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 -r"
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 -r"
63------------------
64
65The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
66as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
67to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
6814 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
69prompt/wait sequence is to be given before trying to read the RAM disk. Since
70the RAM disk dynamically grows as data is being written into it, a size field
71is not required. Bits 11 to 13 are not currently used and may as well be zero.
72These numbers are no magical secrets, as seen below::
73
74  ./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK     0x07FF
75  ./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG          0x8000
76  ./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG            0x4000
77
78Consider a typical two floppy disk setup, where you will have the
79kernel on disk one, and have already put a RAM disk image onto disk #2.
80
81Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
82starts at an offset of 0 kB from the beginning of the floppy.
83The command line equivalent is: "ramdisk_start=0"
84
85You want bit 14 as one, indicating that a RAM disk is to be loaded.
86The command line equivalent is: "load_ramdisk=1"
87
88You want bit 15 as one, indicating that you want a prompt/keypress
89sequence so that you have a chance to switch floppy disks.
90The command line equivalent is: "prompt_ramdisk=1"
91
92Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
93So to create disk one of the set, you would do::
94
95	/usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0
96	/usr/src/linux# rdev /dev/fd0 /dev/fd0
97	/usr/src/linux# rdev -r /dev/fd0 49152
98
99If you make a boot disk that has LILO, then for the above, you would use::
100
101	append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
102
103Since the default start = 0 and the default prompt = 1, you could use::
104
105	append = "load_ramdisk=1"
106
107
1084) An Example of Creating a Compressed RAM Disk
109-----------------------------------------------
110
111To create a RAM disk image, you will need a spare block device to
112construct it on. This can be the RAM disk device itself, or an
113unused disk partition (such as an unmounted swap partition). For this
114example, we will use the RAM disk device, "/dev/ram0".
115
116Note: This technique should not be done on a machine with less than 8 MB
117of RAM. If using a spare disk partition instead of /dev/ram0, then this
118restriction does not apply.
119
120a) Decide on the RAM disk size that you want. Say 2 MB for this example.
121   Create it by writing to the RAM disk device. (This step is not currently
122   required, but may be in the future.) It is wise to zero out the
123   area (esp. for disks) so that maximal compression is achieved for
124   the unused blocks of the image that you are about to create::
125
126	dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
127
128b) Make a filesystem on it. Say ext2fs for this example::
129
130	mke2fs -vm0 /dev/ram0 2048
131
132c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
133   and unmount it again.
134
135d) Compress the contents of the RAM disk. The level of compression
136   will be approximately 50% of the space used by the files. Unused
137   space on the RAM disk will compress to almost nothing::
138
139	dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
140
141e) Put the kernel onto the floppy::
142
143	dd if=zImage of=/dev/fd0 bs=1k
144
145f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
146   that is slightly larger than the kernel, so that you can put another
147   (possibly larger) kernel onto the same floppy later without overlapping
148   the RAM disk image. An offset of 400 kB for kernels about 350 kB in
149   size would be reasonable. Make sure offset+size of ram_image.gz is
150   not larger than the total space on your floppy (usually 1440 kB)::
151
152	dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
153
154g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
155   For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
156   have 2^15 + 2^14 + 400 = 49552::
157
158	rdev /dev/fd0 /dev/fd0
159	rdev -r /dev/fd0 49552
160
161That is it. You now have your boot/root compressed RAM disk floppy. Some
162users may wish to combine steps (d) and (f) by using a pipe.
163
164
165						Paul Gortmaker 12/95
166
167Changelog:
168----------
169
17010-22-04 :
171		Updated to reflect changes in command line options, remove
172		obsolete references, general cleanup.
173		James Nelson (james4765@gmail.com)
174
175
17612-95 :
177		Original Document
178