414 lines
16 KiB
Plaintext
414 lines
16 KiB
Plaintext
Some warnings, first.
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* BIG FAT WARNING *********************************************************
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*
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* If you touch anything on disk between suspend and resume...
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* ...kiss your data goodbye.
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*
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* If you do resume from initrd after your filesystems are mounted...
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* ...bye bye root partition.
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* [this is actually same case as above]
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*
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* If you have unsupported (*) devices using DMA, you may have some
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* problems. If your disk driver does not support suspend... (IDE does),
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* it may cause some problems, too. If you change kernel command line
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* between suspend and resume, it may do something wrong. If you change
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* your hardware while system is suspended... well, it was not good idea;
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* but it will probably only crash.
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*
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* (*) suspend/resume support is needed to make it safe.
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*
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* If you have any filesystems on USB devices mounted before software suspend,
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* they won't be accessible after resume and you may lose data, as though
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* you have unplugged the USB devices with mounted filesystems on them;
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* see the FAQ below for details. (This is not true for more traditional
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* power states like "standby", which normally don't turn USB off.)
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You need to append resume=/dev/your_swap_partition to kernel command
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line. Then you suspend by
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echo shutdown > /sys/power/disk; echo disk > /sys/power/state
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. If you feel ACPI works pretty well on your system, you might try
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echo platform > /sys/power/disk; echo disk > /sys/power/state
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. If you would like to write hibernation image to swap and then suspend
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to RAM (provided your platform supports it), you can try
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echo suspend > /sys/power/disk; echo disk > /sys/power/state
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. If you have SATA disks, you'll need recent kernels with SATA suspend
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support. For suspend and resume to work, make sure your disk drivers
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are built into kernel -- not modules. [There's way to make
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suspend/resume with modular disk drivers, see FAQ, but you probably
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should not do that.]
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If you want to limit the suspend image size to N bytes, do
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echo N > /sys/power/image_size
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before suspend (it is limited to 500 MB by default).
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Article about goals and implementation of Software Suspend for Linux
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Author: Gábor Kuti
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Last revised: 2003-10-20 by Pavel Machek
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Idea and goals to achieve
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Nowadays it is common in several laptops that they have a suspend button. It
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saves the state of the machine to a filesystem or to a partition and switches
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to standby mode. Later resuming the machine the saved state is loaded back to
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ram and the machine can continue its work. It has two real benefits. First we
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save ourselves the time machine goes down and later boots up, energy costs
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are real high when running from batteries. The other gain is that we don't have to
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interrupt our programs so processes that are calculating something for a long
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time shouldn't need to be written interruptible.
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swsusp saves the state of the machine into active swaps and then reboots or
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powerdowns. You must explicitly specify the swap partition to resume from with
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``resume='' kernel option. If signature is found it loads and restores saved
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state. If the option ``noresume'' is specified as a boot parameter, it skips
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the resuming. If the option ``hibernate=nocompress'' is specified as a boot
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parameter, it saves hibernation image without compression.
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In the meantime while the system is suspended you should not add/remove any
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of the hardware, write to the filesystems, etc.
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Sleep states summary
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====================
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There are three different interfaces you can use, /proc/acpi should
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work like this:
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In a really perfect world:
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echo 1 > /proc/acpi/sleep # for standby
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echo 2 > /proc/acpi/sleep # for suspend to ram
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echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative
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echo 4 > /proc/acpi/sleep # for suspend to disk
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echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system
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and perhaps
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echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios
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Frequently Asked Questions
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==========================
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Q: well, suspending a server is IMHO a really stupid thing,
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but... (Diego Zuccato):
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A: You bought new UPS for your server. How do you install it without
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bringing machine down? Suspend to disk, rearrange power cables,
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resume.
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You have your server on UPS. Power died, and UPS is indicating 30
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seconds to failure. What do you do? Suspend to disk.
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Q: Maybe I'm missing something, but why don't the regular I/O paths work?
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A: We do use the regular I/O paths. However we cannot restore the data
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to its original location as we load it. That would create an
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inconsistent kernel state which would certainly result in an oops.
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Instead, we load the image into unused memory and then atomically copy
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it back to it original location. This implies, of course, a maximum
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image size of half the amount of memory.
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There are two solutions to this:
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* require half of memory to be free during suspend. That way you can
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read "new" data onto free spots, then cli and copy
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* assume we had special "polling" ide driver that only uses memory
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between 0-640KB. That way, I'd have to make sure that 0-640KB is free
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during suspending, but otherwise it would work...
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suspend2 shares this fundamental limitation, but does not include user
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data and disk caches into "used memory" by saving them in
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advance. That means that the limitation goes away in practice.
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Q: Does linux support ACPI S4?
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A: Yes. That's what echo platform > /sys/power/disk does.
