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1 | Using the RAM disk block device with Linux |
2 | ------------------------------------------ | |
3 | ||
4 | Contents: | |
5 | ||
6 | 1) Overview | |
7 | 2) Kernel Command Line Parameters | |
8 | 3) Using "rdev -r" | |
086626a7 | 9 | 4) An Example of Creating a Compressed RAM Disk |
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10 | |
11 | ||
12 | 1) Overview | |
13 | ----------- | |
14 | ||
15 | The RAM disk driver is a way to use main system memory as a block device. It | |
16 | is required for initrd, an initial filesystem used if you need to load modules | |
17 | in order to access the root filesystem (see Documentation/initrd.txt). It can | |
18 | also be used for a temporary filesystem for crypto work, since the contents | |
19 | are erased on reboot. | |
20 | ||
21 | The RAM disk dynamically grows as more space is required. It does this by using | |
22 | RAM from the buffer cache. The driver marks the buffers it is using as dirty | |
23 | so that the VM subsystem does not try to reclaim them later. | |
24 | ||
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25 | The RAM disk supports up to 16 RAM disks by default, and can be reconfigured |
26 | to support an unlimited number of RAM disks (at your own risk). Just change | |
27 | the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu | |
28 | and (re)build the kernel. | |
29 | ||
30 | To use RAM disk support with your system, run './MAKEDEV ram' from the /dev | |
31 | directory. RAM disks are all major number 1, and start with minor number 0 | |
32 | for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd. | |
1da177e4 | 33 | |
1da177e4 | 34 | The new RAM disk also has the ability to load compressed RAM disk images, |
086626a7 | 35 | allowing one to squeeze more programs onto an average installation or |
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36 | rescue floppy disk. |
37 | ||
38 | ||
f5abc8e7 | 39 | 2) Parameters |
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40 | --------------------------------- |
41 | ||
f5abc8e7 FF |
42 | 2a) Kernel Command Line Parameters |
43 | ||
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44 | ramdisk_size=N |
45 | ============== | |
46 | ||
47 | This parameter tells the RAM disk driver to set up RAM disks of N k size. The | |
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48 | default is 4096 (4 MB). |
49 | ||
50 | 2b) Module parameters | |
1da177e4 | 51 | |
f5abc8e7 FF |
52 | rd_nr |
53 | ===== | |
54 | /dev/ramX devices created. | |
1da177e4 | 55 | |
f5abc8e7 FF |
56 | max_part |
57 | ======== | |
58 | Maximum partition number. | |
1da177e4 | 59 | |
f5abc8e7 FF |
60 | rd_size |
61 | ======= | |
62 | See ramdisk_size. | |
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63 | |
64 | 3) Using "rdev -r" | |
65 | ------------------ | |
66 | ||
67 | The usage of the word (two bytes) that "rdev -r" sets in the kernel image is | |
68 | as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up | |
69 | to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit | |
70 | 14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a | |
71 | prompt/wait sequence is to be given before trying to read the RAM disk. Since | |
72 | the RAM disk dynamically grows as data is being written into it, a size field | |
73 | is not required. Bits 11 to 13 are not currently used and may as well be zero. | |
74 | These numbers are no magical secrets, as seen below: | |
75 | ||
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76 | ./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF |
77 | ./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000 | |
78 | ./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000 | |
1da177e4 | 79 | |
086626a7 | 80 | Consider a typical two floppy disk setup, where you will have the |
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81 | kernel on disk one, and have already put a RAM disk image onto disk #2. |
82 | ||
83 | Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk | |
84 | starts at an offset of 0 kB from the beginning of the floppy. | |
85 | The command line equivalent is: "ramdisk_start=0" | |
86 | ||
