1Using the RAM disk block device with Linux
2------------------------------------------
3
4Contents:
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
121) Overview
13-----------
14
15The RAM disk driver is a way to use main system memory as a block device.  It
16is required for initrd, an initial filesystem used if you need to load modules
17in order to access the root filesystem (see Documentation/admin-guide/initrd.rst).  It can
18also be used for a temporary filesystem for crypto work, since the contents
19are erased on reboot.
20
21The RAM disk dynamically grows as more space is required. It does this by using
22RAM from the buffer cache. The driver marks the buffers it is using as dirty
23so that the VM subsystem does not try to reclaim them later.
24
25The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
26to support an unlimited number of RAM disks (at your own risk).  Just change
27the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
28and (re)build the kernel.
29
30To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
31directory.  RAM disks are all major number 1, and start with minor number 0
32for /dev/ram0, etc.  If used, modern kernels use /dev/ram0 for an initrd.
33
34The new RAM disk also has the ability to load compressed RAM disk images,
35allowing one to squeeze more programs onto an average installation or
36rescue floppy disk.
37
38
392) Parameters
40---------------------------------
41
422a) Kernel Command Line Parameters
43
44	ramdisk_size=N
45	==============
46
47This parameter tells the RAM disk driver to set up RAM disks of N k size.  The
48default is 4096 (4 MB).
49
502b) Module parameters
51
52	rd_nr
53	=====
54	/dev/ramX devices created.
55
56	max_part
57	========
58	Maximum partition number.
59
60	rd_size
61	=======
62	See ramdisk_size.
63
643) Using "rdev -r"
65------------------
66
67The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
68as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
69to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
7014 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
71prompt/wait sequence is to be given before trying to read the RAM disk. Since
72the RAM disk dynamically grows as data is being written into it, a size field
73is not required. Bits 11 to 13 are not currently used and may as well be zero.
74These numbers are no magical secrets, as seen below:
75
76./arch/x86/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK     0x07FF
77./arch/x86/kernel/setup.c:#define RAMDISK_PROMPT_FLAG          0x8000
78./arch/x86/kernel/setup.c:#define RAMDISK_LOAD_FLAG            0x4000
79
80Consider a typical two floppy disk setup, where you will have the
81kernel on disk one, and have already put a RAM disk image onto disk #2.
82
83Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
84starts at an offset of 0 kB from the beginning of the floppy.
85The command line equivalent is: "ramdisk_start=0"
86
87You want bit 14 as one, indicating that a RAM disk is to be loaded.
88The command line equivalent is: "load_ramdisk=1"
89
90You want bit 15 as one, indicating that you want a prompt/keypress
91sequence so that you have a chance to switch floppy disks.
92The command line equivalent is: "prompt_ramdisk=1"
93
94Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
95So to create disk one of the set, you would do:
96
97	/usr/src/linux# cat arch/x86/boot/zImage > /dev/fd0
98	/usr/src/linux# rdev /dev/fd0 /dev/fd0
99	/usr/src/linux# rdev -r /dev/fd0 49152
100
101If you make a boot disk that has LILO, then for the above, you would use:
102	append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
103Since the default start = 0 and the default prompt = 1, you could use:
104	append = "load_ramdisk=1"
105
106
1074) An Example of Creating a Compressed RAM Disk
108----------------------------------------------
109
110To create a RAM disk image, you will need a spare block device to
111construct it on. This can be the RAM disk device itself, or an
112unused disk partition (such as an unmounted swap partition). For this
113example, we will use the RAM disk device, "/dev/ram0".
114
115Note: This technique should not be done on a machine with less than 8 MB
116of RAM. If using a spare disk partition instead of /dev/ram0, then this
117restriction does not apply.
118
119a) Decide on the RAM disk size that you want. Say 2 MB for this example.
120   Create it by writing to the RAM disk device. (This step is not currently
121   required, but may be in the future.) It is wise to zero out the
122   area (esp. for disks) so that maximal compression is achieved for
123   the unused blocks of the image that you are about to create.
124
125	dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
126
127b) Make a filesystem on it. Say ext2fs for this example.
128
129	mke2fs -vm0 /dev/ram0 2048
130
131c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
132   and unmount it again.
133
134d) Compress the contents of the RAM disk. The level of compression
135   will be approximately 50% of the space used by the files. Unused
136   space on the RAM disk will compress to almost nothing.
137
138	dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
139
140e) Put the kernel onto the floppy
141
142	dd if=zImage of=/dev/fd0 bs=1k
143
144f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
145   that is slightly larger than the kernel, so that you can put another
146   (possibly larger) kernel onto the same floppy later without overlapping
147   the RAM disk image. An offset of 400 kB for kernels about 350 kB in
148   size would be reasonable. Make sure offset+size of ram_image.gz is
149   not larger than the total space on your floppy (usually 1440 kB).
150
151	dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
152
153g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
154   For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
155   have 2^15 + 2^14 + 400 = 49552.
156
157	rdev /dev/fd0 /dev/fd0
158	rdev -r /dev/fd0 49552
159
160That is it. You now have your boot/root compressed RAM disk floppy. Some
161users may wish to combine steps (d) and (f) by using a pipe.
162
163--------------------------------------------------------------------------
164						Paul Gortmaker 12/95
165
166Changelog:
167----------
168
16910-22-04 :	Updated to reflect changes in command line options, remove
170		obsolete references, general cleanup.
171		James Nelson (james4765@gmail.com)
172
173
17412-95 :		Original Document
175