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Q: What is 'suspend2'?
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A: suspend2 is 'Software Suspend 2', a forked implementation of
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suspend-to-disk which is available as separate patches for 2.4 and 2.6
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kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB
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highmem and preemption. It also has a extensible architecture that
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allows for arbitrary transformations on the image (compression,
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encryption) and arbitrary backends for writing the image (eg to swap
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or an NFS share[Work In Progress]). Questions regarding suspend2
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should be sent to the mailing list available through the suspend2
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website, and not to the Linux Kernel Mailing List. We are working
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toward merging suspend2 into the mainline kernel.
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Q: What is the freezing of tasks and why are we using it?
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A: The freezing of tasks is a mechanism by which user space processes and some
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kernel threads are controlled during hibernation or system-wide suspend (on some
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architectures). See freezing-of-tasks.txt for details.
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Q: What is the difference between "platform" and "shutdown"?
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A:
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shutdown: save state in linux, then tell bios to powerdown
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platform: save state in linux, then tell bios to powerdown and blink
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"suspended led"
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"platform" is actually right thing to do where supported, but
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"shutdown" is most reliable (except on ACPI systems).
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Q: I do not understand why you have such strong objections to idea of
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selective suspend.
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A: Do selective suspend during runtime power management, that's okay. But
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it's useless for suspend-to-disk. (And I do not see how you could use
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it for suspend-to-ram, I hope you do not want that).
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Lets see, so you suggest to
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* SUSPEND all but swap device and parents
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* Snapshot
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* Write image to disk
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* SUSPEND swap device and parents
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* Powerdown
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Oh no, that does not work, if swap device or its parents uses DMA,
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you've corrupted data. You'd have to do
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* SUSPEND all but swap device and parents
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* FREEZE swap device and parents
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* Snapshot
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* UNFREEZE swap device and parents
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* Write
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* SUSPEND swap device and parents
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Which means that you still need that FREEZE state, and you get more
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complicated code. (And I have not yet introduce details like system
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devices).
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Q: There don't seem to be any generally useful behavioral
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distinctions between SUSPEND and FREEZE.
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A: Doing SUSPEND when you are asked to do FREEZE is always correct,
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but it may be unnecessarily slow. If you want your driver to stay simple,
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slowness may not matter to you. It can always be fixed later.
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For devices like disk it does matter, you do not want to spindown for
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FREEZE.
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Q: After resuming, system is paging heavily, leading to very bad interactivity.
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A: Try running
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cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null
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after resume. swapoff -a; swapon -a may also be useful.
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Q: What happens to devices during swsusp? They seem to be resumed
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during system suspend?
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A: That's correct. We need to resume them if we want to write image to
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disk. Whole sequence goes like
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Suspend part
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~~~~~~~~~~~~
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running system, user asks for suspend-to-disk
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user processes are stopped
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suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
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with state snapshot
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state snapshot: copy of whole used memory is taken with interrupts disabled
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resume(): devices are woken up so that we can write image to swap
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write image to swap
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suspend(PMSG_SUSPEND): suspend devices so that we can power off
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turn the power off
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Resume part
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~~~~~~~~~~~
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(is actually pretty similar)
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running system, user asks for suspend-to-disk
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user processes are stopped (in common case there are none, but with resume-from-initrd, no one knows)
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read image from disk
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suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
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with image restoration
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image restoration: rewrite memory with image
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resume(): devices are woken up so that system can continue
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thaw all user processes
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Q: What is this 'Encrypt suspend image' for?
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A: First of all: it is not a replacement for dm-crypt encrypted swap.
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It cannot protect your computer while it is suspended. Instead it does
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protect from leaking sensitive data after resume from suspend.
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Think of the following: you suspend while an application is running
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that keeps sensitive data in memory. The application itself prevents
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the data from being swapped out. Suspend, however, must write these
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data to swap to be able to resume later on. Without suspend encryption
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your sensitive data are then stored in plaintext on disk. This means
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that after resume your sensitive data are accessible to all
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applications having direct access to the swap device which was used
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for suspend. If you don't need swap after resume these data can remain
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on disk virtually forever. Thus it can happen that your system gets
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broken in weeks later and sensitive data which you thought were
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encrypted and protected are retrieved and stolen from the swap device.
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To prevent this situation you should use 'Encrypt suspend image'.
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During suspend a temporary key is created and this key is used to
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encrypt the data written to disk. When, during resume, the data was
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read back into memory the temporary key is destroyed which simply
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means that all data written to disk during suspend are then
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inaccessible so they can't be stolen later on. The only thing that
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you must then take care of is that you call 'mkswap' for the swap
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partition used for suspend as early as possible during regular
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boot. This asserts that any temporary key from an oopsed suspend or
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from a failed or aborted resume is erased from the swap device.
