2019-04-19 04:29:24 +08:00
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==========================================
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2005-04-17 06:20:36 +08:00
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Using the RAM disk block device with Linux
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2019-04-19 04:29:24 +08:00
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==========================================
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2005-04-17 06:20:36 +08:00
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2019-04-19 04:29:24 +08:00
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.. Contents:
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1) Overview
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2) Kernel Command Line Parameters
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2020-09-18 09:56:40 +08:00
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3) Using "rdev"
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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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2016-10-18 20:12:27 +08:00
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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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2014-01-24 07:55:27 +08:00
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2) Parameters
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---------------------------------
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2014-01-24 07:55:27 +08:00
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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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2020-09-18 09:56:40 +08:00
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3) Using "rdev"
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---------------
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2005-04-17 06:20:36 +08:00
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2020-09-18 09:56:40 +08:00
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"rdev" is an obsolete, deprecated, antiquated utility that could be used
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to set the boot device in a Linux kernel image.
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2020-09-18 09:56:40 +08:00
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Instead of using rdev, just place the boot device information on the
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kernel command line and pass it to the kernel from the bootloader.
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You can also pass arguments to the kernel by setting FDARGS in
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arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in
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arch/x86/boot/Makefile.
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2020-09-18 09:56:40 +08:00
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Some of the kernel command line boot options that may apply here are::
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ramdisk_start=N
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ramdisk_size=M
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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=N ramdisk_size=M"
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2006-07-14 15:24:10 +08:00
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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) Make sure that you have already specified the boot information in
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FDARGS and FDINITRD or that you use a bootloader to pass kernel
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command line boot options to the kernel.
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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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2019-04-19 04:29:24 +08:00
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2005-04-17 06:20:36 +08:00
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Paul Gortmaker 12/95
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Changelog:
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----------
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2020-09-18 09:56:40 +08:00
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SEPT-2020 :
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Removed usage of "rdev"
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2019-04-19 04:29:24 +08:00
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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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2019-04-19 04:29:24 +08:00
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12-95 :
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Original Document
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