linux - Documentation/admin-guide/blockdev/ramdisk.rst

153 lines · 4.9 KiB

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 RAM Disk
11
12
13	1) Overview
14	-----------
15
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 used if you need to load modules
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 crypto work, since the contents
20	are erased on reboot.
21
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 the buffers it is using as dirty
24	so that the VM subsystem does not try to reclaim them later.
25
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 (at your own risk). Just change
28	the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
29	and (re)build the kernel.
30
31	To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
32	directory. RAM disks are all major number 1, and start with minor number 0
33	for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
34
35	The new RAM disk also has the ability to load compressed RAM disk images,
36	allowing one to squeeze more programs onto an average installation or
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 set up RAM disks of N k size. The
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 utility that could be used
66	to set the boot device in a Linux kernel image.
67
68	Instead of using rdev, just place the boot device information on the
69	kernel command line and pass it to the kernel from the bootloader.
70
71	You can also pass arguments to the kernel by setting FDARGS in
72	arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in
73	arch/x86/boot/Makefile.
74
75	Some of the kernel command line boot options that may apply here are::
76
77	ramdisk_start=N
78	ramdisk_size=M
79
80	If 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
84	4) An Example of Creating a Compressed RAM Disk
85	-----------------------------------------------
86
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 device itself, or an
89	unused disk partition (such as an unmounted swap partition). For this
90	example, we will use the RAM disk device, "/dev/ram0".
91
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 instead of /dev/ram0, then this
94	restriction does not apply.
95
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. (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
104	b) Make a filesystem on it. Say ext2fs for this example::
105
106	mke2fs -vm0 /dev/ram0 2048
107
108	c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
109	and unmount it again.
110
111	d) 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
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, 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
130	g) 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
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 using a pipe.
136
137
138	Paul Gortmaker 12/95
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 command line options, remove
149	obsolete references, general cleanup.
150	James Nelson ([email protected])
151
152	12-95 :
153	Original Document