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"
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"
63 --------------- 63 ---------------
64 64
65 "rdev" is an obsolete, deprecated, antiquated 65 "rdev" is an obsolete, deprecated, antiquated utility that could be used
66 to set the boot device in a Linux kernel image 66 to set the boot device in a Linux kernel image.
67 67
68 Instead of using rdev, just place the boot dev 68 Instead of using rdev, just place the boot device information on the
69 kernel command line and pass it to the kernel 69 kernel command line and pass it to the kernel from the bootloader.
70 70
71 You can also pass arguments to the kernel by s 71 You can also pass arguments to the kernel by setting FDARGS in
72 arch/x86/boot/Makefile and specify in initrd i 72 arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in
73 arch/x86/boot/Makefile. 73 arch/x86/boot/Makefile.
74 74
75 Some of the kernel command line boot options t 75 Some of the kernel command line boot options that may apply here are::
76 76
77 ramdisk_start=N 77 ramdisk_start=N
78 ramdisk_size=M 78 ramdisk_size=M
79 79
80 If you make a boot disk that has LILO, then fo 80 If you make a boot disk that has LILO, then for the above, you would use::
81 81
82 append = "ramdisk_start=N ramdisk_size 82 append = "ramdisk_start=N ramdisk_size=M"
83 83
84 4) An Example of Creating a Compressed RAM Dis 84 4) An Example of Creating a Compressed RAM Disk
85 ---------------------------------------------- 85 -----------------------------------------------
86 86
87 To create a RAM disk image, you will need a sp 87 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 88 construct it on. This can be the RAM disk device itself, or an
89 unused disk partition (such as an unmounted sw 89 unused disk partition (such as an unmounted swap partition). For this
90 example, we will use the RAM disk device, "/de 90 example, we will use the RAM disk device, "/dev/ram0".
91 91
92 Note: This technique should not be done on a m 92 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 93 of RAM. If using a spare disk partition instead of /dev/ram0, then this
94 restriction does not apply. 94 restriction does not apply.
95 95
96 a) Decide on the RAM disk size that you want. 96 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 97 Create it by writing to the RAM disk device. (This step is not currently
98 required, but may be in the future.) It is 98 required, but may be in the future.) It is wise to zero out the
99 area (esp. for disks) so that maximal compr 99 area (esp. for disks) so that maximal compression is achieved for
100 the unused blocks of the image that you are 100 the unused blocks of the image that you are about to create::
101 101
102 dd if=/dev/zero of=/dev/ram0 bs=1k cou 102 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
103 103
104 b) Make a filesystem on it. Say ext2fs for thi 104 b) Make a filesystem on it. Say ext2fs for this example::
105 105
106 mke2fs -vm0 /dev/ram0 2048 106 mke2fs -vm0 /dev/ram0 2048
107 107
108 c) Mount it, copy the files you want to it (eg 108 c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
109 and unmount it again. 109 and unmount it again.
110 110
111 d) Compress the contents of the RAM disk. The 111 d) Compress the contents of the RAM disk. The level of compression
112 will be approximately 50% of the space used 112 will be approximately 50% of the space used by the files. Unused
113 space on the RAM disk will compress to almo 113 space on the RAM disk will compress to almost nothing::
114 114
115 dd if=/dev/ram0 bs=1k count=2048 | gzi 115 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
116 116
117 e) Put the kernel onto the floppy:: 117 e) Put the kernel onto the floppy::
118 118
119 dd if=zImage of=/dev/fd0 bs=1k 119 dd if=zImage of=/dev/fd0 bs=1k
120 120
121 f) Put the RAM disk image onto the floppy, aft 121 f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
122 that is slightly larger than the kernel, so 122 that is slightly larger than the kernel, so that you can put another
123 (possibly larger) kernel onto the same flop 123 (possibly larger) kernel onto the same floppy later without overlapping
124 the RAM disk image. An offset of 400 kB for 124 the RAM disk image. An offset of 400 kB for kernels about 350 kB in
125 size would be reasonable. Make sure offset+ 125 size would be reasonable. Make sure offset+size of ram_image.gz is
126 not larger than the total space on your flo 126 not larger than the total space on your floppy (usually 1440 kB)::
127 127
128 dd if=/tmp/ram_image.gz of=/dev/fd0 bs 128 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
129 129
130 g) Make sure that you have already specified t 130 g) Make sure that you have already specified the boot information in
131 FDARGS and FDINITRD or that you use a bootl 131 FDARGS and FDINITRD or that you use a bootloader to pass kernel
132 command line boot options to the kernel. 132 command line boot options to the kernel.
133 133
134 That is it. You now have your boot/root compre 134 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 135 users may wish to combine steps (d) and (f) by using a pipe.
136 136
137 137
138 138 Paul Gortmaker 12/95
139 139
140 Changelog: 140 Changelog:
141 ---------- 141 ----------
142 142
143 SEPT-2020 : 143 SEPT-2020 :
144 144
145 Removed usage of "rdev" 145 Removed usage of "rdev"
146 146
147 10-22-04 : 147 10-22-04 :
148 Updated to reflect changes in 148 Updated to reflect changes in command line options, remove
149 obsolete references, general c 149 obsolete references, general cleanup.
150 James Nelson (james4765@gmail. 150 James Nelson (james4765@gmail.com)
151 151
152 12-95 : 152 12-95 :
153 Original Document 153 Original Document
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