view Documentation/ramdisk.txt @ 452:c7ed6fe5dca0

kexec: dont initialise regions in reserve_memory()

There is no need to initialise efi_memmap_res and boot_param_res in
reserve_memory() for the initial xen domain as it is done in
machine_kexec_setup_resources() using values from the kexec hypercall.

Signed-off-by: Simon Horman <horms@verge.net.au>
author Keir Fraser <keir.fraser@citrix.com>
date Thu Feb 28 10:55:18 2008 +0000 (2008-02-28)
parents 831230e53067
line source
1 Using the RAM disk block device with Linux
2 ------------------------------------------
4 Contents:
6 1) Overview
7 2) Kernel Command Line Parameters
8 3) Using "rdev -r"
9 4) An Example of Creating a Compressed RAM Disk
12 1) Overview
13 -----------
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.
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.
25 Also, the RAM disk supports up to 16 RAM disks out of the box, and can
26 be reconfigured to support up to 255 RAM disks - change "#define NUM_RAMDISKS"
27 in drivers/block/rd.c. To use RAM disk support with your system, run
28 './MAKEDEV ram' from the /dev directory. RAM disks are all major number 1, and
29 start with minor number 0 for /dev/ram0, etc. If used, modern kernels use
30 /dev/ram0 for an initrd.
32 The old "ramdisk=<ram_size>" has been changed to "ramdisk_size=<ram_size>" to
33 make it clearer. The original "ramdisk=<ram_size>" has been kept around for
34 compatibility reasons, but it may be removed in the future.
36 The new RAM disk also has the ability to load compressed RAM disk images,
37 allowing one to squeeze more programs onto an average installation or
38 rescue floppy disk.
41 2) Kernel Command Line Parameters
42 ---------------------------------
44 ramdisk_size=N
45 ==============
47 This parameter tells the RAM disk driver to set up RAM disks of N k size. The
48 default is 4096 (4 MB) (8192 (8 MB) on S390).
50 ramdisk_blocksize=N
51 ===================
53 This parameter tells the RAM disk driver how many bytes to use per block. The
54 default is 1024 (BLOCK_SIZE).
57 3) Using "rdev -r"
58 ------------------
60 The usage of the word (two bytes) that "rdev -r" sets in the kernel image is
61 as follows. The low 11 bits (0 -> 10) specify an offset (in 1 k blocks) of up
62 to 2 MB (2^11) of where to find the RAM disk (this used to be the size). Bit
63 14 indicates that a RAM disk is to be loaded, and bit 15 indicates whether a
64 prompt/wait sequence is to be given before trying to read the RAM disk. Since
65 the RAM disk dynamically grows as data is being written into it, a size field
66 is not required. Bits 11 to 13 are not currently used and may as well be zero.
67 These numbers are no magical secrets, as seen below:
69 ./arch/i386/kernel/setup.c:#define RAMDISK_IMAGE_START_MASK 0x07FF
70 ./arch/i386/kernel/setup.c:#define RAMDISK_PROMPT_FLAG 0x8000
71 ./arch/i386/kernel/setup.c:#define RAMDISK_LOAD_FLAG 0x4000
73 Consider a typical two floppy disk setup, where you will have the
74 kernel on disk one, and have already put a RAM disk image onto disk #2.
76 Hence you want to set bits 0 to 13 as 0, meaning that your RAM disk
77 starts at an offset of 0 kB from the beginning of the floppy.
78 The command line equivalent is: "ramdisk_start=0"
80 You want bit 14 as one, indicating that a RAM disk is to be loaded.
81 The command line equivalent is: "load_ramdisk=1"
83 You want bit 15 as one, indicating that you want a prompt/keypress
84 sequence so that you have a chance to switch floppy disks.
85 The command line equivalent is: "prompt_ramdisk=1"
87 Putting that together gives 2^15 + 2^14 + 0 = 49152 for an rdev word.
88 So to create disk one of the set, you would do:
90 /usr/src/linux# cat arch/i386/boot/zImage > /dev/fd0
91 /usr/src/linux# rdev /dev/fd0 /dev/fd0
92 /usr/src/linux# rdev -r /dev/fd0 49152
94 If you make a boot disk that has LILO, then for the above, you would use:
95 append = "ramdisk_start=0 load_ramdisk=1 prompt_ramdisk=1"
96 Since the default start = 0 and the default prompt = 1, you could use:
97 append = "load_ramdisk=1"
100 4) An Example of Creating a Compressed RAM Disk
101 ----------------------------------------------
103 To create a RAM disk image, you will need a spare block device to
104 construct it on. This can be the RAM disk device itself, or an
105 unused disk partition (such as an unmounted swap partition). For this
106 example, we will use the RAM disk device, "/dev/ram0".
108 Note: This technique should not be done on a machine with less than 8 MB
109 of RAM. If using a spare disk partition instead of /dev/ram0, then this
110 restriction does not apply.
112 a) Decide on the RAM disk size that you want. Say 2 MB for this example.
113 Create it by writing to the RAM disk device. (This step is not currently
114 required, but may be in the future.) It is wise to zero out the
115 area (esp. for disks) so that maximal compression is achieved for
116 the unused blocks of the image that you are about to create.
118 dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
120 b) Make a filesystem on it. Say ext2fs for this example.
122 mke2fs -vm0 /dev/ram0 2048
124 c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
125 and unmount it again.
127 d) Compress the contents of the RAM disk. The level of compression
128 will be approximately 50% of the space used by the files. Unused
129 space on the RAM disk will compress to almost nothing.
131 dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
133 e) Put the kernel onto the floppy
135 dd if=zImage of=/dev/fd0 bs=1k
137 f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
138 that is slightly larger than the kernel, so that you can put another
139 (possibly larger) kernel onto the same floppy later without overlapping
140 the RAM disk image. An offset of 400 kB for kernels about 350 kB in
141 size would be reasonable. Make sure offset+size of ram_image.gz is
142 not larger than the total space on your floppy (usually 1440 kB).
144 dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
146 g) Use "rdev" to set the boot device, RAM disk offset, prompt flag, etc.
147 For prompt_ramdisk=1, load_ramdisk=1, ramdisk_start=400, one would
148 have 2^15 + 2^14 + 400 = 49552.
150 rdev /dev/fd0 /dev/fd0
151 rdev -r /dev/fd0 49552
153 That is it. You now have your boot/root compressed RAM disk floppy. Some
154 users may wish to combine steps (d) and (f) by using a pipe.
156 --------------------------------------------------------------------------
157 Paul Gortmaker 12/95
159 Changelog:
160 ----------
162 10-22-04 : Updated to reflect changes in command line options, remove
163 obsolete references, general cleanup.
164 James Nelson (james4765@gmail.com)
167 12-95 : Original Document