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1============2Swap suspend3============4 5Some warnings, first.6 7.. warning::8 9 **BIG FAT WARNING**10 11 If you touch anything on disk between suspend and resume...12 ...kiss your data goodbye.13 14 If you do resume from initrd after your filesystems are mounted...15 ...bye bye root partition.16 17 [this is actually same case as above]18 19 If you have unsupported ( ) devices using DMA, you may have some20 problems. If your disk driver does not support suspend... (IDE does),21 it may cause some problems, too. If you change kernel command line22 between suspend and resume, it may do something wrong. If you change23 your hardware while system is suspended... well, it was not good idea;24 but it will probably only crash.25 26 ( ) suspend/resume support is needed to make it safe.27 28 If you have any filesystems on USB devices mounted before software suspend,29 they won't be accessible after resume and you may lose data, as though30 you have unplugged the USB devices with mounted filesystems on them;31 see the FAQ below for details. (This is not true for more traditional32 power states like "standby", which normally don't turn USB off.)33 34Swap partition:35 You need to append resume=/dev/your_swap_partition to kernel command36 line or specify it using /sys/power/resume.37 38Swap file:39 If using a swapfile you can also specify a resume offset using40 resume_offset=<number> on the kernel command line or specify it41 in /sys/power/resume_offset.42 43After preparing then you suspend by::44 45 echo shutdown > /sys/power/disk; echo disk > /sys/power/state46 47- If you feel ACPI works pretty well on your system, you might try::48 49 echo platform > /sys/power/disk; echo disk > /sys/power/state50 51- If you would like to write hibernation image to swap and then suspend52 to RAM (provided your platform supports it), you can try::53 54 echo suspend > /sys/power/disk; echo disk > /sys/power/state55 56- If you have SATA disks, you'll need recent kernels with SATA suspend57 support. For suspend and resume to work, make sure your disk drivers58 are built into kernel -- not modules. [There's way to make59 suspend/resume with modular disk drivers, see FAQ, but you probably60 should not do that.]61 62If you want to limit the suspend image size to N bytes, do::63 64 echo N > /sys/power/image_size65 66before suspend (it is limited to around 2/5 of available RAM by default).67 68- The resume process checks for the presence of the resume device,69 if found, it then checks the contents for the hibernation image signature.70 If both are found, it resumes the hibernation image.71 72- The resume process may be triggered in two ways:73 74 1) During lateinit: If resume=/dev/your_swap_partition is specified on75 the kernel command line, lateinit runs the resume process. If the76 resume device has not been probed yet, the resume process fails and77 bootup continues.78 2) Manually from an initrd or initramfs: May be run from79 the init script by using the /sys/power/resume file. It is vital80 that this be done prior to remounting any filesystems (even as81 read-only) otherwise data may be corrupted.82 83Article about goals and implementation of Software Suspend for Linux84====================================================================85 86Author: Gábor Kuti87Last revised: 2003-10-20 by Pavel Machek88 89Idea and goals to achieve90-------------------------91 92Nowadays it is common in several laptops that they have a suspend button. It93saves the state of the machine to a filesystem or to a partition and switches94to standby mode. Later resuming the machine the saved state is loaded back to95ram and the machine can continue its work. It has two real benefits. First we96save ourselves the time machine goes down and later boots up, energy costs97are real high when running from batteries. The other gain is that we don't have98to interrupt our programs so processes that are calculating something for a long99time shouldn't need to be written interruptible.100 101swsusp saves the state of the machine into active swaps and then reboots or102powerdowns. You must explicitly specify the swap partition to resume from with103`resume=` kernel option. If signature is found it loads and restores saved104state. If the option `noresume` is specified as a boot parameter, it skips105the resuming. If the option `hibernate=nocompress` is specified as a boot106parameter, it saves hibernation image without compression.107 108In the meantime while the system is suspended you should not add/remove any109of the hardware, write to the filesystems, etc.110 111Sleep states summary112====================113 