1 ==========================================
2 Using the RAM disk block device with Linux
3 ==========================================
4
5 .. Contents:
6
7 1) Overview
8 2) Kernel Command Line Parameters
9 3) Using "rdev"
10 4) An Example of Creating a Compressed
11
12
13 1) Overview
14 -----------
15
16 The RAM disk driver is a way to use main syste
17 is required for initrd, an initial filesystem
18 in order to access the root filesystem (see Do
19 also be used for a temporary filesystem for cr
20 are erased on reboot.
21
22 The RAM disk dynamically grows as more space i
23 RAM from the buffer cache. The driver marks th
24 so that the VM subsystem does not try to recla
25
26 The RAM disk supports up to 16 RAM disks by de
27 to support an unlimited number of RAM disks (a
28 the configuration symbol BLK_DEV_RAM_COUNT in
29 and (re)build the kernel.
30
31 To use RAM disk support with your system, run
32 directory. RAM disks are all major number 1,
33 for /dev/ram0, etc. If used, modern kernels u
34
35 The new RAM disk also has the ability to load
36 allowing one to squeeze more programs onto an
37 rescue floppy disk.
38
39
40 2) Parameters
41 ---------------------------------
42
43 2a) Kernel Command Line Parameters
44
45 ramdisk_size=N
46 Size of the ramdisk.
47
48 This parameter tells the RAM disk driver to se
49 default is 4096 (4 MB).
50
51 2b) Module parameters
52
53 rd_nr
54 /dev/ramX devices created.
55
56 max_part
57 Maximum partition number.
58
59 rd_size
60 See ramdisk_size.
61
62 3) Using "rdev"
63 ---------------
64
65 "rdev" is an obsolete, deprecated, antiquated
66 to set the boot device in a Linux kernel image
67
68 Instead of using rdev, just place the boot dev
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
75 Some of the kernel command line boot options t
76
77 ramdisk_start=N
78 ramdisk_size=M
79
80 If you make a boot disk that has LILO, then fo
81
82 append = "ramdisk_start=N ramdisk_size
83
84 4) An Example of Creating a Compressed RAM Dis
85 ----------------------------------------------
86
87 To create a RAM disk image, you will need a sp
88 construct it on. This can be the RAM disk devi
89 unused disk partition (such as an unmounted sw
90 example, we will use the RAM disk device, "/de
91
92 Note: This technique should not be done on a m
93 of RAM. If using a spare disk partition instea
94 restriction does not apply.
95
96 a) Decide on the RAM disk size that you want.
97 Create it by writing to the RAM disk device
98 required, but may be in the future.) It is
99 area (esp. for disks) so that maximal compr
100 the unused blocks of the image that you are
101
102 dd if=/dev/zero of=/dev/ram0 bs=1k cou
103
104 b) Make a filesystem on it. Say ext2fs for thi
105
106 mke2fs -vm0 /dev/ram0 2048
107
108 c) Mount it, copy the files you want to it (eg
109 and unmount it again.
110
111 d) Compress the contents of the RAM disk. The
112 will be approximately 50% of the space used
113 space on the RAM disk will compress to almo
114
115 dd if=/dev/ram0 bs=1k count=2048 | gzi
116
117 e) Put the kernel onto the floppy::
118
119 dd if=zImage of=/dev/fd0 bs=1k
120
121 f) Put the RAM disk image onto the floppy, aft
122 that is slightly larger than the kernel, so
123 (possibly larger) kernel onto the same flop
124 the RAM disk image. An offset of 400 kB for
125 size would be reasonable. Make sure offset+
126 not larger than the total space on your flo
127
128 dd if=/tmp/ram_image.gz of=/dev/fd0 bs
129
130 g) Make sure that you have already specified t
131 FDARGS and FDINITRD or that you use a bootl
132 command line boot options to the kernel.
133
134 That is it. You now have your boot/root compre
135 users may wish to combine steps (d) and (f) by
136
137
138
139
140 Changelog:
141 ----------
142
143 SEPT-2020 :
144
145 Removed usage of "rdev"
146
147 10-22-04 :
148 Updated to reflect changes in
149 obsolete references, general c
150 James Nelson (james4765@gmail.
151
152 12-95 :
153 Original Document
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