Documentation/power/swsusp.txt - linux-mtk - Git at Google

 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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Author: Gábor Kuti
 Last revised: 2003-10-20 by Pavel Machek

 Idea and goals to achieve

 Nowadays it is common in several laptops that they have a suspend button. It
 saves the state of the machine to a filesystem or to a partition and switches
 to standby mode. Later resuming the machine the saved state is loaded back to
 ram and the machine can continue its work. It has two real benefits. First we
 save ourselves the time machine goes down and later boots up, energy costs
 are real high when running from batteries. The other gain is that we don't have to
 interrupt our programs so processes that are calculating something for a long
 time shouldn't need to be written interruptible.

 swsusp saves the state of the machine into active swaps and then reboots or
 powerdowns.  You must explicitly specify the swap partition to resume from with
 ``resume='' kernel option. If signature is found it loads and restores saved
 state. If the option ``noresume'' is specified as a boot parameter, it skips
 the resuming.

 In the meantime while the system is suspended you should not add/remove any
 of the hardware, write to the filesystems, etc.

 Sleep states summary
 ====================

 There are three different interfaces you can use, /proc/acpi should
 work like this:

 In a really perfect world:
 echo 1 > /proc/acpi/sleep       # for standby
 echo 2 > /proc/acpi/sleep       # for suspend to ram
 echo 3 > /proc/acpi/sleep       # for suspend to ram, but with more power conservative
 echo 4 > /proc/acpi/sleep       # for suspend to disk
 echo 5 > /proc/acpi/sleep       # for shutdown unfriendly the system

 and perhaps
 echo 4b > /proc/acpi/sleep      # for suspend to disk via s4bios

 Frequently Asked Questions
 ==========================

 Q: well, suspending a server is IMHO a really stupid thing,
 but... (Diego Zuccato):

 A: You bought new UPS for your server. How do you install it without
 bringing machine down? Suspend to disk, rearrange power cables,
 resume.

 You have your server on UPS. Power died, and UPS is indicating 30
 seconds to failure. What do you do? Suspend to disk.


 Q: Maybe I'm missing something, but why don't the regular I/O paths work?

 A: We do use the regular I/O paths. However we cannot restore the data
 to its original location as we load it. That would create an
 inconsistent kernel state which would certainly result in an oops.
 Instead, we load the image into unused memory and then atomically copy
 it back to it original location. This implies, of course, a maximum
 image size of half the amount of memory.

 There are two solutions to this:

 * require half of memory to be free during suspend. That way you can
 read "new" data onto free spots, then cli and copy

 * assume we had special "polling" ide driver that only uses memory
 between 0-640KB. That way, I'd have to make sure that 0-640KB is free
 during suspending, but otherwise it would work...

 suspend2 shares this fundamental limitation, but does not include user
 data and disk caches into "used memory" by saving them in
 advance. That means that the limitation goes away in practice.

 Q: Does linux support ACPI S4?

 A: Yes. That's what echo platform > /sys/power/disk does.

 Q: What is 'suspend2'?

 A: suspend2 is 'Software Suspend 2', a forked implementation of
 suspend-to-disk which is available as separate patches for 2.4 and 2.6
 kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB
 highmem and preemption. It also has a extensible architecture that
 allows for arbitrary transformations on the image (compression,
 encryption) and arbitrary backends for writing the image (eg to swap
 or an NFS share[Work In Progress]). Questions regarding suspend2
 should be sent to the mailing list available through the suspend2
 website, and not to the Linux Kernel Mailing List. We are working
 toward merging suspend2 into the mainline kernel.

 Q: A kernel thread must voluntarily freeze itself (call 'refrigerator').
 I found some kernel threads that don't do it, and they don't freeze
 so the system can't sleep. Is this a known behavior?

 A: All such kernel threads need to be fixed, one by one. Select the
 place where the thread is safe to be frozen (no kernel semaphores
 should be held at that point and it must be safe to sleep there), and
 add:

        try_to_freeze();

 If the thread is needed for writing the image to storage, you should
 instead set the PF_NOFREEZE process flag when creating the thread (and
 be very carefull).


 Q: What is the difference between between "platform", "shutdown" and
 "firmware" in /sys/power/disk?

 A:

 shutdown: save state in linux, then tell bios to powerdown

 platform: save state in linux, then tell bios to powerdown and blink
           "suspended led"

 firmware: tell bios to save state itself [needs BIOS-specific suspend
 	  partition, and has very little to do with swsusp]

 "platform" is actually right thing to do, but "shutdown" is most
 reliable.

