154 lines
4.9 KiB
ReStructuredText
154 lines
4.9 KiB
ReStructuredText
==========================================
|
|
Using the RAM disk block device with Linux
|
|
==========================================
|
|
|
|
.. Contents:
|
|
|
|
1) Overview
|
|
2) Kernel Command Line Parameters
|
|
3) Using "rdev"
|
|
4) An Example of Creating a Compressed RAM Disk
|
|
|
|
|
|
1) Overview
|
|
-----------
|
|
|
|
The RAM disk driver is a way to use main system memory as a block device. It
|
|
is required for initrd, an initial filesystem used if you need to load modules
|
|
in order to access the root filesystem (see Documentation/admin-guide/initrd.rst). It can
|
|
also be used for a temporary filesystem for crypto work, since the contents
|
|
are erased on reboot.
|
|
|
|
The RAM disk dynamically grows as more space is required. It does this by using
|
|
RAM from the buffer cache. The driver marks the buffers it is using as dirty
|
|
so that the VM subsystem does not try to reclaim them later.
|
|
|
|
The RAM disk supports up to 16 RAM disks by default, and can be reconfigured
|
|
to support an unlimited number of RAM disks (at your own risk). Just change
|
|
the configuration symbol BLK_DEV_RAM_COUNT in the Block drivers config menu
|
|
and (re)build the kernel.
|
|
|
|
To use RAM disk support with your system, run './MAKEDEV ram' from the /dev
|
|
directory. RAM disks are all major number 1, and start with minor number 0
|
|
for /dev/ram0, etc. If used, modern kernels use /dev/ram0 for an initrd.
|
|
|
|
The new RAM disk also has the ability to load compressed RAM disk images,
|
|
allowing one to squeeze more programs onto an average installation or
|
|
rescue floppy disk.
|
|
|
|
|
|
2) Parameters
|
|
---------------------------------
|
|
|
|
2a) Kernel Command Line Parameters
|
|
|
|
ramdisk_size=N
|
|
Size of the ramdisk.
|
|
|
|
This parameter tells the RAM disk driver to set up RAM disks of N k size. The
|
|
default is 4096 (4 MB).
|
|
|
|
2b) Module parameters
|
|
|
|
rd_nr
|
|
/dev/ramX devices created.
|
|
|
|
max_part
|
|
Maximum partition number.
|
|
|
|
rd_size
|
|
See ramdisk_size.
|
|
|
|
3) Using "rdev"
|
|
---------------
|
|
|
|
"rdev" is an obsolete, deprecated, antiquated utility that could be used
|
|
to set the boot device in a Linux kernel image.
|
|
|
|
Instead of using rdev, just place the boot device information on the
|
|
kernel command line and pass it to the kernel from the bootloader.
|
|
|
|
You can also pass arguments to the kernel by setting FDARGS in
|
|
arch/x86/boot/Makefile and specify in initrd image by setting FDINITRD in
|
|
arch/x86/boot/Makefile.
|
|
|
|
Some of the kernel command line boot options that may apply here are::
|
|
|
|
ramdisk_start=N
|
|
ramdisk_size=M
|
|
|
|
If you make a boot disk that has LILO, then for the above, you would use::
|
|
|
|
append = "ramdisk_start=N ramdisk_size=M"
|
|
|
|
4) An Example of Creating a Compressed RAM Disk
|
|
-----------------------------------------------
|
|
|
|
To create a RAM disk image, you will need a spare block device to
|
|
construct it on. This can be the RAM disk device itself, or an
|
|
unused disk partition (such as an unmounted swap partition). For this
|
|
example, we will use the RAM disk device, "/dev/ram0".
|
|
|
|
Note: This technique should not be done on a machine with less than 8 MB
|
|
of RAM. If using a spare disk partition instead of /dev/ram0, then this
|
|
restriction does not apply.
|
|
|
|
a) Decide on the RAM disk size that you want. Say 2 MB for this example.
|
|
Create it by writing to the RAM disk device. (This step is not currently
|
|
required, but may be in the future.) It is wise to zero out the
|
|
area (esp. for disks) so that maximal compression is achieved for
|
|
the unused blocks of the image that you are about to create::
|
|
|
|
dd if=/dev/zero of=/dev/ram0 bs=1k count=2048
|
|
|
|
b) Make a filesystem on it. Say ext2fs for this example::
|
|
|
|
mke2fs -vm0 /dev/ram0 2048
|
|
|
|
c) Mount it, copy the files you want to it (eg: /etc/* /dev/* ...)
|
|
and unmount it again.
|
|
|
|
d) Compress the contents of the RAM disk. The level of compression
|
|
will be approximately 50% of the space used by the files. Unused
|
|
space on the RAM disk will compress to almost nothing::
|
|
|
|
dd if=/dev/ram0 bs=1k count=2048 | gzip -v9 > /tmp/ram_image.gz
|
|
|
|
e) Put the kernel onto the floppy::
|
|
|
|
dd if=zImage of=/dev/fd0 bs=1k
|
|
|
|
f) Put the RAM disk image onto the floppy, after the kernel. Use an offset
|
|
that is slightly larger than the kernel, so that you can put another
|
|
(possibly larger) kernel onto the same floppy later without overlapping
|
|
the RAM disk image. An offset of 400 kB for kernels about 350 kB in
|
|
size would be reasonable. Make sure offset+size of ram_image.gz is
|
|
not larger than the total space on your floppy (usually 1440 kB)::
|
|
|
|
dd if=/tmp/ram_image.gz of=/dev/fd0 bs=1k seek=400
|
|
|
|
g) Make sure that you have already specified the boot information in
|
|
FDARGS and FDINITRD or that you use a bootloader to pass kernel
|
|
command line boot options to the kernel.
|
|
|
|
That is it. You now have your boot/root compressed RAM disk floppy. Some
|
|
users may wish to combine steps (d) and (f) by using a pipe.
|
|
|
|
|
|
Paul Gortmaker 12/95
|
|
|
|
Changelog:
|
|
----------
|
|
|
|
SEPT-2020 :
|
|
|
|
Removed usage of "rdev"
|
|
|
|
10-22-04 :
|
|
Updated to reflect changes in command line options, remove
|
|
obsolete references, general cleanup.
|
|
James Nelson (james4765@gmail.com)
|
|
|
|
12-95 :
|
|
Original Document
|