Root/Documentation/blockdev/ramdisk.txt

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
25The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
26to support an unlimited number of RAM disks (at your own risk). Just change
27the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
28and (re)build the kernel.
29
30To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
31directory. RAM disks are all major number 1, and start with minor number 0
32for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
33
34The new RAM disk also has the ability to load compressed RAM disk images,
35allowing one to squeeze more programs onto an average installation or
36rescue floppy disk.
37
38
392) Kernel Command Line Parameters
40---------------------------------
41
42    ramdisk_size=N
43    ==============
44
45This parameter tells the RAM disk driver to set up RAM disks of N k size. The
46default is 4096 (4 MB) (8192 (8 MB) on S390).
47
48    ramdisk_blocksize=N
49    ===================
50
51This parameter tells the RAM disk driver how many bytes to use per block. The
52default is 1024 (BLOCK_SIZE).
53
54
553) Using "rdev -r"
56------------------
57
58The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
59as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
60to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
6114 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
62prompt/wait sequence is to be given before trying to read the RAM disk. Since
63the RAM disk dynamically grows as data is being written into it, a size field
64is not required. Bits 11 to 13 are not currently used and may as well be zero.
65These numbers are no magical secrets, as seen below:
66
67./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
68./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
69./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000
70
71Consider a typical two floppy disk setup, where you will have the
72kernel on disk one, and have already put a RAM disk image onto disk #2.
73
74Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
75starts at an offset of 0 kB from the beginning of the floppy.
76The command line equivalent is: "ramdisk_start=0"
77
78You want bit 14 as one, indicating that a RAM disk is to be loaded.
79The command line equivalent is: "load_ramdisk=1"
80
81You want bit 15 as one, indicating that you want a prompt/keypress
82sequence so that you have a chance to switch floppy disks.
83The command line equivalent is: "prompt_ramdisk=1"
84
85Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
86So to create disk one of the set, you would do:
87
88    /usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0
89    /usr/src/linux# rdev /dev/fd0 /dev/fd0
90    /usr/src/linux# rdev -r /dev/fd0 49152
91
92If you make a boot disk that has LILO, then for the above, you would use:
93    append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
94Since the default start = 0 and the default prompt = 1, you could use:
95    append = "load_ramdisk=1"
96
97
984) An Example of Creating a Compressed RAM Disk
99----------------------------------------------
100
101To create a RAM disk image, you will need a spare block device to
102construct it on. This can be the RAM disk device itself, or an
103unused disk partition (such as an unmounted swap partition). For this
104example, we will use the RAM disk device, "/dev/ram0".
105
106Note: This technique should not be done on a machine with less than 8 MB
107of RAM. If using a spare disk partition instead of /dev/ram0, then this
108restriction does not apply.
109
110a) Decide on the RAM disk size that you want. Say 2 MB for this example.
111   Create it by writing to the RAM disk device. (This step is not currently
112   required, but may be in the future.) It is wise to zero out the
113   area (esp. for disks) so that maximal compression is achieved for
114   the unused blocks of the image that you are about to create.
115
116    dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
117
118b) Make a filesystem on it. Say ext2fs for this example.
119
120    mke2fs -vm0 /dev/ram0 2048
121
122c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
123   and unmount it again.
124
125d) Compress the contents of the RAM disk. The level of compression
126   will be approximately 50% of the space used by the files. Unused
127   space on the RAM disk will compress to almost nothing.
128
129    dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
130
131e) Put the kernel onto the floppy
132
133    dd if=zImage of=/dev/fd0 bs=1k
134
135f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
136   that is slightly larger than the kernel, so that you can put another
137   (possibly larger) kernel onto the same floppy later without overlapping
138   the RAM disk image. An offset of 400 kB for kernels about 350 kB in
139   size would be reasonable. Make sure offset+size of ram_image.gz is
140   not larger than the total space on your floppy (usually 1440 kB).
141
142    dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
143
144g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
145   For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
146   have 2^15 + 2^14 + 400 = 49552.
147
148    rdev /dev/fd0 /dev/fd0
149    rdev -r /dev/fd0 49552
150
151That is it. You now have your boot/root compressed RAM disk floppy. Some
152users may wish to combine steps (d) and (f) by using a pipe.
153
154--------------------------------------------------------------------------
155                        Paul Gortmaker 12/95
156
157Changelog:
158----------
159
16010-22-04 : Updated to reflect changes in command line options, remove
161        obsolete references, general cleanup.
162        James Nelson (james4765@gmail.com)
163
164
16512-95 : Original Document
166

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