114There are three different interfaces you can use, /proc/acpi should115work like this:116 117In a really perfect world::118 119 echo 1 > /proc/acpi/sleep # for standby120 echo 2 > /proc/acpi/sleep # for suspend to ram121 echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power122 # conservative123 echo 4 > /proc/acpi/sleep # for suspend to disk124 echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system125 126and perhaps::127 128 echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios129 130Frequently Asked Questions131==========================132 133Q:134 well, suspending a server is IMHO a really stupid thing,135 but... (Diego Zuccato):136 137A:138 You bought new UPS for your server. How do you install it without139 bringing machine down? Suspend to disk, rearrange power cables,140 resume.141 142 You have your server on UPS. Power died, and UPS is indicating 30143 seconds to failure. What do you do? Suspend to disk.144 145 146Q:147 Maybe I'm missing something, but why don't the regular I/O paths work?148 149A:150 We do use the regular I/O paths. However we cannot restore the data151 to its original location as we load it. That would create an152 inconsistent kernel state which would certainly result in an oops.153 Instead, we load the image into unused memory and then atomically copy154 it back to it original location. This implies, of course, a maximum155 image size of half the amount of memory.156 157 There are two solutions to this:158 159 * require half of memory to be free during suspend. That way you can160 read "new" data onto free spots, then cli and copy161 162 * assume we had special "polling" ide driver that only uses memory163 between 0-640KB. That way, I'd have to make sure that 0-640KB is free164 during suspending, but otherwise it would work...165 166 suspend2 shares this fundamental limitation, but does not include user167 data and disk caches into "used memory" by saving them in168 advance. That means that the limitation goes away in practice.169 170Q:171 Does linux support ACPI S4?172 173A:174 Yes. That's what echo platform > /sys/power/disk does.175 176Q:177 What is 'suspend2'?178 179A:180 suspend2 is 'Software Suspend 2', a forked implementation of181 suspend-to-disk which is available as separate patches for 2.4 and 2.6182 kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB183 highmem and preemption. It also has a extensible architecture that184 allows for arbitrary transformations on the image (compression,185 encryption) and arbitrary backends for writing the image (eg to swap186 or an NFS share[Work In Progress]). Questions regarding suspend2187 should be sent to the mailing list available through the suspend2188 website, and not to the Linux Kernel Mailing List. We are working189 toward merging suspend2 into the mainline kernel.190 191Q:192 What is the freezing of tasks and why are we using it?193 194A:195 The freezing of tasks is a mechanism by which user space processes and some196 kernel threads are controlled during hibernation or system-wide suspend (on197 some architectures). See freezing-of-tasks.txt for details.198 199Q:200 What is the difference between "platform" and "shutdown"?201 202A:203 shutdown:204 save state in linux, then tell bios to powerdown205 206 platform:207 save state in linux, then tell bios to powerdown and blink208 "suspended led"209 210 "platform" is actually right thing to do where supported, but211 "shutdown" is most reliable (except on ACPI systems).212 213Q:214 I do not understand why you have such strong objections to idea of215 selective suspend.216 217A:218 Do selective suspend during runtime power management, that's okay. But219 it's useless for suspend-to-disk. (And I do not see how you could use220 it for suspend-to-ram, I hope you do not want that).221 222 Lets see, so you suggest to223 224 * SUSPEND all but swap device and parents225 * Snapshot226 * Write image to disk227 * SUSPEND swap device and parents228 * Powerdown229 230 Oh no, that does not work, if swap device or its parents uses DMA,231 you've corrupted data. You'd have to do232 233 * SUSPEND all but swap device and parents234 * FREEZE swap device and parents235 * Snapshot236 * UNFREEZE swap device and parents237 * Write238 * SUSPEND swap device and parents239 240 Which means that you still need that FREEZE state, and you get more241 complicated code. (And I have not yet introduce details like system242 devices).243 244Q:245 There don't seem to be any generally useful behavioral246 distinctions between SUSPEND and FREEZE.247 248A:249 Doing SUSPEND when you are asked to do FREEZE is always correct,250 but it may be unnecessarily slow. If you want your driver to stay