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man: update description of initrd in bootup(7) 

Mention that initramfs is used, not initrd, even though we still call
it that. Also add links and clarify who loads the initramfs.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2019-03-22 13:10:39 +01:00
parent 9b89e602ea
commit 543e6bd4c2
1 changed files with 22 additions and 15 deletions
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37
man/bootup.xml
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@ -23,22 +23,29 @@
<refsect1>
<title>Description</title>
<para>A number of different components are involved in the system
boot. Immediately after power-up, the system BIOS will do minimal
hardware initialization, and hand control over to a boot loader
stored on a persistent storage device. This boot loader will then
invoke an OS kernel from disk (or the network). In the Linux case,
this kernel (optionally) extracts and executes an initial RAM disk
image (initrd), such as generated by
<para>A number of different components are involved in the boot of a Linux system. Immediately after
power-up, the system firmware will do minimal hardware initialization, and hand control over to a boot
loader (e.g.
<citerefentry><refentrytitle>systemd-boot</refentrytitle><manvolnum>7</manvolnum></citerefentry> or
<ulink url="https://www.gnu.org/software/grub/">GRUB</ulink>) stored on a persistent storage device. This
boot loader will then invoke an OS kernel from disk (or the network). On systems using EFI or other types
of firmware, this firmware may also load the kernel directly.</para>
<para>The kernel (optionally) mounts an in-memory file system, often generated by
<citerefentry project='die-net'><refentrytitle>dracut</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
which looks for the root file system (possibly using
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
for this). After the root file system is found and mounted, the
initrd hands over control to the host's system manager (such as
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>)
stored on the OS image, which is then responsible for probing all
remaining hardware, mounting all necessary file systems and
spawning all configured services.</para>
which looks for the root file system. Nowadays this is usually implemented as an initramfs — a compressed
archive which is extracted when the kernel boots up into a lightweight in-memory file system based on
tmpfs, but in the past normal file systems using an in-memory block device (ramdisk) were used, and the
name "initrd" is still used to describe both concepts. It's the boot loader or the firmware that loads
both the kernel and initrd/initramfs images into memory, but the kernel which interprets it as a file
system. <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> may
be used to manage services in the initrd, similarly to the real system.</para>
<para>After the root file system is found and mounted, the initrd hands over control to the host's system
manager (such as
<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>) stored in
the root file system, which is then responsible for probing all remaining hardware, mounting all
necessary file systems and spawning all configured services.</para>
<para>On shutdown, the system manager stops all services, unmounts
all file systems (detaching the storage technologies backing