1) Overview
2) Kernel Command Line Parameters
- 3) Using "rdev -r" With New Kernels
- 4) An Example of Creating a Compressed RAM Disk
+ 3) Using "rdev -r"
+ 4) An Example of Creating a Compressed RAM Disk
1) Overview
-----------
-As of kernel v1.3.48, the RAM disk driver was substantially changed.
+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 older versions would grab a chunk of memory off the top before
-handing the remainder to the kernel at boot time. Thus a size parameter
-had to be specified via "ramdisk=1440" or "rdev -r /dev/fd0 1440" so
-that the driver knew how much memory to grab.
+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.
-Now 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 with a new "BH_Protected" flag so that the kernel does
-not try to reuse them later. This means that the old size parameter
-is no longer used, new command line parameters exist, and the behavior
-of the "rdev -r" or "ramsize" (usually a symbolic link to "rdev")
-command has changed.
+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.
-Also, the new RAM disk supports up to 16 RAM disks out of the box, and can
-be reconfigured in rd.c to support up to 255 RAM disks. To use multiple
-RAM disk support with your system, run 'mknod /dev/ramX b 1 X' and chmod
-(to change its permissions) it to your liking. The default /dev/ram(disk)
-uses minor #1, so start with ram2 and go from there.
-
-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 will probably be removed in 2.1.x.
+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
+allowing one to squeeze more programs onto an average installation or
rescue floppy disk.
-Notes: You may have "/dev/ram" or "/dev/ramdisk" or both. They are
-equivalent from the standpoint of this document. Also, the new RAM disk
-is a config option. When running "make config", make sure you enable
-RAM disk support for the kernel with which you intend to use the RAM disk.
-
2) Kernel Command Line Parameters
---------------------------------
- ramdisk_start=NNN
- =================
-
-To allow a kernel image to reside on a floppy disk along with a compressed
-RAM disk image, the "ramdisk_start=<offset>" command was added. The kernel
-can't be included into the compressed RAM disk filesystem image, because
-it needs to be stored starting at block zero so that the BIOS can load the
-boot sector and then the kernel can bootstrap itself to get going.
-
-Note: If you are using an uncompressed RAM disk image, then the kernel can
-be a part of the filesystem image that is being loaded into the RAM disk,
-and the floppy can be booted with LILO, or the two can be separate as
-is done for the compressed images.
-
-If you are using a two-disk boot/root setup (kernel on #1, RAM disk image
-on #2) then the RAM disk would start at block zero, and an offset of
-zero would be used. Since this is the default value, you would not need
-to actually use the command at all.
-
-If instead, you have a "zImage" of about 350 kB, and a "fs_image.gz" of
-say about 1 MB, and you want them both on the same disk, then you
-would use an offset. If you stored the "fs_image.gz" onto the floppy
-starting at an offset of 400 kB, you would use "ramdisk_start=400".
-
-
- load_ramdisk=N
+ ramdisk_size=N
==============
-This parameter tells the kernel whether it is to try to load a
-RAM disk image or not. Specifying "load_ramdisk=1" will tell the
-kernel to load a floppy into the RAM disk. The default value is
-zero, meaning that the kernel should not try to load a RAM disk.
-
+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).
- prompt_ramdisk=N
- ================
+ ramdisk_blocksize=N
+ ===================
-This parameter tells the kernel whether or not to give you a prompt
-asking you to insert the floppy containing the RAM disk image. In
-a single floppy configuration the RAM disk image is on the same floppy
-as the kernel that just finished loading/booting and so a prompt
-is not needed. In this case one can use "prompt_ramdisk=0". In a
-two floppy configuration, you will need the chance to switch disks,
-and thus "prompt_ramdisk=1" can be used. Since this is the default
-value, it doesn't really need to be specified.
+This parameter tells the RAM disk driver how many bytes to use per block. The
+default is 1024 (BLOCK_SIZE).
- 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).
-3) Using "rdev -r" With New Kernels
------------------------------------
+3) Using "rdev -r"
+------------------
-The usage of the word (two bytes) that "rdev -r" sets in the kernel image
-has changed. 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 no longer required. Bits 11
-to 13 are not currently used and may as well be zero. These numbers
-are no magical secrets, as seen below:
+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
+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
append = "load_ramdisk=1"
-4) An Example of Creating a Compressed RAM Disk
+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/ram".
+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/ram, then this
+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.
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/ram bs=1k count=2048
+ 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/ram 2048
+ mke2fs -vm0 /dev/ram0 2048
c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
and unmount it again.
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/ram bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
+ dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
e) Put the kernel onto the floppy
users may wish to combine steps (d) and (f) by using a pipe.
--------------------------------------------------------------------------
- Paul Gortmaker 12/95
+ 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
git://git.onelab.eu / linux-2.6.git / blobdiff