1Using the RAM disk block device with Linux 2------------------------------------------ 3 4Contents: 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 11 121) Overview 13----------- 14 15The RAM disk driver is a way to use main system memory as a block device. It 16is required for initrd, an initial filesystem used if you need to load modules 17in order to access the root filesystem (see Documentation/initrd.txt). It can 18also be used for a temporary filesystem for crypto work, since the contents 19are erased on reboot. 20 21The RAM disk dynamically grows as more space is required. It does this by using 22RAM from the buffer cache. The driver marks the buffers it is using as dirty 23so that the VM subsystem does not try to reclaim them later. 24 25The RAM disk supports up to 16 RAM disks by default, and can be reconfigured 26to support an unlimited number of RAM disks (at your own risk). Just change 27the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu 28and (re)build the kernel. 29 30To use RAM disk support with your system, run './MAKEDEV ram' from the /dev 31directory. RAM disks are all major number 1, and start with minor number 0 32for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd. 33 34The new RAM disk also has the ability to load compressed RAM disk images, 35allowing one to squeeze more programs onto an average installation or 36rescue floppy disk. 37 38 392) Kernel Command Line Parameters 40--------------------------------- 41 42 ramdisk_size=N 43 ============== 44 45This parameter tells the RAM disk driver to set up RAM disks of N k size. The 46default is 4096 (4 MB) (8192 (8 MB) on S390). 47 48 ramdisk_blocksize=N 49 =================== 50 51This parameter tells the RAM disk driver how many bytes to use per block. The 52default is 1024 (BLOCK_SIZE). 53 54 553) Using "rdev -r" 56------------------ 57 58The usage of the word (two bytes) that "rdev -r" sets in the kernel image is 59as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up 60to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit 6114 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a 62prompt/wait sequence is to be given before trying to read the RAM disk. Since 63the RAM disk dynamically grows as data is being written into it, a size field 64is not required. Bits 11 to 13 are not currently used and may as well be zero. 65These numbers are no magical secrets, as seen below: 66 67./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF 68./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000 69./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000 70 71Consider a typical two floppy disk setup, where you will have the 72kernel on disk one, and have already put a RAM disk image onto disk #2. 73 74Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk 75starts at an offset of 0 kB from the beginning of the floppy. 76The command line equivalent is: "ramdisk_start=0" 77 78You want bit 14 as one, indicating that a RAM disk is to be loaded. 79The command line equivalent is: "load_ramdisk=1" 80 81You want bit 15 as one, indicating that you want a prompt/keypress 82sequence so that you have a chance to switch floppy disks. 83The command line equivalent is: "prompt_ramdisk=1" 84 85Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word. 86So to create disk one of the set, you would do: 87 88 /usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0 89 /usr/src/linux# rdev /dev/fd0 /dev/fd0 90 /usr/src/linux# rdev -r /dev/fd0 49152 91 92If you make a boot disk that has LILO, then for the above, you would use: 93 append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1" 94Since the default start = 0 and the default prompt = 1, you could use: 95 append = "load_ramdisk=1" 96 97 984) An Example of Creating a Compressed RAM Disk 99---------------------------------------------- 100 101To create a RAM disk image, you will need a spare block device to 102construct it on. This can be the RAM disk device itself, or an 103unused disk partition (such as an unmounted swap partition). For this 104example, we will use the RAM disk device, "/dev/ram0". 105 106Note: This technique should not be done on a machine with less than 8 MB 107of RAM. If using a spare disk partition instead of /dev/ram0, then this 108restriction does not apply. 109 110a) Decide on the RAM disk size that you want. Say 2 MB for this example. 111 Create it by writing to the RAM disk device. (This step is not currently 112 required, but may be in the future.) It is wise to zero out the 113 area (esp. for disks) so that maximal compression is achieved for 114 the unused blocks of the image that you are about to create. 115 116 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048 117 118b) Make a filesystem on it. Say ext2fs for this example. 119 120 mke2fs -vm0 /dev/ram0 2048 121 122c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...) 123 and unmount it again. 124 125d) Compress the contents of the RAM disk. The level of compression 126 will be approximately 50% of the space used by the files. Unused 127 space on the RAM disk will compress to almost nothing. 128 129 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz 130 131e) Put the kernel onto the floppy 132 133 dd if=zImage of=/dev/fd0 bs=1k 134 135f) Put the RAM disk image onto the floppy, after the kernel. Use an offset 136 that is slightly larger than the kernel, so that you can put another 137 (possibly larger) kernel onto the same floppy later without overlapping 138 the RAM disk image. An offset of 400 kB for kernels about 350 kB in 139 size would be reasonable. Make sure offset+size of ram_image.gz is 140 not larger than the total space on your floppy (usually 1440 kB). 141 142 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400 143 144g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc. 145 For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would 146 have 2^15 + 2^14 + 400 = 49552. 147 148 rdev /dev/fd0 /dev/fd0 149 rdev -r /dev/fd0 49552 150 151That is it. You now have your boot/root compressed RAM disk floppy. Some 152users may wish to combine steps (d) and (f) by using a pipe. 153 154-------------------------------------------------------------------------- 155 Paul Gortmaker 12/95 156 157Changelog: 158---------- 159 16010-22-04 : Updated to reflect changes in command line options, remove 161 obsolete references, general cleanup. 162 James Nelson (james4765@gmail.com) 163 164 16512-95 : Original Document 166