Suspend to disk HOWTO
Copyright (C) 2006 Pavel Machek <email@example.com>
Copyright (C) 2006 Rafael J. Wysocki <firstname.lastname@example.org>
I. Quick start
1) Necessary items
(a) This package
(b) Linux kernel supporting the swsusp userland interface (2.6.17 or above)
(c) Swap partition or a swap file, approximately as big as 1/2 of RAM
(d) Special device file for the snapshot device (character, 10, 231), e.g.
crw-r--r-- 1 root root 10, 231 Jan 13 21:21 /dev/snapshot
(e) libx86, a hardware-independent library for executing real-mode x86 code
(f) [optionally] Markus F.X.J. Oberhumer's lzo library (for lzo image compression)
(g) [optionally] libgcrypt (for image encryption)
(h) [optionally] libsplashy (for user space splash)
2) Basic steps
(a) Configure, build and install the kernel
The kernel has to be configured to support software suspend. It is also
highly recommended to configure it to support RAM disks, initial RAM disks,
loopback block devices, and the ext2 filesystem, at least for initial testing.
Later on, you can use initramfs instead of initrd (please see Section II of
this document for details).
If you already use initrd or initramfs, you'll need to integrate the resume
binary with it eventually. However, it's better to postpone this until you
have verified that the suspend utilities work on your system. Thus it is
recommended to initially build the kernel with all of the drivers needed to
mount the root filesystem (most importantly the drivers needed to access
your hard disk drive) compiled in. Then, you will only need the initrd to run
the resume binary itself and you will be able to use the resume initrd
installation script provided in this package.
The kernel installation procedure will depend on what Linux distribution
you use and should be documented elsewhere.
(b) Install libx86 developement packages
If libx86 is not installed on your system (Debian: libx86-dev) you can download
the tarball from http://www.codon.org.uk/~mjg59/libx86/downloads/. Unpack it,
change into the resulting directory. Then run
or if you're building for an x86_64
$ make BACKEND=x86emu
Next, become root and run
# make install DESTDIR=/usr/local
(c) [optionally] Install the Markus F.X.J. Oberhumer's lzo library
If you want to use the image lzo compression functionality and lzo it's not
installed on your system, download the tarball from
http://www.oberhumer.com/opensource/lzo, unpack it, go to the directory
containing the source and run
Next, become root and run
# make install
[This will put the lzo files into /usr/local/include/lzo, the library
(d) [optionally] Install libgrypt
If you want to use the image encryption functionality of the suspend
tools, you'll need libgcrypt to build them. If you have installed
GnuPG, libgcrypt is present on your system, but you'll also need
the development package for it.
On a SUSE system you can verify if the package has been installed by running
$ rpm -q libgcrypt-devel
and it should give you the version of the package if so (there are similar
methods for any other distribution). If not, you'll need to install it.
The majority of modern Linux distributions provide a binary package with it
and you should install it in a way that's appropriate for your distribution.
Still, if your distribution doesn't provide the binary package, you can refer
to the documentation at http://www.gnupg.org/ for installation instructions.
(d) [optionally] install libsplashy-dev (for user space splash screens)
For now you need the experimental version of splashy, which can be
found at svn://svn.debian.org/svn/splashy/branches/0.3 .
(e) Configure, build and install the suspend utilities
- Edit Makefile and the conf/uswsusp.conf file to reflect the configuration
of your system (please refer to Section II of this document for details). The
value of the RESUME_DEVICE constant in Makefile must be the same as the value
of the "resume device" parameter in the configuration file and it must be the
full path of a swap partition device file (eg. /dev/hda3).
[There should be just one resume partition, for now. You'll need at most 1/2
of your RAM of free space on it, but in some cases it may be smaller, too.
The s2disk tool may be configured to create quite small snapshot images but
then some contents of the RAM will have to be swapped out before suspend.
It also is possible to use a swap file for suspending, but this only works with
the 2.6.18-mm3 kernel (or later). In such a case the resume partition is the
partition that holds the swap file, and there is an additional configuration
parameter "resume offset" that has to be set, as described below.]