87 | You want bit 14 as one, indicating that a RAM disk is to be loaded. | |
88 | The command line equivalent is: "load_ramdisk=1" | |
89 | ||
90 | You want bit 15 as one, indicating that you want a prompt/keypress | |
91 | sequence so that you have a chance to switch floppy disks. | |
92 | The command line equivalent is: "prompt_ramdisk=1" | |
93 | ||
94 | Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word. | |
95 | So to create disk one of the set, you would do: | |
96 | ||
25eb650a | 97 | /usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0 |
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98 | /usr/src/linux# rdev /dev/fd0 /dev/fd0 |
99 | /usr/src/linux# rdev -r /dev/fd0 49152 | |
100 | ||
101 | If you make a boot disk that has LILO, then for the above, you would use: | |
102 | append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1" | |
103 | Since the default start = 0 and the default prompt = 1, you could use: | |
104 | append = "load_ramdisk=1" | |
105 | ||
106 | ||
086626a7 | 107 | 4) An Example of Creating a Compressed RAM Disk |
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108 | ---------------------------------------------- |
109 | ||
110 | To create a RAM disk image, you will need a spare block device to | |
111 | construct it on. This can be the RAM disk device itself, or an | |
086626a7 | 112 | unused disk partition (such as an unmounted swap partition). For this |
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113 | example, we will use the RAM disk device, "/dev/ram0". |
114 | ||
115 | Note: This technique should not be done on a machine with less than 8 MB | |
116 | of RAM. If using a spare disk partition instead of /dev/ram0, then this | |
117 | restriction does not apply. | |
118 | ||
119 | a) Decide on the RAM disk size that you want. Say 2 MB for this example. | |
120 | Create it by writing to the RAM disk device. (This step is not currently | |
121 | required, but may be in the future.) It is wise to zero out the | |
122 | area (esp. for disks) so that maximal compression is achieved for | |
123 | the unused blocks of the image that you are about to create. | |
124 | ||
125 | dd if=/dev/zero of=/dev/ram0 bs=1k count=2048 | |
126 | ||
127 | b) Make a filesystem on it. Say ext2fs for this example. | |
128 | ||
129 | mke2fs -vm0 /dev/ram0 2048 | |
130 | ||
131 | c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...) | |
132 | and unmount it again. | |
133 | ||
134 | d) Compress the contents of the RAM disk. The level of compression | |
135 | will be approximately 50% of the space used by the files. Unused | |
136 | space on the RAM disk will compress to almost nothing. | |
137 | ||
138 | dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz | |
139 | ||
140 | e) Put the kernel onto the floppy | |
141 | ||
142 | dd if=zImage of=/dev/fd0 bs=1k | |
143 | ||
144 | f) Put the RAM disk image onto the floppy, after the kernel. Use an offset | |
145 | that is slightly larger than the kernel, so that you can put another | |
146 | (possibly larger) kernel onto the same floppy later without overlapping | |
147 | the RAM disk image. An offset of 400 kB for kernels about 350 kB in | |
148 | size would be reasonable. Make sure offset+size of ram_image.gz is | |
149 | not larger than the total space on your floppy (usually 1440 kB). | |
150 | ||
151 | dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400 | |
152 | ||
153 | g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc. | |
154 | For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would | |
155 | have 2^15 + 2^14 + 400 = 49552. | |
156 | ||
157 | rdev /dev/fd0 /dev/fd0 | |
158 | rdev -r /dev/fd0 49552 | |
159 | ||
160 | That is it. You now have your boot/root compressed RAM disk floppy. Some | |
161 | users may wish to combine steps (d) and (f) by using a pipe. | |
162 | ||
163 | -------------------------------------------------------------------------- | |
164 | Paul Gortmaker 12/95 | |
165 | ||
166 | Changelog: | |
167 | ---------- | |
168 | ||
169 | 10-22-04 : Updated to reflect changes in command line options, remove | |
170 | obsolete references, general cleanup. | |
171 | James Nelson (james4765@gmail.com) | |
172 | ||
173 | ||
174 | 12-95 : Original Document |