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.

diff --git a/man/bootup.xml b/man/bootup.xml
index 386af9e4ded4ef7d59dbe9cc56fed9eff6c5f2ea..9468a613196994d2b3da41119a93013cda5b4950 100644 (file)
--- a/man/bootup.xml
+++ b/man/bootup.xml
@@ -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