path: root/Documentation/ramdisk.txt
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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/ramdisk.txt
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+Using the RAM disk block device with Linux
+ 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/initrd.txt). 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.
+Also, the RAM disk supports up to 16 RAM disks out of the box, and can
+be reconfigured to support up to 255 RAM disks - change "#define NUM_RAMDISKS"
+in drivers/block/rd.c. 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 old "ramdisk=<ram_size>" has been changed to "ramdisk_size=<ram_size>" to
+make it clearer. The original "ramdisk=<ram_size>" has been kept around for
+compatibility reasons, but it may be removed in the future.
+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) 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) (8192 (8 MB) on S390).
+ ramdisk_blocksize=N
+ ===================
+This parameter tells the RAM disk driver how many bytes to use per block. The
+default is 512.
+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/i386/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
+./arch/i386/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
+./arch/i386/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/i386/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
+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