- Build the suspend tools in the usual way:
$ make s2disk
$ make resume
- To install the s2disk tool, become root and run:
# make install-minimal
This will also create the snapshot device file in /dev/, if needed.
- To create and install an initrd image containing the resume binary, become
root and run:
# make install-resume-new-initrd
This will place the file resume-initrd in the /boot directory.
[If you already use an initrd or initramfs, the resume binary will need to be
integrated with it, but initially it's better to use a separate "resume"
initrd image. Unfortunately the procedure of installing the resume binary
in the exsiting initrd or initramfs image depends on the Linux distribution
used, but one exaple is given at the end of Section II.]
You will also need to make the kernel use the resume initrd image.
Unfortunately this depends on the Linux distribution too, but if you
use GRUB and the entry in the GRUB configuration file (usually
/boot/grub/menu.lst) corresponding to your newly installed kernel is similar
to following one:
title Linux (2.6.18-rc2)
kernel /vmlinuz-2.6.18-rc2 root=/dev/md1 vga=792 ...
where "..." represents some additional kernel command line options, you will
only need to add the line:
right next to the "kernel" line.
[The paths in the above example correspond to the situation in which there is
a separate boot partition. However, if the /boot directory is located in the
root partition, the only difference will be the appearance of /boot in the
title Linux (2.6.18-rc2)
kernel /boot/vmlinuz-2.6.18-rc2 root=/dev/md1 vga=792 ...
where we have shown the complete configuration. The (hd0,0) identifies the
partition that GRUB will use to read the kernel and the initrd image from,
but please refer to the GRUB documentation for more details.]
(e) To suspend to disk, run
NOTE: For this to work you must be running _exactly_ the same kernel that
will be used for resuming. Otherwise the kernel will refuse to accept the
To resume, just boot the kernel you were running when the image was created
and if the initrd is properly installed, the resume tool will be run
automatically out of it.
II. Installation of suspend tools
1) Kernel configuration
(a) The kernel must support the swsusp userland interface, so it's most
recommended to use the 2.6.17 kernel or above.
(b) The kernel has to be configured to support software suspend, i.e.
the kernel configuration file (.config) has to contain
(c) The kernel should be configured with CONFIG_BLK_DEV_INITRD=y, which will
allow you to run the resume binary out of an iniramfs/initrd image.
[It is possible to use the suspend tools without any initramfs/initrd
images, but it's dangerous and will not be documented here.]
(d) It is recommended to configure the kernel with CONFIG_BLK_DEV_RAM=y and
set CONFIG_BLK_DEV_RAM_SIZE to at lease 2048 Kbytes. [This will make it
possible to use initrd images.]
(e) It is recommended to configure the kernel with CONFIG_BLK_DEV_LOOP=y, or
the resume initrd installation script provided in this package will not work.
Also the loopback device support may be needed for manual configuration
of the initrd image.
(f) The automatic resume initrd installation script provided in this package
installs an ext2 filesystem in the initrd image, so the kernel has to be
configure with CONFIG_EXT2_FS=y for it to work.
(g) For initial testing it's better to compile all of the drivers needed to
mount the root filesystem into the kernel, install the kernel without
an initrd/initramfs, and use the automatically-generated resume initrd
(see below) with it.
2) Installation of optional packages
If you want to use the compression and/or encryption capabilities of the
suspend tools, you'll need the Marc Lehmann's libLZF and/or libgcrypt,
respectively. Short instructions related to these packages have been
given in Secion I above.
If you want a user space splash screen you will need a splashy theme,
you also have to copy the theme files to your initramfs/initrd.
3) Configuration and compilation of suspend tools
(a) [optional] If you want to use the compression, encryption and/or
user space splash capabilities of the suspend tools and you have installed
the necessary packages referred to in subsection 2), you need to set
CONFIG_COMPRESS, CONFIG_ENCRYPT and/or CONFIG_SPLASHY in the Makefile to "yes".
(b) Compile the s2disk and resume binaries by running:
$ make s2disk
$ make resume
in the directory where you have placed the sources.
4) Create the configuration file for the s2disk tool. Its default name
is /etc/uswsusp.conf, but this can be customized by changing CONFIG_FILE
in config.h. It is also possible to specify the configuration file name
from the command line, by using the -f option, eg.
