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Based on kernel version 2.6.27. Page generated on 2008-10-13 09:53 EST.

1	Using the RAM disk block device with Linux
2	------------------------------------------
3	
4	Contents:
5	
6		1) Overview
7		2) Kernel Command Line Parameters
8		3) Using "rdev -r"
9		4) An Example of Creating a Compressed RAM Disk
10	
11	
12	1) Overview
13	-----------
14	
15	The RAM disk driver is a way to use main system memory as a block device.  It
16	is required for initrd, an initial filesystem used if you need to load modules
17	in order to access the root filesystem (see Documentation/initrd.txt).  It can
18	also be used for a temporary filesystem for crypto work, since the contents
19	are erased on reboot.
20	
21	The RAM disk dynamically grows as more space is required. It does this by using
22	RAM from the buffer cache. The driver marks the buffers it is using as dirty
23	so that the VM subsystem does not try to reclaim them later.
24	
25	The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
26	to support an unlimited number of RAM disks (at your own risk).  Just change
27	the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
28	and (re)build the kernel.
29	
30	To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
31	directory.  RAM disks are all major number 1, and start with minor number 0
32	for /dev/ram0, etc.  If used, modern kernels use /dev/ram0 for an initrd.
33	
34	The new RAM disk also has the ability to load compressed RAM disk images,
35	allowing one to squeeze more programs onto an average installation or
36	rescue floppy disk.
37	
38	
39	2) Kernel Command Line Parameters
40	---------------------------------
41	
42		ramdisk_size=N
43		==============
44	
45	This parameter tells the RAM disk driver to set up RAM disks of N k size.  The
46	default is 4096 (4 MB) (8192 (8 MB) on S390).
47	
48		ramdisk_blocksize=N
49		===================
50	
51	This parameter tells the RAM disk driver how many bytes to use per block.  The
52	default is 1024 (BLOCK_SIZE).
53	
54	
55	3) Using "rdev -r"
56	------------------
57	
58	The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
59	as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
60	to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
61	14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
62	prompt/wait sequence is to be given before trying to read the RAM disk. Since
63	the RAM disk dynamically grows as data is being written into it, a size field
64	is not required. Bits 11 to 13 are not currently used and may as well be zero.
65	These numbers are no magical secrets, as seen below:
66	
67	./arch/i386/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK     0x07FF
68	./arch/i386/kernel/setup.c:#define RAMDISK_PROMPT_FLAG          0x8000
69	./arch/i386/kernel/setup.c:#define RAMDISK_LOAD_FLAG            0x4000
70	
71	Consider a typical two floppy disk setup, where you will have the
72	kernel on disk one, and have already put a RAM disk image onto disk #2.
73	
74	Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
75	starts at an offset of 0 kB from the beginning of the floppy.
76	The command line equivalent is: "ramdisk_start=0"
77	
78	You want bit 14 as one, indicating that a RAM disk is to be loaded.
79	The command line equivalent is: "load_ramdisk=1"
80	
81	You want bit 15 as one, indicating that you want a prompt/keypress
82	sequence so that you have a chance to switch floppy disks.
83	The command line equivalent is: "prompt_ramdisk=1"
84	
85	Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
86	So to create disk one of the set, you would do:
87	
88		/usr/src/linux# cat arch/i386/boot/zImage > /dev/fd0
89		/usr/src/linux# rdev /dev/fd0 /dev/fd0
90		/usr/src/linux# rdev -r /dev/fd0 49152
91	
92	If you make a boot disk that has LILO, then for the above, you would use:
93		append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
94	Since the default start = 0 and the default prompt = 1, you could use:
95		append = "load_ramdisk=1"
96	
97	
98	4) An Example of Creating a Compressed RAM Disk
99	----------------------------------------------
100	
101	To create a RAM disk image, you will need a spare block device to
102	construct it on. This can be the RAM disk device itself, or an
103	unused disk partition (such as an unmounted swap partition). For this
104	example, we will use the RAM disk device, "/dev/ram0".
105	
106	Note: This technique should not be done on a machine with less than 8 MB
107	of RAM. If using a spare disk partition instead of /dev/ram0, then this
108	restriction does not apply.
109	
110	a) Decide on the RAM disk size that you want. Say 2 MB for this example.
111	   Create it by writing to the RAM disk device. (This step is not currently
112	   required, but may be in the future.) It is wise to zero out the
113	   area (esp. for disks) so that maximal compression is achieved for
114	   the unused blocks of the image that you are about to create.
115	
116		dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
117	
118	b) Make a filesystem on it. Say ext2fs for this example.
119	
120		mke2fs -vm0 /dev/ram0 2048
121	
122	c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
123	   and unmount it again.
124	
125	d) Compress the contents of the RAM disk. The level of compression
126	   will be approximately 50% of the space used by the files. Unused
127	   space on the RAM disk will compress to almost nothing.
128	
129		dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
130	
131	e) Put the kernel onto the floppy
132	
133		dd if=zImage of=/dev/fd0 bs=1k
134	
135	f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
136	   that is slightly larger than the kernel, so that you can put another
137	   (possibly larger) kernel onto the same floppy later without overlapping
138	   the RAM disk image. An offset of 400 kB for kernels about 350 kB in
139	   size would be reasonable. Make sure offset+size of ram_image.gz is
140	   not larger than the total space on your floppy (usually 1440 kB).
141	
142		dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
143	
144	g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
145	   For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
146	   have 2^15 + 2^14 + 400 = 49552.
147	
148		rdev /dev/fd0 /dev/fd0
149		rdev -r /dev/fd0 49552
150	
151	That is it. You now have your boot/root compressed RAM disk floppy. Some
152	users may wish to combine steps (d) and (f) by using a pipe.
153	
154	--------------------------------------------------------------------------
155							Paul Gortmaker 12/95
156	
157	Changelog:
158	----------
159	
160	10-22-04 :	Updated to reflect changes in command line options, remove
161			obsolete references, general cleanup.
162			James Nelson (james4765[AT]gmail[DOT]com)
163	
164	
165	12-95 :		Original Document
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