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As a rule of thumb use encrypted swap to protect your data while your
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system is shut down or suspended. Additionally use the encrypted
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suspend image to prevent sensitive data from being stolen after
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resume.
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Q: Can I suspend to a swap file?
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A: Generally, yes, you can. However, it requires you to use the "resume=" and
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"resume_offset=" kernel command line parameters, so the resume from a swap file
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cannot be initiated from an initrd or initramfs image. See
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swsusp-and-swap-files.txt for details.
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Q: Is there a maximum system RAM size that is supported by swsusp?
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A: It should work okay with highmem.
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Q: Does swsusp (to disk) use only one swap partition or can it use
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multiple swap partitions (aggregate them into one logical space)?
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A: Only one swap partition, sorry.
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Q: If my application(s) causes lots of memory & swap space to be used
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(over half of the total system RAM), is it correct that it is likely
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to be useless to try to suspend to disk while that app is running?
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A: No, it should work okay, as long as your app does not mlock()
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it. Just prepare big enough swap partition.
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Q: What information is useful for debugging suspend-to-disk problems?
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A: Well, last messages on the screen are always useful. If something
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is broken, it is usually some kernel driver, therefore trying with as
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little as possible modules loaded helps a lot. I also prefer people to
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suspend from console, preferably without X running. Booting with
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init=/bin/bash, then swapon and starting suspend sequence manually
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usually does the trick. Then it is good idea to try with latest
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vanilla kernel.
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Q: How can distributions ship a swsusp-supporting kernel with modular
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disk drivers (especially SATA)?
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A: Well, it can be done, load the drivers, then do echo into
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/sys/power/disk/resume file from initrd. Be sure not to mount
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anything, not even read-only mount, or you are going to lose your
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data.
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Q: How do I make suspend more verbose?
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A: If you want to see any non-error kernel messages on the virtual
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terminal the kernel switches to during suspend, you have to set the
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kernel console loglevel to at least 4 (KERN_WARNING), for example by
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doing
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# save the old loglevel
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read LOGLEVEL DUMMY < /proc/sys/kernel/printk
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# set the loglevel so we see the progress bar.
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# if the level is higher than needed, we leave it alone.
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if [ $LOGLEVEL -lt 5 ]; then
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echo 5 > /proc/sys/kernel/printk
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fi
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IMG_SZ=0
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read IMG_SZ < /sys/power/image_size
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echo -n disk > /sys/power/state
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RET=$?
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#
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# the logic here is:
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# if image_size > 0 (without kernel support, IMG_SZ will be zero),
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# then try again with image_size set to zero.
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if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
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echo 0 > /sys/power/image_size
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echo -n disk > /sys/power/state
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RET=$?
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fi
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# restore previous loglevel
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echo $LOGLEVEL > /proc/sys/kernel/printk
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exit $RET
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Q: Is this true that if I have a mounted filesystem on a USB device and
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I suspend to disk, I can lose data unless the filesystem has been mounted
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with "sync"?
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A: That's right ... if you disconnect that device, you may lose data.
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In fact, even with "-o sync" you can lose data if your programs have
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information in buffers they haven't written out to a disk you disconnect,
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or if you disconnect before the device finished saving data you wrote.
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Software suspend normally powers down USB controllers, which is equivalent
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to disconnecting all USB devices attached to your system.
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Your system might well support low-power modes for its USB controllers
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while the system is asleep, maintaining the connection, using true sleep
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modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the
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/sys/power/state file; write "standby" or "mem".) We've not seen any
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hardware that can use these modes through software suspend, although in
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theory some systems might support "platform" modes that won't break the
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USB connections.
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Remember that it's always a bad idea to unplug a disk drive containing a
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mounted filesystem. That's true even when your system is asleep! The
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safest thing is to unmount all filesystems on removable media (such USB,
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Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
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before suspending; then remount them after resuming.
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There is a work-around for this problem. For more information, see
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Documentation/usb/persist.txt.
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Q: Can I suspend-to-disk using a swap partition under LVM?
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A: No. You can suspend successfully, but you'll not be able to
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resume. uswsusp should be able to work with LVM. See suspend.sf.net.
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Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
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compiled with the similar configuration files. Anyway I found that
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suspend to disk (and resume) is much slower on 2.6.16 compared to
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2.6.15. Any idea for why that might happen or how can I speed it up?
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A: This is because the size of the suspend image is now greater than
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for 2.6.15 (by saving more data we can get more responsive system
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after resume).
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There's the /sys/power/image_size knob that controls the size of the
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image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
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root), the 2.6.15 behavior should be restored. If it is still too
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slow, take a look at suspend.sf.net -- userland suspend is faster and
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supports LZF compression to speed it up further.
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