Documentation/admin-guide/blockdev/ramdisk.rst

Source file repositories/reference/linux-study-clean/Documentation/admin-guide/blockdev/ramdisk.rst

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Documentation/admin-guide/blockdev/ramdisk.rst
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Support Tooling And Documentation
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Support Tooling And Documentation: documentation
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Repository support layer: documentation, build tooling, samples, user-space helper tools, generated initramfs support, licenses, and validation utilities.

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==========================================
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.

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