 Q: I do not understand why you have such strong objections to idea of
 selective suspend.

 A: Do selective suspend during runtime power management, that's okay. But
 it's useless for suspend-to-disk. (And I do not see how you could use
 it for suspend-to-ram, I hope you do not want that).

 Lets see, so you suggest to

 * SUSPEND all but swap device and parents
 * Snapshot
 * Write image to disk
 * SUSPEND swap device and parents
 * Powerdown

 Oh no, that does not work, if swap device or its parents uses DMA,
 you've corrupted data. You'd have to do

 * SUSPEND all but swap device and parents
 * FREEZE swap device and parents
 * Snapshot
 * UNFREEZE swap device and parents
 * Write
 * SUSPEND swap device and parents

 Which means that you still need that FREEZE state, and you get more
 complicated code. (And I have not yet introduce details like system
 devices).

 Q: There don't seem to be any generally useful behavioral
 distinctions between SUSPEND and FREEZE.

 A: Doing SUSPEND when you are asked to do FREEZE is always correct,
 but it may be unneccessarily slow. If you want your driver to stay simple,
 slowness may not matter to you. It can always be fixed later.

 For devices like disk it does matter, you do not want to spindown for
 FREEZE.

 Q: After resuming, system is paging heavily, leading to very bad interactivity.

 A: Try running

 cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null

 after resume. swapoff -a; swapon -a may also be useful.

 Q: What happens to devices during swsusp? They seem to be resumed
 during system suspend?

 A: That's correct. We need to resume them if we want to write image to
 disk. Whole sequence goes like

       Suspend part
       ~~~~~~~~~~~~
       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: Why can't we suspend to a swap file?

 A: Because accessing swap file needs the filesystem mounted, and
 filesystem might do something wrong (like replaying the journal)
 during mount.

 There are few ways to get that fixed:

 1) Probably could be solved by modifying every filesystem to support
 some kind of "really read-only!" option. Patches welcome.

 2) suspend2 gets around that by storing absolute positions in on-disk
 image (and blocksize), with resume parameter pointing directly to
 suspend header.

 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" or "firmware" 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.

 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.
	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
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	Author: Gábor Kuti
	Last revised: 2003-10-20 by Pavel Machek

	Idea and goals to achieve

	Nowadays it is common in several laptops that they have a suspend button. It
	saves the state of the machine to a filesystem or to a partition and switches
	to standby mode. Later resuming the machine the saved state is loaded back to
	ram and the machine can continue its work. It has two real benefits. First we
	save ourselves the time machine goes down and later boots up, energy costs
	are real high when running from batteries. The other gain is that we don't have to
	interrupt our programs so processes that are calculating something for a long
	time shouldn't need to be written interruptible.

	swsusp saves the state of the machine into active swaps and then reboots or
	powerdowns. You must explicitly specify the swap partition to resume from with
	``resume='' kernel option. If signature is found it loads and restores saved
	state. If the option ``noresume'' is specified as a boot parameter, it skips
	the resuming.

	In the meantime while the system is suspended you should not add/remove any
	of the hardware, write to the filesystems, etc.

	Sleep states summary
	====================

	There are three different interfaces you can use, /proc/acpi should
	work like this:

	In a really perfect world:
	echo 1 > /proc/acpi/sleep # for standby
	echo 2 > /proc/acpi/sleep # for suspend to ram
	echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative
	echo 4 > /proc/acpi/sleep # for suspend to disk
	echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system

	and perhaps
	echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios

	Frequently Asked Questions
	==========================

	Q: well, suspending a server is IMHO a really stupid thing,
	but... (Diego Zuccato):

	A: You bought new UPS for your server. How do you install it without
	bringing machine down? Suspend to disk, rearrange power cables,
	resume.

	You have your server on UPS. Power died, and UPS is indicating 30
	seconds to failure. What do you do? Suspend to disk.


	Q: Maybe I'm missing something, but why don't the regular I/O paths work?

	A: We do use the regular I/O paths. However we cannot restore the data
	to its original location as we load it. That would create an
	inconsistent kernel state which would certainly result in an oops.
	Instead, we load the image into unused memory and then atomically copy
	it back to it original location. This implies, of course, a maximum
	image size of half the amount of memory.

