1 ========================================== 2 Using 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" 10 4) An Example of Creating a Compressed RAM Disk 11 12 13 1) Overview 14 ----------- 15 16 The RAM disk driver is a way to use main system memory as a block device. It 17 is required for initrd, an initial filesystem used if you need to load modules 18 in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can 19 also be used for a temporary filesystem for crypto work, since the contents 20 are erased on reboot. 21 22 The RAM disk dynamically grows as more space is required. It does this by using 23 RAM from the buffer cache. The driver marks the buffers it is using as dirty 24 so that the VM subsystem does not try to reclaim them later. 25 26 The RAM disk supports up to 16 RAM disks by default, and can be reconfigured 27 to support an unlimited number of RAM disks (at your own risk). Just change 28 the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu 29 and (re)build the kernel. 30 31 To use RAM disk support with your system, run './MAKEDEV ram' from the /dev 32 directory. RAM disks are all major number 1, and start with minor number 0 33 for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd. 34 35 The new RAM disk also has the ability to load compressed RAM disk images, 36 allowing one to squeeze more programs onto an average installation or 37 rescue floppy disk. 38 39 40 2) Parameters 41 --------------------------------- 42 43 2a) Kernel Command Line Parameters 44 45 ramdisk_size=N 46 Size of the ramdisk. 47 48 This parameter tells the RAM disk driver to set up RAM disks of N k size. The 49 default is 4096 (4 MB). 50 51 2b) 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 62 3) Using "rdev" 63 --------------- 64 65 "rdev" is an obsolete, deprecated, antiquated utility that could be used 66 to set the boot device in a Linux kernel image. 67 68 Instead of using rdev, just place the boot device information on the 69 kernel command line and pass it to the kernel from the bootloader. 70 71 You can also pass arguments to the kernel by setting FDARGS in 72 arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in 73 arch/x86/boot/Makefile. 74 75 Some of the kernel command line boot options that may apply here are:: 76 77 ramdisk_start=N 78 ramdisk_size=M 79 80 If you make a boot disk that has LILO, then for the above, you would use:: 81 82 append = "ramdisk_start=N ramdisk_size=M" 83 84 4) An Example of Creating a Compressed RAM Disk 85 ----------------------------------------------- 86 87 To create a RAM disk image, you will need a spare block device to 88 construct it on. This can be the RAM disk device itself, or an 89 unused disk partition (such as an unmounted swap partition). For this 90 example, we will use the RAM disk device, "/dev/ram0". 91 92 Note: This technique should not be done on a machine with less than 8 MB 93 of RAM. If using a spare disk partition instead of /dev/ram0, then this 94 restriction does not apply. 95 96 a) Decide on the RAM disk size that you want. Say 2 MB for this example. 97 Create it by writing to the RAM disk device. (This step is not currently 98 required, but may be in the future.) It is wise to zero out the 99 area (esp. for disks) so that maximal compression is achieved for 100 the unused blocks of the image that you are about to create:: 101 102 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048 103 104 b) Make a filesystem on it. Say ext2fs for this example:: 105 106 mke2fs -vm0 /dev/ram0 2048 107 108 c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...) 109 and unmount it again. 110 111 d) Compress the contents of the RAM disk. The level of compression 112 will be approximately 50% of the space used by the files. Unused 113 space on the RAM disk will compress to almost nothing:: 114 115 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz 116 117 e) Put the kernel onto the floppy:: 118 119 dd if=zImage of=/dev/fd0 bs=1k 120 121 f) Put the RAM disk image onto the floppy, after the kernel. Use an offset 122 that is slightly larger than the kernel, so that you can put another 123 (possibly larger) kernel onto the same floppy later without overlapping 124 the RAM disk image. An offset of 400 kB for kernels about 350 kB in 125 size would be reasonable. Make sure offset+size of ram_image.gz is 126 not larger than the total space on your floppy (usually 1440 kB):: 127 128 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400 129 130 g) Make sure that you have already specified the boot information in 131 FDARGS and FDINITRD or that you use a bootloader to pass kernel 132 command line boot options to the kernel. 133 134 That is it. You now have your boot/root compressed RAM disk floppy. Some 135 users may wish to combine steps (d) and (f) by using a pipe. 136 137 138 Paul Gortmaker 12/95 139 140 Changelog: 141 ---------- 142 143 SEPT-2020 : 144 145 Removed usage of "rdev" 146 147 10-22-04 : 148 Updated to reflect changes in command line options, remove 149 obsolete references, general cleanup. 150 James Nelson (james4765@gmail.com) 151 152 12-95 : 153 Original Document
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