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153 lines
4.9 KiB
153 lines
4.9 KiB
========================================== |
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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" |
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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" |
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--------------- |
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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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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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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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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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Paul Gortmaker 12/95 |
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Changelog: |
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---------- |
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SEPT-2020 : |
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Removed usage of "rdev" |
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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 ([email protected]) |
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12-95 : |
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Original Document
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