# s2disk -f <config_file>
The configuration file must contain the resume device specification, eg.
resume device = /dev/<your_swap_partition>
Optionally, it can contain the snapshot device specification, eg.
snapshot device = /dev/snapshot
and the following parameters:
resume offset = <offset_of_the_swap_header>
image size = <preferred_suspend_image_size_in_bytes>
shutdown method = <reboot, platform>
suspend loglevel = <kernel_console_loglevel_during_suspend>
compute checksum = <y/n>
compress = <y/n>
encrypt = <y/n>
RSA key file = <path>
max loglevel = <ignored>
early writeout = <y/n>
splash = <y/n>
The "resume offset" parameter is necessary if a swap file is used for
suspending. In such a case the device identified by the "resume device"
parameter is regarded as the partition that contains the swap file, and
"resume offset" must be equal to the offset from the beginning of this
partition at which the swap file's header is located, in <PAGE_SIZE> units.
The value of this parameter for given swap file can be determined by the
swap-offset program (has to be run as root) included in this package. [For
this feature to work, you will need an -mm kernel, 2.6.18-mm3 or newer.]
The "image size" parameter may be used to limit the size of the system
snapshot image created by the s2disk tool, but it's not mandatory. Namely,
the s2disk tool will do its best to limit the image size as required by
this parameter, but if that's not possible, it will suspend the system anyway,
with a bigger image. If "image size" is set to 0, the snapshot image will be
as small as possible.
The "shutdown method" parameter defines the operation that will be carried out
after the suspend image has been created and the machine is ready to be powered
off. If it is set to "reboot", the machine will be rebooted immediately.
If it is set to "platform", the machine will be shut down using special
power management operations available from the kernel that may be necessary
for the hardware to be properly reinitialized after the resume, and may cause
the system to resume faster (this is the recommended shutdown method on the
majority of systems). For any other value the machine will be powered off.
The default is to use the "platform" method. [For this feature to work,
you will need an -mm kernel, 2.6.18-mm3 or newer.]
If the "compute checksum" parameter is set to 'y', the s2disk and resume
tools will use the MD5 algorithm to verify the image integrity.
If the "compress" parameter is set to 'y', the s2disk and resume tools will
use the LZF compression algorithm to compress/decompress the image.
If the "encrypt" parameter is set to 'y', the s2disk and resume tools will
use the AES encryption algorithm to encrypt/decrypt the image. If the
"RSA key file" option is also used, the s2disk tool will generate a random
key for the AES encryption that will be passed to the resume tool
within the image header with the help of the RSA cipher. However, for this
purpose you will need an additional RSA key file the path to which should
be the value of "RSA key file" parameter. For more details refer to Section
IV ("Advanced encryption") of this document.
If the "early writeout" parameter is set to 'y', the s2disk
utility will start syncing the resume device early in the process of writing
the image to it. [This has been reported to speed up the suspend on some
boxes and eliminates the "fast progress meter and long fsync wait" effect.]
The "splash" parameter is used to make s2disk and/or resume use a splash system
(when set to 'y'). Currently the bootsplash.org and splashy splash systems
are supported. Note that for both systems your initrd or initramfs will
need additional files. See the documentation of your splash system for
The resume tool can use the same configuration file that is used by the
s2disk tool, but it will ignore most of the above parameters. It will use the
value of "suspend loglevel" as the kernel console loglevel during resume.
Additionally it will use the value of "max loglevel" as the kernel console
loglevel to switch to in case the resume fails (this parameter is ignored by
the s2disk tool).
It is not necessary to set "compute checksum = y" and/or "compress = y",
and/or "encrypt = y" for the resume tool. The appropriate information related
to these options will be passed to it in the image header by the s2disk tool.
However, the resume tool will only use the splash system if "splash = y" is set
for it explicitly.
In order to create the configuration file for s2disk and/or resume, it is
recommended to edit the file uswsusp.conf in the conf/ subdirectory of the
package source. Then this file will be automatically placed in /etc and
in the resume initrd file by scripts provided in this package.