simple,251 slowness may not matter to you. It can always be fixed later.252 253 For devices like disk it does matter, you do not want to spindown for254 FREEZE.255 256Q:257 After resuming, system is paging heavily, leading to very bad interactivity.258 259A:260 Try running::261 262 cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u | while read file263 do264 test -f "$file" && cat "$file" > /dev/null265 done266 267 after resume. swapoff -a; swapon -a may also be useful.268 269Q:270 What happens to devices during swsusp? They seem to be resumed271 during system suspend?272 273A:274 That's correct. We need to resume them if we want to write image to275 disk. Whole sequence goes like276 277 **Suspend part**278 279 running system, user asks for suspend-to-disk280 281 user processes are stopped282 283 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere284 with state snapshot285 286 state snapshot: copy of whole used memory is taken with interrupts287 disabled288 289 resume(): devices are woken up so that we can write image to swap290 291 write image to swap292 293 suspend(PMSG_SUSPEND): suspend devices so that we can power off294 295 turn the power off296 297 **Resume part**298 299 (is actually pretty similar)300 301 running system, user asks for suspend-to-disk302 303 user processes are stopped (in common case there are none,304 but with resume-from-initrd, no one knows)305 306 read image from disk307 308 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere309 with image restoration310 311 image restoration: rewrite memory with image312 313 resume(): devices are woken up so that system can continue314 315 thaw all user processes316 317Q:318 What is this 'Encrypt suspend image' for?319 320A:321 First of all: it is not a replacement for dm-crypt encrypted swap.322 It cannot protect your computer while it is suspended. Instead it does323 protect from leaking sensitive data after resume from suspend.324 325 Think of the following: you suspend while an application is running326 that keeps sensitive data in memory. The application itself prevents327 the data from being swapped out. Suspend, however, must write these328 data to swap to be able to resume later on. Without suspend encryption329 your sensitive data are then stored in plaintext on disk. This means330 that after resume your sensitive data are accessible to all331 applications having direct access to the swap device which was used332 for suspend. If you don't need swap after resume these data can remain333 on disk virtually forever. Thus it can happen that your system gets334 broken in weeks later and sensitive data which you thought were335 encrypted and protected are retrieved and stolen from the swap device.336 To prevent this situation you should use 'Encrypt suspend image'.337 338 During suspend a temporary key is created and this key is used to339 encrypt the data written to disk. When, during resume, the data was340 read back into memory the temporary key is destroyed which simply341 means that all data written to disk during suspend are then342 inaccessible so they can't be stolen later on. The only thing that343 you must then take care of is that you call 'mkswap' for the swap344 partition used for suspend as early as possible during regular345 boot. This asserts that any temporary key from an oopsed suspend or346 from a failed or aborted resume is erased from the swap device.347 348 As a rule of thumb use encrypted swap to protect your data while your349 system is shut down or suspended. Additionally use the encrypted350 suspend image to prevent sensitive data from being stolen after351 resume.352 353Q:354 Can I suspend to a swap file?355 356A:357 Generally, yes, you can. However, it requires you to use the "resume=" and358 "resume_offset=" kernel command line parameters, so the resume from a swap359 file cannot be initiated from an initrd or initramfs image. See360 swsusp-and-swap-files.txt for details.361 362Q:363 Is there a maximum system RAM size that is supported by swsusp?364 365A:366 It should work okay with highmem.367 368Q:369 Does swsusp (to disk) use only one swap partition or can it use370 multiple swap partitions (aggregate them into one logical space)?371 372A:373 Only one swap partition, sorry.374 375Q:376 If my application(s) causes lots of memory & swap space to be used377 (over half of the total system RAM), is it correct that it is likely378 to be useless to try to suspend to disk while that app is running?379 380A:381 No, it should work okay, as long as your app does not mlock()382 it. Just prepare big enough swap partition.383 384Q:385 What information is useful for debugging suspend-to-disk problems?386 387A:388 