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1==========================================
2Using 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 RAM Disk
11
12
131) Overview
14-----------
15
16The RAM disk driver is a way to use main system memory as a block device. It
17is required for initrd, an initial filesystem used if you need to load modules
18in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can
19also be used for a temporary filesystem for crypto work, since the contents
20are erased on reboot.
21
22The RAM disk dynamically grows as more space is required. It does this by using
23RAM from the buffer cache. The driver marks the buffers it is using as dirty
24so that the VM subsystem does not try to reclaim them later.
25
26The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
27to support an unlimited number of RAM disks (at your own risk). Just change
28the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
29and (re)build the kernel.
30
31To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
32directory. RAM disks are all major number 1, and start with minor number 0
33for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
34
35The new RAM disk also has the ability to load compressed RAM disk images,
36allowing one to squeeze more programs onto an average installation or
37rescue floppy disk.
38
39
402) Parameters
41---------------------------------
42
432a) Kernel Command Line Parameters
44
45 ramdisk_size=N
46 Size of the ramdisk.
47
48This parameter tells the RAM disk driver to set up RAM disks of N k size. The
49default is 4096 (4 MB).
50
512b) 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
623) Using "rdev"
63---------------
64
65"rdev" is an obsolete, deprecated, antiquated utility that could be used
66to set the boot device in a Linux kernel image.
67
68Instead of using rdev, just place the boot device information on the
69kernel command line and pass it to the kernel from the bootloader.
70
71You can also pass arguments to the kernel by setting FDARGS in
72arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in
73arch/x86/boot/Makefile.
74
75Some of the kernel command line boot options that may apply here are::
76
77 ramdisk_start=N
78 ramdisk_size=M
79
80If you make a boot disk that has LILO, then for the above, you would use::
81
82 append = "ramdisk_start=N ramdisk_size=M"
83
844) An Example of Creating a Compressed RAM Disk
85-----------------------------------------------
86
87To create a RAM disk image, you will need a spare block device to
88construct it on. This can be the RAM disk device itself, or an
89unused disk partition (such as an unmounted swap partition). For this
90example, we will use the RAM disk device, "/dev/ram0".
91
92Note: This technique should not be done on a machine with less than 8 MB
93of RAM. If using a spare disk partition instead of /dev/ram0, then this
94restriction does not apply.
95
96a) 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. (This step is not currently
98 required, but may be in the future.) It is wise to zero out the
99 area (esp. for disks) so that maximal compression is achieved for
100 the unused blocks of the image that you are about to create::
101
102 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
103
104b) Make a filesystem on it. Say ext2fs for this example::
105
106 mke2fs -vm0 /dev/ram0 2048
107
108c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
109 and unmount it again.
110
111d) Compress the contents of the RAM disk. The level of compression
112 will be approximately 50% of the space used by the files. Unused
113 space on the RAM disk will compress to almost nothing::
114
115 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
116
117e) Put the kernel onto the floppy::
118
119 dd if=zImage of=/dev/fd0 bs=1k
120
121f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
122 that is slightly larger than the kernel, so that you can put another
123 (possibly larger) kernel onto the same floppy later without overlapping
124 the RAM disk image. An offset of 400 kB for kernels about 350 kB in
125 size would be reasonable. Make sure offset+size of ram_image.gz is
126 not larger than the total space on your floppy (usually 1440 kB)::
127
128 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
129
130g) Make sure that you have already specified the boot information in
131 FDARGS and FDINITRD or that you use a bootloader to pass kernel
132 command line boot options to the kernel.
133
134That is it. You now have your boot/root compressed RAM disk floppy. Some
135users may wish to combine steps (d) and (f) by using a pipe.
136
137
138 Paul Gortmaker 12/95
139
140Changelog:
141----------
142
143SEPT-2020 :
144
145 Removed usage of "rdev"
146
14710-22-04 :
148 Updated to reflect changes in command line options, remove
149 obsolete references, general cleanup.
150 James Nelson (james4765@gmail.com)
151
15212-95 :
153 Original Document