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/Documentation/ramdisk.txt

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  1Using the RAM disk block device with Linux
  2------------------------------------------
  3
  4Contents:
  5
  6	1) Overview
  7	2) Kernel Command Line Parameters
  8	3) Using "rdev -r"
  9	4) An Example of Creating a Compressed RAM Disk 
 10
 11
 121) Overview
 13-----------
 14
 15The RAM disk driver is a way to use main system memory as a block device.  It
 16is required for initrd, an initial filesystem used if you need to load modules
 17in order to access the root filesystem (see Documentation/initrd.txt).  It can
 18also be used for a temporary filesystem for crypto work, since the contents
 19are erased on reboot.
 20
 21The RAM disk dynamically grows as more space is required. It does this by using
 22RAM from the buffer cache. The driver marks the buffers it is using as dirty
 23so that the VM subsystem does not try to reclaim them later.
 24
 25Also, the RAM disk supports up to 16 RAM disks out of the box, and can
 26be reconfigured to support up to 255 RAM disks - change "#define NUM_RAMDISKS"
 27in drivers/block/rd.c.  To use RAM disk support with your system, run
 28'./MAKEDEV ram' from the /dev directory.  RAM disks are all major number 1, and
 29start with minor number 0 for /dev/ram0, etc.  If used, modern kernels use
 30/dev/ram0 for an initrd.
 31
 32The old "ramdisk=<ram_size>" has been changed to "ramdisk_size=<ram_size>" to
 33make it clearer.  The original "ramdisk=<ram_size>" has been kept around for
 34compatibility reasons, but it may be removed in the future.
 35
 36The new RAM disk also has the ability to load compressed RAM disk images,
 37allowing one to squeeze more programs onto an average installation or 
 38rescue floppy disk.
 39
 40
 412) Kernel Command Line Parameters
 42---------------------------------
 43
 44	ramdisk_size=N
 45	==============
 46
 47This parameter tells the RAM disk driver to set up RAM disks of N k size.  The
 48default is 4096 (4 MB) (8192 (8 MB) on S390).
 49
 50	ramdisk_blocksize=N
 51	===================
 52
 53This parameter tells the RAM disk driver how many bytes to use per block.  The
 54default is 512.
 55
 56
 573) Using "rdev -r"
 58------------------
 59
 60The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
 61as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
 62to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
 6314 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
 64prompt/wait sequence is to be given before trying to read the RAM disk. Since
 65the RAM disk dynamically grows as data is being written into it, a size field
 66is not required. Bits 11 to 13 are not currently used and may as well be zero.
 67These numbers are no magical secrets, as seen below:
 68
 69./arch/i386/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK     0x07FF
 70./arch/i386/kernel/setup.c:#define RAMDISK_PROMPT_FLAG          0x8000
 71./arch/i386/kernel/setup.c:#define RAMDISK_LOAD_FLAG            0x4000
 72
 73Consider a typical two floppy disk setup, where you will have the 
 74kernel on disk one, and have already put a RAM disk image onto disk #2.
 75
 76Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
 77starts at an offset of 0 kB from the beginning of the floppy.
 78The command line equivalent is: "ramdisk_start=0"
 79
 80You want bit 14 as one, indicating that a RAM disk is to be loaded.
 81The command line equivalent is: "load_ramdisk=1"
 82
 83You want bit 15 as one, indicating that you want a prompt/keypress
 84sequence so that you have a chance to switch floppy disks.
 85The command line equivalent is: "prompt_ramdisk=1"
 86
 87Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
 88So to create disk one of the set, you would do:
 89
 90	/usr/src/linux# cat arch/i386/boot/zImage > /dev/fd0
 91	/usr/src/linux# rdev /dev/fd0 /dev/fd0
 92	/usr/src/linux# rdev -r /dev/fd0 49152
 93
 94If you make a boot disk that has LILO, then for the above, you would use:
 95	append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
 96Since the default start = 0 and the default prompt = 1, you could use:
 97	append = "load_ramdisk=1"
 98
 99
1004) An Example of Creating a Compressed RAM Disk 
101----------------------------------------------
102
103To create a RAM disk image, you will need a spare block device to
104construct it on. This can be the RAM disk device itself, or an
105unused disk partition (such as an unmounted swap partition). For this 
106example, we will use the RAM disk device, "/dev/ram0".
107
108Note: This technique should not be done on a machine with less than 8 MB
109of RAM. If using a spare disk partition instead of /dev/ram0, then this
110restriction does not apply.
111
112a) Decide on the RAM disk size that you want. Say 2 MB for this example.
113   Create it by writing to the RAM disk device. (This step is not currently
114   required, but may be in the future.) It is wise to zero out the
115   area (esp. for disks) so that maximal compression is achieved for
116   the unused blocks of the image that you are about to create.
117
118	dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
119
120b) Make a filesystem on it. Say ext2fs for this example.
121
122	mke2fs -vm0 /dev/ram0 2048
123
124c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
125   and unmount it again.
126
127d) Compress the contents of the RAM disk. The level of compression
128   will be approximately 50% of the space used by the files. Unused
129   space on the RAM disk will compress to almost nothing.
130
131	dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
132
133e) Put the kernel onto the floppy
134
135	dd if=zImage of=/dev/fd0 bs=1k
136
137f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
138   that is slightly larger than the kernel, so that you can put another
139   (possibly larger) kernel onto the same floppy later without overlapping
140   the RAM disk image. An offset of 400 kB for kernels about 350 kB in
141   size would be reasonable. Make sure offset+size of ram_image.gz is
142   not larger than the total space on your floppy (usually 1440 kB).
143
144	dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
145
146g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
147   For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
148   have 2^15 + 2^14 + 400 = 49552.
149
150	rdev /dev/fd0 /dev/fd0
151	rdev -r /dev/fd0 49552
152
153That is it. You now have your boot/root compressed RAM disk floppy. Some
154users may wish to combine steps (d) and (f) by using a pipe.
155
156--------------------------------------------------------------------------
157						Paul Gortmaker 12/95
158
159Changelog:
160----------
161
16210-22-04 :	Updated to reflect changes in command line options, remove
163		obsolete references, general cleanup.
164		James Nelson (james4765@gmail.com)
165
166
16712-95 :		Original Document