5) Install s2disk
To install the s2disk tool, run:
# make install-minimal
as root. This will place the s2disk binary in the destination directory
defined in Makefile (/usr/local/sbin by default). It will also
create the special device file for the suspend device in /dev, if needed,
and copy the configuration file from the conf/ subdirectory of the source
package to /etc (or somewhere else, depending on the settings in Makefile).
If the configuration file already exists, it will not be overwritten and
a new file with the suffix ".new" appended to its name will be created.
If CONFIG_ENCRYPT is set to "yes" in Makefile, the suspend-keygen tool
used for generating RSA keys for s2disk (see Section IV of this document
for details) will also be built and installed in the destination directory
specified in Makefile (/usr/local/sbin by default).
If you don't want to use "make install-minimal", you can copy the s2disk
binary and the configuration file to wherever you like, but you'll also
need to make sure the snapshot device file is present (if you have set
CONFIG_ENCRYPT=yes in Makefile, you may also need to copy the suspend-keygen
binary to its final destination manually).
You can also use a tool called "checkinstall" to prepare a package file
according to the distribution of your choice (checkinstall supports
Slackware, RPM distributions and DEB distributions).
To download it: http://asic-linux.com.mx/~izto/checkinstall/download.php
To create your package, run "make" and then (as root user) simply
type "checkinstall" and answer the questions. This will not install
anything: instead it will create a package file you can install with rpm,
dpkg or whatever tool your distribution provides.
4) Create snapshot device file
If you don't use "make install-minimal", you may need to create the snapshot
# mknod /dev/snapshot c 10 231
If the name of this file is equal to SNAPSHOT_DEVICE in swsusp.h, it is not
necessary to specify it in the configuration file.
Now you should be able to use the s2disk tool. It is recommended, however,
to put the tool into a directory located directly on the root filesystem,
for safe testing.
5) Install resume
It is recommended to place the resume tool on an initrd/initramfs image.
For the latter, see instructions for yaird under (6) below. For an initrd,
this may be done using "make install-resume-new-initrd", but I'll describe the
full procedure in case you want to do something in a different way. For
simplicity I will only cover the fastest method of setting it up that will
allow you to get the resume tool up and running quickly. [The described
procedure reflects the actions performed by "make install-resume-new-initrd".]
(a) Configure the kernel to support RAM disks, initial RAM disks, loopback
block devices, and the ext2 filesystem, and install it as described in 1).
(b) Create the /initrd directory if it doesn't exist already.
(c) Create a blank file, as large as to accommodate all files you'll
be placing on the initrd filesystem. In the simplest case that will be the
resume tool itself, some directories and some device files.
The resume tool is linked statically, so you won't need any libraries to run it.
On my box the size of it was smaller that 2 MB, so I could do:
$ dd if=/dev/zero of=initrd bs=2000k count=1
(d) Run mke2fs on this file, eg.
$ /sbin/mke2fs -F -m0 initrd
(e) Become root and mount the filesystem, eg.
# mount -t ext2 -o loop initrd /mnt
(f) Create the console device, the snapshot device, and the resume device on it, eg.
# mkdir /mnt/dev
# mknod /mnt/dev/console c 5 1
# mknod /mnt/dev/snapshot c 10 231
# mknod /mnt/dev/hda2 b 3 2
[The initrd filesystem will be mounted as the root filesystem, so the paths
with respect to it must be the same as in your "normal" root filesystem.]
(g) [optional] If you want to use the RSA encryption mechanism described in
Section IV, you'll need to create the random and urandom device files on the
# mknod /dev/random c 1 8
# mknod /dev/urandom c 1 9
(h) Create the proc and etc directories on it (the resume tool will use them), eg.
# mkdir /mnt/proc
# mkdir /mnt/etc
(i) Copy the resume tool to the initrd filesystem and link it to linuxrc, eg.
# cp resume /mnt/
# cd /mnt
# ln -s resume linuxrc
(j) Put the configuration file for the resume tool in the etc directory on
the initrd filesystem, eg.
