Some warnings, first.
- BIG FAT WARNING *****
- If you touch anything on disk between suspend and resume…
...kiss your data goodbye.- If you do resume from initrd after your filesystems are mounted…
...bye bye root partition.[this is actually same case as above]- If you have unsupported (*) devices using DMA, you may have some
- problems. If your disk driver does not support suspend… (IDE does),
- it may cause some problems, too. If you change kernel command line
- between suspend and resume, it may do something wrong. If you change
- your hardware while system is suspended… well, it was not good idea;
- but it will probably only crash.
- (*) suspend/resume support is needed to make it safe.
- If you have any filesystems on USB devices mounted before software suspend,
- they won’t be accessible after resume and you may lose data, as though
- you have unplugged the USB devices with mounted filesystems on them;
- see the FAQ below for details. (This is not true for more traditional
- power states like “standby”, which normally don’t turn USB off.)
You need to append resume=/dev/your_swap_partition to kernel command
line. Then you suspend by
echo shutdown > /sys/power/disk; echo disk > /sys/power/state
. If you feel ACPI works pretty well on your system, you might try
echo platform > /sys/power/disk; echo disk > /sys/power/state
. If you have SATA disks, you’ll need recent kernels with SATA suspend
support. For suspend and resume to work, make sure your disk drivers
are built into kernel – not modules. [There’s way to make
suspend/resume with modular disk drivers, see FAQ, but you probably
should not do that.]
If you want to limit the suspend image size to N bytes, do
echo N > /sys/power/image_size
before suspend (it is limited to 500 MB by default).
Article about goals and implementation of Software Suspend for Linux
1 | Author: GÂábor Kuti |
running system, user asks for suspend-to-disk
user processes are stopped
suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
with state snapshot
state snapshot: copy of whole used memory is taken with interrupts disabled
resume(): devices are woken up so that we can write image to swap
write image to swap
suspend(PMSG_SUSPEND): suspend devices so that we can power off
turn the power off
Resume part
~~~~~~~~~~~
(is actually pretty similar)
running system, user asks for suspend-to-disk
user processes are stopped (in common case there are none, but with resume-from-initrd, noone knows)
read image from disk
suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
with image restoration
image restoration: rewrite memory with image
resume(): devices are woken up so that system can continue
thaw all user processes
Q: What is this ‘Encrypt suspend image’ for?
A: First of all: it is not a replacement for dm-crypt encrypted swap.
It cannot protect your computer while it is suspended. Instead it does
protect from leaking sensitive data after resume from suspend.
Think of the following: you suspend while an application is running
that keeps sensitive data in memory. The application itself prevents
the data from being swapped out. Suspend, however, must write these
data to swap to be able to resume later on. Without suspend encryption
your sensitive data are then stored in plaintext on disk. This means
that after resume your sensitive data are accessible to all
applications having direct access to the swap device which was used
for suspend. If you don’t need swap after resume these data can remain
on disk virtually forever. Thus it can happen that your system gets
broken in weeks later and sensitive data which you thought were
encrypted and protected are retrieved and stolen from the swap device.
To prevent this situation you should use ‘Encrypt suspend image’.
During suspend a temporary key is created and this key is used to
encrypt the data written to disk. When, during resume, the data was
read back into memory the temporary key is destroyed which simply
means that all data written to disk during suspend are then
inaccessible so they can’t be stolen later on. The only thing that
you must then take care of is that you call ‘mkswap’ for the swap
partition used for suspend as early as possible during regular
boot. This asserts that any temporary key from an oopsed suspend or
from a failed or aborted resume is erased from the swap device.
As a rule of thumb use encrypted swap to protect your data while your
system is shut down or suspended. Additionally use the encrypted
suspend image to prevent sensitive data from being stolen after
resume.
Q: Can I suspend to a swap file?
A: Generally, yes, you can. However, it requires you to use the “resume=” and
“resume_offset=” kernel command line parameters, so the resume from a swap file
cannot be initiated from an initrd or initramfs image. See
swsusp-and-swap-files.txt for details.