	There are two solutions to this:

	* require half of memory to be free during suspend. That way you can
	read "new" data onto free spots, then cli and copy

	* assume we had special "polling" ide driver that only uses memory
	between 0-640KB. That way, I'd have to make sure that 0-640KB is free
	during suspending, but otherwise it would work...

	suspend2 shares this fundamental limitation, but does not include user
	data and disk caches into "used memory" by saving them in
	advance. That means that the limitation goes away in practice.

	Q: Does linux support ACPI S4?

	A: Yes. That's what echo platform > /sys/power/disk does.

	Q: What is 'suspend2'?

	A: suspend2 is 'Software Suspend 2', a forked implementation of
	suspend-to-disk which is available as separate patches for 2.4 and 2.6
	kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB
	highmem and preemption. It also has a extensible architecture that
	allows for arbitrary transformations on the image (compression,
	encryption) and arbitrary backends for writing the image (eg to swap
	or an NFS share[Work In Progress]). Questions regarding suspend2
	should be sent to the mailing list available through the suspend2
	website, and not to the Linux Kernel Mailing List. We are working
	toward merging suspend2 into the mainline kernel.

	Q: A kernel thread must voluntarily freeze itself (call 'refrigerator').
	I found some kernel threads that don't do it, and they don't freeze
	so the system can't sleep. Is this a known behavior?

	A: All such kernel threads need to be fixed, one by one. Select the
	place where the thread is safe to be frozen (no kernel semaphores
	should be held at that point and it must be safe to sleep there), and
	add:

	try_to_freeze();

	If the thread is needed for writing the image to storage, you should
	instead set the PF_NOFREEZE process flag when creating the thread (and
	be very carefull).


	Q: What is the difference between between "platform", "shutdown" and
	"firmware" in /sys/power/disk?

	A:

	shutdown: save state in linux, then tell bios to powerdown

	platform: save state in linux, then tell bios to powerdown and blink
	"suspended led"

	firmware: tell bios to save state itself [needs BIOS-specific suspend
	partition, and has very little to do with swsusp]

	"platform" is actually right thing to do, but "shutdown" is most
	reliable.

	Q: I do not understand why you have such strong objections to idea of
	selective suspend.

	A: Do selective suspend during runtime power management, that's okay. But
	it's useless for suspend-to-disk. (And I do not see how you could use
	it for suspend-to-ram, I hope you do not want that).

	Lets see, so you suggest to

	* SUSPEND all but swap device and parents
	* Snapshot
	* Write image to disk
	* SUSPEND swap device and parents
	* Powerdown

	Oh no, that does not work, if swap device or its parents uses DMA,
	you've corrupted data. You'd have to do

	* SUSPEND all but swap device and parents
	* FREEZE swap device and parents
	* Snapshot
	* UNFREEZE swap device and parents
	* Write
	* SUSPEND swap device and parents

	Which means that you still need that FREEZE state, and you get more
	complicated code. (And I have not yet introduce details like system
	devices).

	Q: There don't seem to be any generally useful behavioral
	distinctions between SUSPEND and FREEZE.

	A: Doing SUSPEND when you are asked to do FREEZE is always correct,
	but it may be unneccessarily slow. If you want your driver to stay simple,
	slowness may not matter to you. It can always be fixed later.

	For devices like disk it does matter, you do not want to spindown for
	FREEZE.

	Q: After resuming, system is paging heavily, leading to very bad interactivity.

	A: Try running

	cat `cat /proc/[0-9]/maps \| grep / \| sed 's:. /:/:' \| sort -u` > /dev/null

	after resume. swapoff -a; swapon -a may also be useful.

	Q: What happens to devices during swsusp? They seem to be resumed
	during system suspend?

	A: That's correct. We need to resume them if we want to write image to
	disk. Whole sequence goes like

	Suspend part
	~~~~~~~~~~~~
	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: Why can't we suspend to a swap file?

	A: Because accessing swap file needs the filesystem mounted, and
	filesystem might do something wrong (like replaying the journal)
	during mount.

	There are few ways to get that fixed:

	1) Probably could be solved by modifying every filesystem to support
	some kind of "really read-only!" option. Patches welcome.

	2) suspend2 gets around that by storing absolute positions in on-disk
	image (and blocksize), with resume parameter pointing directly to
	suspend header.

	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" or "firmware" 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.

	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.