gpt-auto-generator: automatically find the root disk of the system

When run in an initrd and no root= argument is set (or is set to
root=gpt-auto) we will automatically look for the root partition on the
same disk the EFI ESP is located on.

Since we look for swap, /home and /srv on the disk the root partition is
located on, we hence have a fully discoverable chain:

    Firmware discovers the EFI ESP partition → the initrd discovers the
    root partition → the host OS discovers swap, /home, and /srv.

Note that this requires an EFI boot loader that sets the
LoaderDevicePartUUID EFI variable, such as Gummiboot.

1 files changed, 39 insertions, 19 deletions

diff --git a/man/systemd-gpt-auto-generator.xml b/man/systemd-gpt-auto-generator.xml
index 75313994d7..a753fb0bef 100644
--- a/man/systemd-gpt-auto-generator.xml
+++ b/man/systemd-gpt-auto-generator.xml
@@ -43,7 +43,8 @@
         <refnamediv>
                 <refname>systemd-gpt-auto-generator</refname>
                 <refpurpose>Generator for automatically discovering
-                and mounting <filename>/home</filename> and <filename>/srv</filename>, as well as
+                and mounting root, <filename>/home</filename> and
+                <filename>/srv</filename> partitions, as well as
                 discovering and enabling swap partitions, based on GPT
                 partition type GUIDs.</refpurpose>
         </refnamediv>
@@ -56,7 +57,7 @@
                 <title>Description</title>
 
                 <para><filename>systemd-gpt-auto-generator</filename>
-                is a unit generator that automatically discovers
+                is a unit generator that automatically discovers root,
                 <filename>/home</filename>, <filename>/srv</filename>
                 and swap partitions and creates mount and swap units
                 for them, based on the the partition type GUIDs of
@@ -67,11 +68,14 @@
                 <citerefentry><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>),
                 or where the mount points are non-empty.</para>
 
-                <para>This generator will only look for partitions on
-                the same physical disk the root file system is stored
-                on. This generator has no effect on systems where the
-                root file system is distributed on multiple disks, for
-                example via btrfs RAID.</para>
+                <para>This generator will only look for root
+                partitions on the same physical disk the EFI System
+                Partition (ESP) is located on. It will only look for
+                the other partitions on the same physical disk the
+                root file system is located on. These partitions will
+                not be search on systems where the root file system is
+                distributed on multiple disks, for example via btrfs
+                RAID.</para>
 
                 <para><filename>systemd-gpt-auto-generator</filename>
                 is useful for centralizing file system configuration
@@ -87,30 +91,40 @@
                         <title>Partition Type GUIDs</title>
                         <tgroup cols='3' align='left' colsep='1' rowsep='1'>
                                 <colspec colname="guid" />
-                                <colspec colname="location" />
+                                <colspec colname="name" />
                                 <colspec colname="explanation" />
                                 <thead>
                                         <row>
                                                 <entry>Partition Type GUID</entry>
-                                                <entry>Location</entry>
+                                                <entry>Name</entry>
                                                 <entry>Explanation</entry>
                                         </row>
                                 </thead>
                                 <tbody>
                                         <row>
-                                                <entry>0657fd6d-a4ab-43c4-84e50933c84b4f4f</entry>
-                                                <entry>Swap</entry>
-                                                <entry>All swap partitions are enabled.</entry>
+                                                <entry>44479540-f297-41b2-9af7d131d5f0458a</entry>
+                                                <entry><filename>Root Partition (x86)</filename></entry>
+                                                <entry>On 32bit x86 systems the first x86 root partition on the disk the EFI ESP is located on is mounted to the root directory <filename>/</filename>.</entry>
+                                        </row>
+                                        <row>
+                                                <entry>4f68bce3-e8cd-4db1-96e7fbcaf984b709</entry>
+                                                <entry><filename>Root Partition (x86-64)</filename></entry>
+                                                <entry>On 64bit x86 systems the first x86-64 root partition on the disk the EFI ESP is located on is mounted to the root directory <filename>/</filename>.</entry>
                                         </row>
                                         <row>
                                                 <entry>933ac7e1-2eb4-4f13-b8440e14e2aef915</entry>
-                                                <entry><filename>/home</filename></entry>
-                                                <entry>The first home partition on the disk is mounted to <filename>/home</filename>.</entry>
+                                                <entry>Home Partition</entry>
+                                                <entry>The first home partition on the disk the root partition is located on is mounted to <filename>/home</filename>.</entry>
                                         </row>
                                         <row>
                                                 <entry>3b8f8425-20e0-4f3b-907f1a25a76f98e8</entry>
-                                                <entry><filename>/srv</filename></entry>
-                                                <entry>The first server data partition on the disk is mounted to <filename>/srv</filename>.</entry>
+                                                <entry>Server Data Partition</entry>
+                                                <entry>The first server data partition on the disk the root partition is located on is mounted to <filename>/srv</filename>.</entry>
+                                        </row>
+                                        <row>
+                                                <entry>0657fd6d-a4ab-43c4-84e50933c84b4f4f</entry>
+                                                <entry>Swap</entry>
+                                                <entry>All swap partitions located on the disk the root partition is located on are enabled.</entry>
                                         </row>
                                 </tbody>
                         </tgroup>
@@ -128,8 +142,13 @@
 
                 <para>Also note that
                 <citerefentry><refentrytitle>systemd-efi-boot-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
-                will mount the EFI System Partition to
-                <filename>/boot</filename> is not otherwise mounted.</para>
+                will mount the EFI System Partition (ESP) to
+                <filename>/boot</filename> if not otherwise mounted.</para>
+
+                <para>When using this generator in conjunction with
+                btrfs file systems make sure to set the correct
+                default subvolumes on them, using <command>btrfs
+                subvolume set-default</command>.</para>
 
                 <para><filename>systemd-gpt-auto-generator</filename>
                 implements the <ulink
@@ -147,7 +166,8 @@
                         <citerefentry><refentrytitle>systemd-efi-boot-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                         <citerefentry><refentrytitle>systemd-cryptsetup@.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
                         <citerefentry><refentrytitle>cryptsetup</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
-                        <citerefentry><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+                        <citerefentry><refentrytitle>fstab</refentrytitle><manvolnum>5</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>btrfs</refentrytitle><manvolnum>8</manvolnum></citerefentry>
                 </para>
         </refsect1>