1 ========================================== 1 ==========================================
2 Using the RAM disk block device with Linux 2 Using the RAM disk block device with Linux
3 ========================================== 3 ==========================================
4 4
5 .. Contents: 5 .. Contents:
6 6
7 1) Overview 7 1) Overview
8 2) Kernel Command Line Parameters 8 2) Kernel Command Line Parameters
9 3) Using "rdev" !! 9 3) Using "rdev -r"
10 4) An Example of Creating a Compressed 10 4) An Example of Creating a Compressed RAM Disk
11 11
12 12
13 1) Overview 13 1) Overview
14 ----------- 14 -----------
15 15
16 The RAM disk driver is a way to use main syste 16 The RAM disk driver is a way to use main system memory as a block device. It
17 is required for initrd, an initial filesystem 17 is required for initrd, an initial filesystem used if you need to load modules
18 in order to access the root filesystem (see Do 18 in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can
19 also be used for a temporary filesystem for cr 19 also be used for a temporary filesystem for crypto work, since the contents
20 are erased on reboot. 20 are erased on reboot.
21 21
22 The RAM disk dynamically grows as more space i 22 The RAM disk dynamically grows as more space is required. It does this by using
23 RAM from the buffer cache. The driver marks th 23 RAM from the buffer cache. The driver marks the buffers it is using as dirty
24 so that the VM subsystem does not try to recla 24 so that the VM subsystem does not try to reclaim them later.
25 25
26 The RAM disk supports up to 16 RAM disks by de 26 The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
27 to support an unlimited number of RAM disks (a 27 to support an unlimited number of RAM disks (at your own risk). Just change
28 the configuration symbol BLK_DEV_RAM_COUNT in 28 the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
29 and (re)build the kernel. 29 and (re)build the kernel.
30 30
31 To use RAM disk support with your system, run 31 To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
32 directory. RAM disks are all major number 1, 32 directory. RAM disks are all major number 1, and start with minor number 0
33 for /dev/ram0, etc. If used, modern kernels u 33 for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
34 34
35 The new RAM disk also has the ability to load 35 The new RAM disk also has the ability to load compressed RAM disk images,
36 allowing one to squeeze more programs onto an 36 allowing one to squeeze more programs onto an average installation or
37 rescue floppy disk. 37 rescue floppy disk.
38 38
39 39
40 2) Parameters 40 2) Parameters
41 --------------------------------- 41 ---------------------------------
42 42
43 2a) Kernel Command Line Parameters 43 2a) Kernel Command Line Parameters
44 44
45 ramdisk_size=N 45 ramdisk_size=N
46 Size of the ramdisk. 46 Size of the ramdisk.
47 47
48 This parameter tells the RAM disk driver to se 48 This parameter tells the RAM disk driver to set up RAM disks of N k size. The
49 default is 4096 (4 MB). 49 default is 4096 (4 MB).
50 50
51 2b) Module parameters 51 2b) Module parameters
52 52
53 rd_nr 53 rd_nr
54 /dev/ramX devices created. 54 /dev/ramX devices created.
55 55
56 max_part 56 max_part
57 Maximum partition number. 57 Maximum partition number.
58 58
59 rd_size 59 rd_size
60 See ramdisk_size. 60 See ramdisk_size.
61 61
62 3) Using "rdev" !! 62 3) Using "rdev -r"
63 --------------- !! 63 ------------------
64 64
65 "rdev" is an obsolete, deprecated, antiquated !! 65 The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
66 to set the boot device in a Linux kernel image !! 66 as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
>> 67 to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
>> 68 14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
>> 69 prompt/wait sequence is to be given before trying to read the RAM disk. Since
>> 70 the RAM disk dynamically grows as data is being written into it, a size field
>> 71 is not required. Bits 11 to 13 are not currently used and may as well be zero.
>> 72 These numbers are no magical secrets, as seen below::
>> 73
>> 74 ./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
>> 75 ./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
>> 76 ./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000
>> 77
>> 78 Consider a typical two floppy disk setup, where you will have the
>> 79 kernel on disk one, and have already put a RAM disk image onto disk #2.
>> 80
>> 81 Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
>> 82 starts at an offset of 0 kB from the beginning of the floppy.
>> 83 The command line equivalent is: "ramdisk_start=0"
>> 84
>> 85 You want bit 14 as one, indicating that a RAM disk is to be loaded.
>> 86 The command line equivalent is: "load_ramdisk=1"
>> 87
>> 88 You want bit 15 as one, indicating that you want a prompt/keypress
>> 89 sequence so that you have a chance to switch floppy disks.