# cp /etc/uswsusp.conf /mnt/etc/
(k) Unmount the initrd filesystem and move the file containing it to your /boot
# umount /mnt
# mv initrd /boot/resume-initrd
(l) In the GRUB configuration file add the initrd to the configuration of the
newly installed kernel, eg.
(if you use LILO, please refer to its documentation).
(m) If you already have the resume initrd file which is called resume-initrd
and located in the /boot directory, you can install the resume tool on it
by using "make install-resume-on-initrd".
6) Install resume on initramfs with yaird
For an initramfs made by yaird, you can make yaird load the resume binary.
For this purpose, copy install the resume binary in /usr/local/sbin (for
example) and edit /etc/yaird/Templates.cfg to reflect your configuration.
Namely, find "Template RESUME" in there and make it read as follows:
!if [ -z "$noresume" ]
! # for suspend2 (>= 2.2-rc8)
! if [ -w /proc/suspend2/do_resume ]; then
! echo > /proc/suspend2/do_resume
! # for suspend2 (< 2.2-rc8)
! if [ -w /proc/software_suspend/do_resume ]; then
! echo > /proc/software_suspend/do_resume
! if [ -x /usr/local/sbin/resume ]
! mkcdev /dev/snapshot misc/snapshot
Here, the two "FILE" lines and the last if-then statement are new.
Furthermore, yaird needs to know what the resume partition is. You can either
edit /etc/yaird/Default.cfg and add that to the RESUME goal, or add an option
"resume" to your swap partition in /etc/fstab, as follows
/dev/hda3 none swap sw,resume 0 0
With all the above done, run yaird (or if you are a lazy Debian user, just
Before you start using the s2disk and resume tools for real, you should test
your setup. For this purpose it is recommended to boot the new kernel with
the "init=/bin/bash" command line argument. Then, the kernel should run bash
instead of init and you should get a (root) command prompt with the root
filesystem mounted read-only and with no other filesystems.
If this happens, mount the sysfs and proc filesystems and enable swap:
# mount /sys
# mount /proc
# swapon -a
Next, go to the directory where the s2disk tool is and do:
If this fails, the tool will return to the command prompt with a (hopefully
instructive) message. Otherwise you should be able to see some messages
from it on the screen and finally the progress meter, and the box should be
powered off. If that happens, you can start it again and let the resume
utility run from the initrd. If everything is fine, it will read the image
and restore the system state from it, and you will get the command prompt
back. If so, your setup works. Otherwise there's something wrong and you'll
need to find out what and why.
The next step is to check whether your device drivers suspend and resume
correctly. To do this, it is reasonable to boot the kernel to the runlevel 2
and run the suspend tool. If it suspends successfully, the resume tool is
able to restore the system memory state, and you get the command prompt back,
the drivers are most probably fine. Otherwise please report the problem.
Finally, you can start X and try to suspend from an xterm. If this works,
IV. Advanced encryption
If the s2disk and resume tools are compiled with the encryption support, the
RSA cipher may be used to pass the AES encryption key from the s2disk
tool to the resume tool within the suspend image. This way the s2disk tool
need not ask the user for a passphrase.
For this purpose you need to generate the RSA key pair using the
suspend-keygen tool provided along with the s2disk and resume tools. The
output file of suspend-keygen should be saved as /etc/uswsusp.key (or
something else pointed to by the "RSA key file =" configuration parameter of
s2disk). This file contains the public modulus (n), public exponent (e), the
AES-encrypted private exponent (d), the AES-encrypted primes (p), (q) and
the (AES-encrypted) component (u) of the RSA key pair. The components (d),
(p), (q), (u) of the RSA key pair are encrypted with a key derived from a
Then, the s2disk utility will load the contents of this file, generate a random
session key (k) and initialization vector (i) for the image encryption and use
(n, e) to encrypt these values with RSA. The encrypted (k, i) as well as the
contents of the RSA key file will be saved in the image header.
The resume utility will read (n, e) and (AES-encrypted) (d, p, q, u) as well
as (RSA-encrypted) (k, i) from the image header. Then it will ask the user
for the passphrase needed to decrypt (d, p, q, u). Next, if the user provides
the right passphrase, the resume tool will decrypt (k, i) needed for decrypting