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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 | ||
25 | Also, the RAM disk supports up to 16 RAM disks out of the box, and can | |
26 | be reconfigured to support up to 255 RAM disks - change "#define NUM_RAMDISKS" | |
27 | in 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 | |
29 | start with minor number 0 for /dev/ram0, etc. If used, modern kernels use | |
30 | /dev/ram0 for an initrd. | |
31 | ||
32 | The old "ramdisk=<ram_size>" has been changed to "ramdisk_size=<ram_size>" to | |
33 | make it clearer. The original "ramdisk=<ram_size>" has been kept around for | |
34 | compatibility reasons, but it may be removed in the future. | |
35 | ||
36 | The new RAM disk also has the ability to load compressed RAM disk images, | |
086626a7 | 37 | allowing one to squeeze more programs onto an average installation or |
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38 | rescue floppy disk. |
39 | ||
40 | ||
41 | 2) Kernel Command Line Parameters | |
42 | --------------------------------- | |
43 | ||
44 | ramdisk_size=N | |
45 | ============== | |
46 | ||
47 | This parameter tells the RAM disk driver to set up RAM disks of N k size. The | |
48 | default is 4096 (4 MB) (8192 (8 MB) on S390). | |
49 | ||
50 | ramdisk_blocksize=N | |
51 | =================== | |
52 | ||
53 | This parameter tells the RAM disk driver how many bytes to use per block. The | |
086626a7 | 54 | default is 1024 (BLOCK_SIZE). |
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55 | |
56 | ||
57 | 3) Using "rdev -r" | |
58 | ------------------ | |
59 | ||
60 | The usage of the word (two bytes) that "rdev -r" sets in the kernel image is | |
61 | as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up | |
62 | to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit | |
63 | 14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a | |
64 | prompt/wait sequence is to be given before trying to read the RAM disk. Since | |
65 | the RAM disk dynamically grows as data is being written into it, a size field | |
66 | is not required. Bits 11 to 13 are not currently used and may as well be zero. | |
67 | These 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 | ||
086626a7 | 73 | Consider a typical two floppy disk setup, where you will have the |
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74 | kernel on disk one, and have already put a RAM disk image onto disk #2. |
75 | ||
76 | Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk | |
77 | starts at an offset of 0 kB from the beginning of the floppy. | |
78 | The command line equivalent is: "ramdisk_start=0" | |
79 | ||
80 | You want bit 14 as one, indicating that a RAM disk is to be loaded. | |
81 | The command line equivalent is: "load_ramdisk=1" | |
82 | ||
83 | You want bit 15 as one, indicating that you want a prompt/keypress | |
84 | sequence so that you have a chance to switch floppy disks. | |
85 | The command line equivalent is: "prompt_ramdisk=1" | |
86 | ||
87 | Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word. | |
88 | So 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 | ||
94 | If 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" | |
96 | Since the default start = 0 and the default prompt = 1, you could use: | |
97 | append = "load_ramdisk=1" | |
98 | ||
99 | ||
086626a7 | 100 | 4) An Example of Creating a Compressed RAM Disk |
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101 | ---------------------------------------------- |
102 | ||
103 | To create a RAM disk image, you will need a spare block device to | |
104 | construct it on. This can be the RAM disk device itself, or an | |
086626a7 | 105 | unused disk partition (such as an unmounted swap partition). For this |
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106 | example, we will use the RAM disk device, "/dev/ram0". |
107 | ||
108 | Note: This technique should not be done on a machine with less than 8 MB | |
109 | of RAM. If using a spare disk partition instead of /dev/ram0, then this | |
110 | restriction does not apply. | |
111 | ||
112 | a) 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 | ||
120 | b) Make a filesystem on it. Say ext2fs for this example. | |
121 | ||
122 | mke2fs -vm0 /dev/ram0 2048 | |
123 | ||
124 | c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...) | |
125 | and unmount it again. | |
126 | ||
127 | d) 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 | ||
133 | e) Put the kernel onto the floppy | |
134 | ||
135 | dd if=zImage of=/dev/fd0 bs=1k | |
136 | ||
137 | f) 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 | ||
146 | g) 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 | ||
153 | That is it. You now have your boot/root compressed RAM disk floppy. Some | |
154 | users may wish to combine steps (d) and (f) by using a pipe. | |
155 | ||
156 | -------------------------------------------------------------------------- | |
157 | Paul Gortmaker 12/95 | |
158 | ||
159 | Changelog: | |
160 | ---------- | |
161 | ||
162 | 10-22-04 : Updated to reflect changes in command line options, remove | |
163 | obsolete references, general cleanup. | |
164 | James Nelson (james4765@gmail.com) | |
165 | ||
166 | ||
167 | 12-95 : Original Document |