 0 kB from the beginning of the floppy. 85 The command line equivalent is: "ramdisk_start=0" 86 87 You want bit 14 as one, indicating that a RAM disk is to be loaded. 88 The command line equivalent is: "load_ramdisk=1" 89 90 You want bit 15 as one, indicating that you want a prompt/keypress 91 sequence so that you have a chance to switch floppy disks. 92 The command line equivalent is: "prompt_ramdisk=1" 93 94 Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word. 95 So to create disk one of the set, you would do: 96 97 /usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0 98 /usr/src/linux# rdev /dev/fd0 /dev/fd0 99 /usr/src/linux# rdev -r /dev/fd0 49152 100 101 If you make a boot disk that has LILO, then for the above, you would use: 102 append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1" 103 Since the default start = 0 and the default prompt = 1, you could use: 104 append = "load_ramdisk=1" 105 106 107 4) An Example of Creating a Compressed RAM Disk 108 ---------------------------------------------- 109 110 To create a RAM disk image, you will need a spare block device to 111 construct it on. This can be the RAM disk device itself, or an 112 unused disk partition (such as an unmounted swap partition). For this 113 example, we will use the RAM disk device, "/dev/ram0". 114 115 Note: This technique should not be done on a machine with less than 8 MB 116 of RAM. If using a spare disk partition instead of /dev/ram0, then this 117 restriction does not apply. 118 119 a) Decide on the RAM disk size that you want. Say 2 MB for this example. 120 Create it by writing to the RAM disk device. (This step is not currently 121 required, but may be in the future.) It is wise to zero out the 122 area (esp. for disks) so that maximal compression is achieved for 123 the unused blocks of the image that you are about to create. 124 125 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048 126 127 b) Make a filesystem on it. Say ext2fs for this example. 128 129 mke2fs -vm0 /dev/ram0 2048 130 131 c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...) 132 and unmount it again. 133 134 d) Compress the contents of the RAM disk. The level of compression 135 will be approximately 50% of the space used by the files. Unused 136 space on the RAM disk will compress to almost nothing. 137 138 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz 139 140 e) Put the kernel onto the floppy 141 142 dd if=zImage of=/dev/fd0 bs=1k 143 144 f) Put the RAM disk image onto the floppy, after the kernel. Use an offset 145 that is slightly larger than the kernel, so that you can put another 146 (possibly larger) kernel onto the same floppy later without overlapping 147 the RAM disk image. An offset of 400 kB for kernels about 350 kB in 148 size would be reasonable. Make sure offset+size of ram_image.gz is 149 not larger than the total space on your floppy (usually 1440 kB). 150 151 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400 152 153 g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc. 154 For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would 155 have 2^15 + 2^14 + 400 = 49552. 156 157 rdev /dev/fd0 /dev/fd0 158 rdev -r /dev/fd0 49552 159 160 That is it. You now have your boot/root compressed RAM disk floppy. Some 161 users may wish to combine steps (d) and (f) by using a pipe. 162 163 -------------------------------------------------------------------------- 164 Paul Gortmaker 12/95 165 166 Changelog: 167 ---------- 168 169 10-22-04 : Updated to reflect changes in command line options, remove 170 obsolete references, general cleanup. 171 James Nelson (james4765@gmail.com) 172 173 174 12-95 : Original Document