ostree/man/ostree-prepare-root.xml
Colin Walters 1e4cb30c68 man: Add ostree-prepare-root
Add an overdue man page that describes this.  Prep for also
documenting composefs things here.
2023-07-13 17:24:52 -04:00

123 lines
4.8 KiB
XML

<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<refentry id="ostree">
<refentryinfo>
<title>ostree prepare-root</title>
<productname>OSTree</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Colin</firstname>
<surname>Walters</surname>
<email>walters@verbum.org</email>
</author>
</authorgroup>g
</refentryinfo>
<refmeta>
<refentrytitle>ostree prepare-root</refentrytitle>
<manvolnum>1</manvolnum>
</refmeta>
<refnamediv>
<refname>ostree-prepare-root</refname>
<refpurpose>Change the view of a mounted root filesystem to an ostree deployment</refpurpose>
</refnamediv>
<refsynopsisdiv>
<cmdsynopsis>
<command>ostree prepare-root</command> <arg choice="req">TARGET</arg>
</cmdsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
At its core, ostree operates on an existing mounted filesystem. Tooling such
as <literal>ostree admin deploy</literal> will create a new directory that can be
used as a bootable target. This tool is designed to run in an initramfs and
set up "remapping" mounts as a view into that filesystem.
</para>
<para>
As of more recently, this tool also has optional support for composefs, which
creates a distinct mount point layered on top of the underlying filesystem.
</para>
<para>
The most common pattern today is to use systemd in an initramfs. The systemd
unit shipped upstream is ordered in this way:
<literal>After=sysroot.mount</literal> and <literal>Before=initrd-root-fs.target</literal>
</para>
<para>
When it runs, the mounted filesystem at the provided <literal>TARGET</literal> (usually <literal>/sysroot</literal>)
will be changed such that what appears at <literal>/sysroot</literal> is actually the
"deployment root" - i.e. a particular versioned subdirectory. What was formerly the
"physical root" i.e. the real root of the filesystem will appear as <literal>/sysroot/sysroot</literal>.
</para>
<para>
For <literal>/var</literal>, by default a bind mount is created from the deployment root to <literal>/sysroot/var</literal>.
</para>
<para>
A read-only bind mount is created over <literal>/sysroot/usr</literal>. The immutable bit is set on the deployment
root, so this provides basic protection for filesystem mutation. If the <literal>sysroot.readonly</literal>
option is enabled, instead a writable bind mount for <literal>/sysroot/etc</literal>, and everything else
is mounted read-only.
</para>
<para>
Finally, when higher level tooling such as systemd performs a switch-root operation, what
was <literal>/sysroot</literal> becomes <literal>/</literal> and after the transition into
the real root, the system will be booted into the "deployment", which is a versioned immutable
filesystem tree. The ostree tooling running in the real root thereafter performs further changes
by operating on <literal>/sysroot</literal> which is now the "physical root".
</para>
</refsect1>
<refsect1>
<title>systemd</title>
<para>
As mentioned above, this tool comes with a systemd unit file <literal>ostree-prepare-root.service</literal>
and it is primarily expected to be invoked this way.
</para>
</refsect1>
<refsect1>
<title>Composefs</title>
<para>
The default for ostree is to create a plain hardlinked filesystem tree.
composefs support is currently experimental; see the upstream <literal>doc/composefs.md</literal>
for more information on using it.
</para>
</refsect1>
</refentry>