>> 90 The command line equivalent is: "prompt_ramdisk=1"
>> 91
>> 92 Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
>> 93 So to create disk one of the set, you would do::
>> 94
>> 95 /usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0
>> 96 /usr/src/linux# rdev /dev/fd0 /dev/fd0
>> 97 /usr/src/linux# rdev -r /dev/fd0 49152
67 98
68 Instead of using rdev, just place the boot dev !! 99 If you make a boot disk that has LILO, then for the above, you would use::
69 kernel command line and pass it to the kernel <<
70 <<
71 You can also pass arguments to the kernel by s <<
72 arch/x86/boot/Makefile and specify in initrd i <<
73 arch/x86/boot/Makefile. <<
74 100
75 Some of the kernel command line boot options t !! 101 append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
76 102
77 ramdisk_start=N !! 103 Since the default start = 0 and the default prompt = 1, you could use::
78 ramdisk_size=M <<
79 104
80 If you make a boot disk that has LILO, then fo !! 105 append = "load_ramdisk=1"
81 106
82 append = "ramdisk_start=N ramdisk_size <<
83 107
84 4) An Example of Creating a Compressed RAM Dis 108 4) An Example of Creating a Compressed RAM Disk
85 ---------------------------------------------- 109 -----------------------------------------------
86 110
87 To create a RAM disk image, you will need a sp 111 To create a RAM disk image, you will need a spare block device to
88 construct it on. This can be the RAM disk devi 112 construct it on. This can be the RAM disk device itself, or an
89 unused disk partition (such as an unmounted sw 113 unused disk partition (such as an unmounted swap partition). For this
90 example, we will use the RAM disk device, "/de 114 example, we will use the RAM disk device, "/dev/ram0".
91 115
92 Note: This technique should not be done on a m 116 Note: This technique should not be done on a machine with less than 8 MB
93 of RAM. If using a spare disk partition instea 117 of RAM. If using a spare disk partition instead of /dev/ram0, then this
94 restriction does not apply. 118 restriction does not apply.
95 119
96 a) Decide on the RAM disk size that you want. 120 a) Decide on the RAM disk size that you want. Say 2 MB for this example.
97 Create it by writing to the RAM disk device 121 Create it by writing to the RAM disk device. (This step is not currently
98 required, but may be in the future.) It is 122 required, but may be in the future.) It is wise to zero out the
99 area (esp. for disks) so that maximal compr 123 area (esp. for disks) so that maximal compression is achieved for
100 the unused blocks of the image that you are 124 the unused blocks of the image that you are about to create::
101 125
102 dd if=/dev/zero of=/dev/ram0 bs=1k cou 126 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
103 127
104 b) Make a filesystem on it. Say ext2fs for thi 128 b) Make a filesystem on it. Say ext2fs for this example::
105 129
106 mke2fs -vm0 /dev/ram0 2048 130 mke2fs -vm0 /dev/ram0 2048
107 131
108 c) Mount it, copy the files you want to it (eg 132 c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
109 and unmount it again. 133 and unmount it again.
110 134
111 d) Compress the contents of the RAM disk. The 135 d) Compress the contents of the RAM disk. The level of compression
112 will be approximately 50% of the space used 136 will be approximately 50% of the space used by the files. Unused
113 space on the RAM disk will compress to almo 137 space on the RAM disk will compress to almost nothing::
114 138
115 dd if=/dev/ram0 bs=1k count=2048 | gzi 139 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
116 140
117 e) Put the kernel onto the floppy:: 141 e) Put the kernel onto the floppy::
118 142
119 dd if=zImage of=/dev/fd0 bs=1k 143 dd if=zImage of=/dev/fd0 bs=1k
120 144
121 f) Put the RAM disk image onto the floppy, aft 145 f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
122 that is slightly larger than the kernel, so 146 that is slightly larger than the kernel, so that you can put another
123 (possibly larger) kernel onto the same flop 147 (possibly larger) kernel onto the same floppy later without overlapping
124 the RAM disk image. An offset of 400 kB for 148 the RAM disk image. An offset of 400 kB for kernels about 350 kB in
125 size would be reasonable. Make sure offset+ 149 size would be reasonable. Make sure offset+size of ram_image.gz is
126 not larger than the total space on your flo 150 not larger than the total space on your floppy (usually 1440 kB)::
127 151
128 dd if=/tmp/ram_image.gz of=/dev/fd0 bs 152 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
129 153
130 g) Make sure that you have already specified t !! 154 g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
131 FDARGS and FDINITRD or that you use a bootl !! 155 For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
132 command line boot options to the kernel. !! 156 have 2^15 + 2^14 + 400 = 49552::
>> 157
>> 158 rdev /dev/fd0 /dev/fd0
>> 159 rdev -r /dev/fd0 49552
133 160
134 That is it. You now have your boot/root compre 161 That is it. You now have your boot/root compressed RAM disk floppy. Some
135 users may wish to combine steps (d) and (f) by 162 users may wish to combine steps (d) and (f) by using a pipe.
136 163
137 164
138 165 Paul Gortmaker 12/95
139 166
140 Changelog: 167 Changelog:
141 ---------- 168 ----------
142 169
143 SEPT-2020 : <<
144 <<
145 Removed usage of "rdev" <<
146 <<
147 10-22-04 : 170 10-22-04 :
148 Updated to reflect changes in 171 Updated to reflect changes in command line options, remove
149 obsolete references, general c 172 obsolete references, general cleanup.
150 James Nelson (james4765@gmail. 173 James Nelson (james4765@gmail.com)
>> 174
151 175
152 12-95 : 176 12-95 :
153 Original Document 177 Original Document
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