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diff --git a/Documentation/blockdev/ramdisk.txt b/Documentation/blockdev/ramdisk.txt deleted file mode 100644 index 501e12e0323e..000000000000 --- a/Documentation/blockdev/ramdisk.txt +++ /dev/null @@ -1,174 +0,0 @@ -Using the RAM disk block device with Linux ------------------------------------------- - -Contents: - - 1) Overview - 2) Kernel Command Line Parameters - 3) Using "rdev -r" - 4) An Example of Creating a Compressed RAM Disk - - -1) Overview ------------ - -The RAM disk driver is a way to use main system memory as a block device. It -is required for initrd, an initial filesystem used if you need to load modules -in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can -also be used for a temporary filesystem for crypto work, since the contents -are erased on reboot. - -The RAM disk dynamically grows as more space is required. It does this by using -RAM from the buffer cache. The driver marks the buffers it is using as dirty -so that the VM subsystem does not try to reclaim them later. - -The RAM disk supports up to 16 RAM disks by default, and can be reconfigured -to support an unlimited number of RAM disks (at your own risk). Just change -the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu -and (re)build the kernel. - -To use RAM disk support with your system, run './MAKEDEV ram' from the /dev -directory. RAM disks are all major number 1, and start with minor number 0 -for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd. - -The new RAM disk also has the ability to load compressed RAM disk images, -allowing one to squeeze more programs onto an average installation or -rescue floppy disk. - - -2) Parameters ---------------------------------- - -2a) Kernel Command Line Parameters - - ramdisk_size=N - ============== - -This parameter tells the RAM disk driver to set up RAM disks of N k size. The -default is 4096 (4 MB). - -2b) Module parameters - - rd_nr - ===== - /dev/ramX devices created. - - max_part - ======== - Maximum partition number. - - rd_size - ======= - See ramdisk_size. - -3) Using "rdev -r" ------------------- - -The usage of the word (two bytes) that "rdev -r" sets in the kernel image is -as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up -to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit -14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a -prompt/wait sequence is to be given before trying to read the RAM disk. Since -the RAM disk dynamically grows as data is being written into it, a size field -is not required. Bits 11 to 13 are not currently used and may as well be zero. -These numbers are no magical secrets, as seen below: - -./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF -./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000 -./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000 - -Consider a typical two floppy disk setup, where you will have the -kernel on disk one, and have already put a RAM disk image onto disk #2. - -Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk -starts at an offset of 0 kB from the beginning of the floppy. -The command line equivalent is: "ramdisk_start=0" - -You want bit 14 as one, indicating that a RAM disk is to be loaded. -The command line equivalent is: "load_ramdisk=1" - -You want bit 15 as one, indicating that you want a prompt/keypress -sequence so that you have a chance to switch floppy disks. -The command line equivalent is: "prompt_ramdisk=1" - -Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word. -So to create disk one of the set, you would do: - - /usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0 - /usr/src/linux# rdev /dev/fd0 /dev/fd0 - /usr/src/linux# rdev -r /dev/fd0 49152 - -If you make a boot disk that has LILO, then for the above, you would use: - append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1" -Since the default start = 0 and the default prompt = 1, you could use: - append = "load_ramdisk=1" - - -4) An Example of Creating a Compressed RAM Disk ----------------------------------------------- - -To create a RAM disk image, you will need a spare block device to -construct it on. This can be the RAM disk device itself, or an -unused disk partition (such as an unmounted swap partition). For this -example, we will use the RAM disk device, "/dev/ram0". - -Note: This technique should not be done on a machine with less than 8 MB -of RAM. If using a spare disk partition instead of /dev/ram0, then this -restriction does not apply. - -a) Decide on the RAM disk size that you want. Say 2 MB for this example. - Create it by writing to the RAM disk device. (This step is not currently - required, but may be in the future.) It is wise to zero out the - area (esp. for disks) so that maximal compression is achieved for - the unused blocks of the image that you are about to create. - - dd if=/dev/zero of=/dev/ram0 bs=1k count=2048 - -b) Make a filesystem on it. Say ext2fs for this example. - - mke2fs -vm0 /dev/ram0 2048 - -c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...) - and unmount it again. - -d) Compress the contents of the RAM disk. The level of compression - will be approximately 50% of the space used by the files. Unused - space on the RAM disk will compress to almost nothing. - - dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz - -e) Put the kernel onto the floppy - - dd if=zImage of=/dev/fd0 bs=1k - -f) Put the RAM disk image onto the floppy, after the kernel. Use an offset - that is slightly larger than the kernel, so that you can put another - (possibly larger) kernel onto the same floppy later without overlapping - the RAM disk image. An offset of 400 kB for kernels about 350 kB in - size would be reasonable. Make sure offset+size of ram_image.gz is - not larger than the total space on your floppy (usually 1440 kB). - - dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400 - -g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc. - For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would - have 2^15 + 2^14 + 400 = 49552. - - rdev /dev/fd0 /dev/fd0 - rdev -r /dev/fd0 49552 - -That is it. You now have your boot/root compressed RAM disk floppy. Some -users may wish to combine steps (d) and (f) by using a pipe. - --------------------------------------------------------------------------- - Paul Gortmaker 12/95 - -Changelog: ----------- - -10-22-04 : Updated to reflect changes in command line options, remove - obsolete references, general cleanup. - James Nelson (james4765@gmail.com) - - -12-95 : Original Document |