Well, last messages on the screen are always useful. If something389 is broken, it is usually some kernel driver, therefore trying with as390 little as possible modules loaded helps a lot. I also prefer people to391 suspend from console, preferably without X running. Booting with392 init=/bin/bash, then swapon and starting suspend sequence manually393 usually does the trick. Then it is good idea to try with latest394 vanilla kernel.395 396Q:397 How can distributions ship a swsusp-supporting kernel with modular398 disk drivers (especially SATA)?399 400A:401 Well, it can be done, load the drivers, then do echo into402 /sys/power/resume file from initrd. Be sure not to mount403 anything, not even read-only mount, or you are going to lose your404 data.405 406Q:407 How do I make suspend more verbose?408 409A:410 If you want to see any non-error kernel messages on the virtual411 terminal the kernel switches to during suspend, you have to set the412 kernel console loglevel to at least 4 (KERN_WARNING), for example by413 doing::414 415 # save the old loglevel416 read LOGLEVEL DUMMY < /proc/sys/kernel/printk417 # set the loglevel so we see the progress bar.418 # if the level is higher than needed, we leave it alone.419 if [ $LOGLEVEL -lt 5 ]; then420 echo 5 > /proc/sys/kernel/printk421 fi422 423 IMG_SZ=0424 read IMG_SZ < /sys/power/image_size425 echo -n disk > /sys/power/state426 RET=$?427 #428 # the logic here is:429 # if image_size > 0 (without kernel support, IMG_SZ will be zero),430 # then try again with image_size set to zero.431 if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size432 echo 0 > /sys/power/image_size433 echo -n disk > /sys/power/state434 RET=$?435 fi436 437 # restore previous loglevel438 echo $LOGLEVEL > /proc/sys/kernel/printk439 exit $RET440 441Q:442 Is this true that if I have a mounted filesystem on a USB device and443 I suspend to disk, I can lose data unless the filesystem has been mounted444 with "sync"?445 446A:447 That's right ... if you disconnect that device, you may lose data.448 In fact, even with "-o sync" you can lose data if your programs have449 information in buffers they haven't written out to a disk you disconnect,450 or if you disconnect before the device finished saving data you wrote.451 452 Software suspend normally powers down USB controllers, which is equivalent453 to disconnecting all USB devices attached to your system.454 455 Your system might well support low-power modes for its USB controllers456 while the system is asleep, maintaining the connection, using true sleep457 modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the458 /sys/power/state file; write "standby" or "mem".) We've not seen any459 hardware that can use these modes through software suspend, although in460 theory some systems might support "platform" modes that won't break the461 USB connections.462 463 Remember that it's always a bad idea to unplug a disk drive containing a464 mounted filesystem. That's true even when your system is asleep! The465 safest thing is to unmount all filesystems on removable media (such USB,466 Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)467 before suspending; then remount them after resuming.468 469 There is a work-around for this problem. For more information, see470 Documentation/driver-api/usb/persist.rst.471 472Q:473 Can I suspend-to-disk using a swap partition under LVM?474 475A:476 Yes and No. You can suspend successfully, but the kernel will not be able477 to resume on its own. You need an initramfs that can recognize the resume478 situation, activate the logical volume containing the swap volume (but not479 touch any filesystems!), and eventually call::480 481 echo -n "$major:$minor" > /sys/power/resume482 483 where $major and $minor are the respective major and minor device numbers of484 the swap volume.485 486 uswsusp works with LVM, too. See http://suspend.sourceforge.net/487 488Q:489 I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were490 compiled with the similar configuration files. Anyway I found that491 suspend to disk (and resume) is much slower on 2.6.16 compared to492 2.6.15. Any idea for why that might happen or how can I speed it up?493 494A:495 This is because the size of the suspend image is now greater than496 for 2.6.15 (by saving more data we can get more responsive system497 after resume).498 499 There's the /sys/power/image_size knob that controls the size of the500 image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as501 root), the 2.6.15 behavior should be restored. If it is still too502 slow, take a look at suspend.sf.net -- userland suspend is faster and503 supports LZF compression to speed it up further.504