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78 lines
3.3 KiB
XML
78 lines
3.3 KiB
XML
<?xml version='1.0' encoding='utf-8'?>
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<!-- -*- DocBook -*- -->
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<chapter id="ch-initramfs">
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<?dbhtml filename="ch-initramfs.html" ?>
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<title>Managing the initial ramfs (initramfs) archive</title>
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<para>
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The booting in Debian is a two-stage process, involving the initial RAM
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filesystem (initramfs for short, sometimes it is also referred to as initrd,
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which stands for initial RAM disk). First, the bootloader loads the kernel and
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initramfs into memory, and passes the execution control to the kernel. After
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basic initialization the kernel extracts the initramfs archive and mounts it as
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a temporary root filesystem. initramfs contains kernel modules and userspace
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programs required to initialize the physical or logical device(s) containing
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the real root filesystem. The <command>init</command> script on the initramfs
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loads modules and performs other neccessary initialization steps. At the end
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of this stage <command>run-init</command> deletes the initramfs from memory,
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mounts the real root filesystem and passes control to the
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<command>/sbin/init</command> program on it.
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</para>
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<para>
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Two major goals are achieved with such setup: the kernel size is kept under
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control by allowing most of the drivers to be compiled as modules (in a
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initramfs-less setup the drivers neccessary for the boot-time initialization of
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the root device must be compiled into it) and allow the setups which require
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initialization which cannot be done in-kernel, but is performed by userspace
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utilities.
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</para>
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<section id="s-initramfs-gen-tools"><title>Initramfs generation tools</title>
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<para>
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Since initramfs usually needs to be customized for the particular
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hardware/device configuration and kernel version, they are not included as a
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part of any package, but are generated on the fly at kernel installation time.
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Currently there are two tools in Debian capable of generating an initramfs:
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<command>update-initramfs</command> provided by
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<package>initramfs-tools</package> (default) and
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<command>dracut-update-initramfs</command> provided by the
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<package>dracut</package> package (experimental).
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</para>
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</section>
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<section id="s-initramfs-regen"><title>Regenerating the initramfs</title>
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<para>
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If changes are desired after the corresponding <package>linux-image</package>
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has been installed, the initramfs needs to be regenerated. This is achieved by
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the command
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</para>
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<screen>
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<prompt>#</prompt> <userinput>dpkg-reconfigure linux-image-3.2.0-2-686-pae</userinput>
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</screen>
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<para>
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where <package>linux-image-3.2.0-2-686-pae</package> is the name of the kernel
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package for which the initramfs regeneration is requested.
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</para>
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</section>
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<section id="s-initramfs-exam"><title>Examining the initramfs contents</title>
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<para>
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Occasionally it is useful to examine the contents of initramfs to diagnose a
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problem or for educational purposes. They are compressed
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<literal>cpio</literal> archives, which may be extracted using the command
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</para>
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<screen>
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<prompt>$</prompt> <userinput>zcat /boot/initrd.img-3.2.0-2-686-pae | cpio -i</userinput>
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</screen>
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<para>
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It will unpack the contents of the initramfs into the current directory.
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</para>
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<para>
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It is also possible to list the contents of an initramfs using the
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<command>cpio</command> <option>-t</option> option or the command
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</para>
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<screen>
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<prompt>$</prompt> <userinput>lsinitramfs /boot/initrd.img-3.2.0-2-686-pae</userinput>
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</screen>
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</section>
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</chapter>
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