178 lines
6.2 KiB
ReStructuredText
178 lines
6.2 KiB
ReStructuredText
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==========================================
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Using the RAM disk block device with Linux
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==========================================
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.. Contents:
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1) Overview
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2) Kernel Command Line Parameters
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3) Using "rdev -r"
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4) An Example of Creating a Compressed RAM Disk
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1) Overview
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-----------
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The RAM disk driver is a way to use main system memory as a block device. It
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is required for initrd, an initial filesystem used if you need to load modules
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in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can
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also be used for a temporary filesystem for crypto work, since the contents
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are erased on reboot.
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The RAM disk dynamically grows as more space is required. It does this by using
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RAM from the buffer cache. The driver marks the buffers it is using as dirty
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so that the VM subsystem does not try to reclaim them later.
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The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
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to support an unlimited number of RAM disks (at your own risk). Just change
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the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
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and (re)build the kernel.
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To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
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directory. RAM disks are all major number 1, and start with minor number 0
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for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
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The new RAM disk also has the ability to load compressed RAM disk images,
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allowing one to squeeze more programs onto an average installation or
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rescue floppy disk.
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2) Parameters
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---------------------------------
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2a) Kernel Command Line Parameters
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ramdisk_size=N
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Size of the ramdisk.
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This parameter tells the RAM disk driver to set up RAM disks of N k size. The
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default is 4096 (4 MB).
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2b) Module parameters
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rd_nr
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/dev/ramX devices created.
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max_part
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Maximum partition number.
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rd_size
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See ramdisk_size.
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3) Using "rdev -r"
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------------------
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The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
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as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
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to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
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14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
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prompt/wait sequence is to be given before trying to read the RAM disk. Since
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the RAM disk dynamically grows as data is being written into it, a size field
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is not required. Bits 11 to 13 are not currently used and may as well be zero.
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These numbers are no magical secrets, as seen below::
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./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
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./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
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./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000
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Consider a typical two floppy disk setup, where you will have the
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kernel on disk one, and have already put a RAM disk image onto disk #2.
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Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
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starts at an offset of 0 kB from the beginning of the floppy.
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The command line equivalent is: "ramdisk_start=0"
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You want bit 14 as one, indicating that a RAM disk is to be loaded.
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The command line equivalent is: "load_ramdisk=1"
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You want bit 15 as one, indicating that you want a prompt/keypress
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sequence so that you have a chance to switch floppy disks.
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The command line equivalent is: "prompt_ramdisk=1"
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Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
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So to create disk one of the set, you would do::
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/usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0
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/usr/src/linux# rdev /dev/fd0 /dev/fd0
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/usr/src/linux# rdev -r /dev/fd0 49152
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If you make a boot disk that has LILO, then for the above, you would use::
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append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
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Since the default start = 0 and the default prompt = 1, you could use::
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append = "load_ramdisk=1"
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4) An Example of Creating a Compressed RAM Disk
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-----------------------------------------------
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To create a RAM disk image, you will need a spare block device to
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construct it on. This can be the RAM disk device itself, or an
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unused disk partition (such as an unmounted swap partition). For this
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example, we will use the RAM disk device, "/dev/ram0".
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Note: This technique should not be done on a machine with less than 8 MB
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of RAM. If using a spare disk partition instead of /dev/ram0, then this
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restriction does not apply.
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a) Decide on the RAM disk size that you want. Say 2 MB for this example.
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Create it by writing to the RAM disk device. (This step is not currently
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required, but may be in the future.) It is wise to zero out the
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area (esp. for disks) so that maximal compression is achieved for
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the unused blocks of the image that you are about to create::
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dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
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b) Make a filesystem on it. Say ext2fs for this example::
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mke2fs -vm0 /dev/ram0 2048
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c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
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and unmount it again.
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d) Compress the contents of the RAM disk. The level of compression
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will be approximately 50% of the space used by the files. Unused
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space on the RAM disk will compress to almost nothing::
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dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
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e) Put the kernel onto the floppy::
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dd if=zImage of=/dev/fd0 bs=1k
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f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
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that is slightly larger than the kernel, so that you can put another
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(possibly larger) kernel onto the same floppy later without overlapping
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the RAM disk image. An offset of 400 kB for kernels about 350 kB in
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size would be reasonable. Make sure offset+size of ram_image.gz is
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not larger than the total space on your floppy (usually 1440 kB)::
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dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
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g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
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For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
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have 2^15 + 2^14 + 400 = 49552::
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rdev /dev/fd0 /dev/fd0
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rdev -r /dev/fd0 49552
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That is it. You now have your boot/root compressed RAM disk floppy. Some
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users may wish to combine steps (d) and (f) by using a pipe.
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Paul Gortmaker 12/95
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Changelog:
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----------
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10-22-04 :
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Updated to reflect changes in command line options, remove
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obsolete references, general cleanup.
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James Nelson (james4765@gmail.com)
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12-95 :
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Original Document
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