ramdisk.txt 6.3 KB

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  1. Using the RAM disk block device with Linux
  2. ------------------------------------------
  3. Contents:
  4. 1) Overview
  5. 2) Kernel Command Line Parameters
  6. 3) Using "rdev -r"
  7. 4) An Example of Creating a Compressed RAM Disk
  8. 1) Overview
  9. -----------
  10. The RAM disk driver is a way to use main system memory as a block device. It
  11. is required for initrd, an initial filesystem used if you need to load modules
  12. in order to access the root filesystem (see Documentation/initrd.txt). It can
  13. also be used for a temporary filesystem for crypto work, since the contents
  14. are erased on reboot.
  15. The RAM disk dynamically grows as more space is required. It does this by using
  16. RAM from the buffer cache. The driver marks the buffers it is using as dirty
  17. so that the VM subsystem does not try to reclaim them later.
  18. Also, the RAM disk supports up to 16 RAM disks out of the box, and can
  19. be reconfigured to support up to 255 RAM disks - change "#define NUM_RAMDISKS"
  20. in drivers/block/rd.c. To use RAM disk support with your system, run
  21. './MAKEDEV ram' from the /dev directory. RAM disks are all major number 1, and
  22. start with minor number 0 for /dev/ram0, etc. If used, modern kernels use
  23. /dev/ram0 for an initrd.
  24. The old "ramdisk=<ram_size>" has been changed to "ramdisk_size=<ram_size>" to
  25. make it clearer. The original "ramdisk=<ram_size>" has been kept around for
  26. compatibility reasons, but it may be removed in the future.
  27. The new RAM disk also has the ability to load compressed RAM disk images,
  28. allowing one to squeeze more programs onto an average installation or
  29. rescue floppy disk.
  30. 2) Kernel Command Line Parameters
  31. ---------------------------------
  32. ramdisk_size=N
  33. ==============
  34. This parameter tells the RAM disk driver to set up RAM disks of N k size. The
  35. default is 4096 (4 MB) (8192 (8 MB) on S390).
  36. ramdisk_blocksize=N
  37. ===================
  38. This parameter tells the RAM disk driver how many bytes to use per block. The
  39. default is 1024 (BLOCK_SIZE).
  40. 3) Using "rdev -r"
  41. ------------------
  42. The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
  43. as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
  44. to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
  45. 14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
  46. prompt/wait sequence is to be given before trying to read the RAM disk. Since
  47. the RAM disk dynamically grows as data is being written into it, a size field
  48. is not required. Bits 11 to 13 are not currently used and may as well be zero.
  49. These numbers are no magical secrets, as seen below:
  50. ./arch/i386/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
  51. ./arch/i386/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
  52. ./arch/i386/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000
  53. Consider a typical two floppy disk setup, where you will have the
  54. kernel on disk one, and have already put a RAM disk image onto disk #2.
  55. Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
  56. starts at an offset of 0 kB from the beginning of the floppy.
  57. The command line equivalent is: "ramdisk_start=0"
  58. You want bit 14 as one, indicating that a RAM disk is to be loaded.
  59. The command line equivalent is: "load_ramdisk=1"
  60. You want bit 15 as one, indicating that you want a prompt/keypress
  61. sequence so that you have a chance to switch floppy disks.
  62. The command line equivalent is: "prompt_ramdisk=1"
  63. Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
  64. So to create disk one of the set, you would do:
  65. /usr/src/linux# cat arch/i386/boot/zImage > /dev/fd0
  66. /usr/src/linux# rdev /dev/fd0 /dev/fd0
  67. /usr/src/linux# rdev -r /dev/fd0 49152
  68. If you make a boot disk that has LILO, then for the above, you would use:
  69. append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
  70. Since the default start = 0 and the default prompt = 1, you could use:
  71. append = "load_ramdisk=1"
  72. 4) An Example of Creating a Compressed RAM Disk
  73. ----------------------------------------------
  74. To create a RAM disk image, you will need a spare block device to
  75. construct it on. This can be the RAM disk device itself, or an
  76. unused disk partition (such as an unmounted swap partition). For this
  77. example, we will use the RAM disk device, "/dev/ram0".
  78. Note: This technique should not be done on a machine with less than 8 MB
  79. of RAM. If using a spare disk partition instead of /dev/ram0, then this
  80. restriction does not apply.
  81. a) Decide on the RAM disk size that you want. Say 2 MB for this example.
  82. Create it by writing to the RAM disk device. (This step is not currently
  83. required, but may be in the future.) It is wise to zero out the
  84. area (esp. for disks) so that maximal compression is achieved for
  85. the unused blocks of the image that you are about to create.
  86. dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
  87. b) Make a filesystem on it. Say ext2fs for this example.
  88. mke2fs -vm0 /dev/ram0 2048
  89. c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
  90. and unmount it again.
  91. d) Compress the contents of the RAM disk. The level of compression
  92. will be approximately 50% of the space used by the files. Unused
  93. space on the RAM disk will compress to almost nothing.
  94. dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
  95. e) Put the kernel onto the floppy
  96. dd if=zImage of=/dev/fd0 bs=1k
  97. f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
  98. that is slightly larger than the kernel, so that you can put another
  99. (possibly larger) kernel onto the same floppy later without overlapping
  100. the RAM disk image. An offset of 400 kB for kernels about 350 kB in
  101. size would be reasonable. Make sure offset+size of ram_image.gz is
  102. not larger than the total space on your floppy (usually 1440 kB).
  103. dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
  104. g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
  105. For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
  106. have 2^15 + 2^14 + 400 = 49552.
  107. rdev /dev/fd0 /dev/fd0
  108. rdev -r /dev/fd0 49552
  109. That is it. You now have your boot/root compressed RAM disk floppy. Some
  110. users may wish to combine steps (d) and (f) by using a pipe.
  111. --------------------------------------------------------------------------
  112. Paul Gortmaker 12/95
  113. Changelog:
  114. ----------
  115. 10-22-04 : Updated to reflect changes in command line options, remove
  116. obsolete references, general cleanup.
  117. James Nelson (james4765@gmail.com)
  118. 12-95 : Original Document