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42 <h1>mini-DebConf Cambridge 2014</h1>
43 <h2>initramfs-tools</h2>
48 <div class="presentation">
51 <h1>initramfs-tools</h1>
52 <object data="tux-debian.svg" width="35%" align="right"></object>
53 <h3>Ben Hutchings</h3>
57 <h1>What is the initramfs for?</h1>
58 <ul class="incremental">
60 Huge variety of disk drivers, net drivers and filesystems that
61 may be needed to mount root
64 Generic kernel shouldn't have all these built-in
67 Root might also need RAID, LVM, dm-crypt, which generally need
68 to be configured by userland
71 Kernel needs a way to load modules and run scripts but
72 it can't read them from the root filesystem
75 Solution: boot loader provides a bundle of files
76 (initramfs image) to the kernel
82 <img src="yo-dawg.jpeg" width="90%" />
86 <h1>How the kernel uses an initramfs</h1>
87 <ul class="incremental">
89 Kernel <em>always</em> mounts an initial root filesystem using
93 If the boot loader provides an initramfs image, kernel
94 unpacks it into this filesystem, then runs <tt>/init</tt>
97 The initramfs init system is then responsible for mounting
98 the real root filesystem and running the real <tt>init</tt>
99 <ul class="incremental">
101 Although it is not <em>required</em> to hand over at all
102 - e.g. debian-installer
111 <ul class="incremental">
113 Started in Ubuntu in 2005 and adopted in Debian later that year
116 <tt>mkinitramfs</tt> builds an initramfs image using the
117 included scripts, necessary kernel modules, udev, and utilities
118 from klibc-utils or busybox
121 Uses build-time and boot-time hook scripts for
124 <li>Used by btrfs-tools, cryptsetup, lvm2, mdadm, etc.</li>
128 Supports dependencies within each set of scripts
134 <h1>The init system</h1>
135 <ul class="incremental">
137 Mount <tt>devtmpfs</tt>, <tt>devpts</tt>, <tt>procfs</tt>,
138 <tt>sysfs</tt>, <tt>tmpfs</tt> at <tt>/run</tt>
141 Run <tt>init-top</tt> scripts - includes starting udev
144 Load explicitly configured modules
147 Run <tt>init-premount</tt> scripts
150 Run <tt>local-top</tt> or <tt>nfs-top</tt> scripts
154 Run <tt>local-premount</tt> or <tt>nfs-premount</tt> scripts
155 - e.g. btrfs scan, resume from disk
158 Mount root filesystem
164 <h1>Problems (1)</h1>
166 Root filesystem is not enough. Depenency-based boot means
167 there's no clean split between root and <tt>/usr</tt>. So
168 initramfs should mount it (implemented in 0.117). However:
170 <ul class="incremental">
172 initscripts and mountall won't remount it read-write if it's
173 already mounted read-only
174 <ul class="incremental">
176 Current workaround: only do this for systemd
181 fsck will refuse to operate on <tt>/usr</tt> if already
183 <ul class="incremental">
185 So we need to fsck it from the initramfs
186 <ul class="incremental">
188 But <tt>e2fsck</tt> complains if the superblock
189 time is newer than system time, and system time
190 may not be correct yet
191 <ul class="incremental">
193 So we need to fix the system time in the
203 Selecting the right fsck to install is harder than
210 <h1>Problems (2)</h1>
211 <p class="incremental">
212 Device discovery by kernel is asynchronous. But that's
213 why we have udev, right?
215 <p class="incremental">
216 Unfortunately not everything needed for device setup is
217 or can be in udev hooks (e.g. cryptsetup requires interaction)
219 <p class="incremental">
220 Lots of hacks used (<tt>while ... do sleep... done</tt>,
221 <tt>udevadm settle</tt> in too many places), and they still
223 (<a href="https://bugs.debian.org/616689">#616689</a> and merged
226 <p class="incremental">
227 Ubuntu has a somewhat better approach but we still haven't merged
230 <p class="incremental">
231 ...in fact, Debian and Ubuntu branches have been diverging for
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