Q: Is there a maximum system RAM size that is supported by swsusp?
A: It should work okay with highmem.
Q: Does swsusp (to disk) use only one swap partition or can it use
multiple swap partitions (aggregate them into one logical space)?
A: Only one swap partition, sorry.
Q: If my application(s) causes lots of memory & swap space to be used
(over half of the total system RAM), is it correct that it is likely
to be useless to try to suspend to disk while that app is running?
A: No, it should work okay, as long as your app does not mlock()
it. Just prepare big enough swap partition.
Q: What information is useful for debugging suspend-to-disk problems?
A: Well, last messages on the screen are always useful. If something
is broken, it is usually some kernel driver, therefore trying with as
little as possible modules loaded helps a lot. I also prefer people to
suspend from console, preferably without X running. Booting with
init=/bin/bash, then swapon and starting suspend sequence manually
usually does the trick. Then it is good idea to try with latest
vanilla kernel.
Q: How can distributions ship a swsusp-supporting kernel with modular
disk drivers (especially SATA)?
A: Well, it can be done, load the drivers, then do echo into
/sys/power/disk/resume file from initrd. Be sure not to mount
anything, not even read-only mount, or you are going to lose your
data.
Q: How do I make suspend more verbose?
A: If you want to see any non-error kernel messages on the virtual
terminal the kernel switches to during suspend, you have to set the
kernel console loglevel to at least 4 (KERN_WARNING), for example by
doing
# save the old loglevel
read LOGLEVEL DUMMY < /proc/sys/kernel/printk
# set the loglevel so we see the progress bar.
# if the level is higher than needed, we leave it alone.
if [ $LOGLEVEL -lt 5 ]; then
echo 5 > /proc/sys/kernel/printk
fi
IMG_SZ=0
read IMG_SZ < /sys/power/image_size
echo -n disk > /sys/power/state
RET=$?
#
# the logic here is:
# if image_size > 0 (without kernel support, IMG_SZ will be zero),
# then try again with image_size set to zero.
if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
echo 0 > /sys/power/image_size
echo -n disk > /sys/power/state
RET=$?
fi
# restore previous loglevel
echo $LOGLEVEL > /proc/sys/kernel/printk
exit $RET
Q: Is this true that if I have a mounted filesystem on a USB device and
I suspend to disk, I can lose data unless the filesystem has been mounted
with “sync”?
A: That’s right … if you disconnect that device, you may lose data.
In fact, even with “-o sync” you can lose data if your programs have
information in buffers they haven’t written out to a disk you disconnect,
or if you disconnect before the device finished saving data you wrote.
Software suspend normally powers down USB controllers, which is equivalent
to disconnecting all USB devices attached to your system.
Your system might well support low-power modes for its USB controllers
while the system is asleep, maintaining the connection, using true sleep
modes like “suspend-to-RAM” or “standby”. (Don’t write “disk” to the
/sys/power/state file; write “standby” or “mem”.) We’ve not seen any
hardware that can use these modes through software suspend, although in
theory some systems might support “platform” modes that won’t break the
USB connections.
Remember that it’s always a bad idea to unplug a disk drive containing a
mounted filesystem. That’s true even when your system is asleep! The
safest thing is to unmount all filesystems on removable media (such USB,
Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
before suspending; then remount them after resuming.
There is a work-around for this problem. For more information, see
Documentation/usb/persist.txt.
Q: Can I suspend-to-disk using a swap partition under LVM?
A: No. You can suspend successfully, but you’ll not be able to
resume. uswsusp should be able to work with LVM. See suspend.sf.net.
Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
compiled with the similar configuration files. Anyway I found that
suspend to disk (and resume) is much slower on 2.6.16 compared to
2.6.15. Any idea for why that might happen or how can I speed it up?
A: This is because the size of the suspend image is now greater than
for 2.6.15 (by saving more data we can get more responsive system
after resume).
There’s the /sys/power/image_size knob that controls the size of the
image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
root), the 2.6.15 behavior should be restored. If it is still too
slow, take a look at suspend.sf.net – userland suspend is faster and
supports LZF compression to speed it up further.
