1Some warnings, first. 2 3 * BIG FAT WARNING ********************************************************* 4 * 5 * If you touch anything on disk between suspend and resume... 6 * ...kiss your data goodbye. 7 * 8 * If you do resume from initrd after your filesystems are mounted... 9 * ...bye bye root partition. 10 * [this is actually same case as above] 11 * 12 * If you have unsupported (*) devices using DMA, you may have some 13 * problems. If your disk driver does not support suspend... (IDE does), 14 * it may cause some problems, too. If you change kernel command line 15 * between suspend and resume, it may do something wrong. If you change 16 * your hardware while system is suspended... well, it was not good idea; 17 * but it will probably only crash. 18 * 19 * (*) suspend/resume support is needed to make it safe. 20 * 21 * If you have any filesystems on USB devices mounted before software suspend, 22 * they won't be accessible after resume and you may lose data, as though 23 * you have unplugged the USB devices with mounted filesystems on them; 24 * see the FAQ below for details. (This is not true for more traditional 25 * power states like "standby", which normally don't turn USB off.) 26 27You need to append resume=/dev/your_swap_partition to kernel command 28line. Then you suspend by 29 30echo shutdown > /sys/power/disk; echo disk > /sys/power/state 31 32. If you feel ACPI works pretty well on your system, you might try 33 34echo platform > /sys/power/disk; echo disk > /sys/power/state 35 36. If you would like to write hibernation image to swap and then suspend 37to RAM (provided your platform supports it), you can try 38 39echo suspend > /sys/power/disk; echo disk > /sys/power/state 40 41. If you have SATA disks, you'll need recent kernels with SATA suspend 42support. For suspend and resume to work, make sure your disk drivers 43are built into kernel -- not modules. [There's way to make 44suspend/resume with modular disk drivers, see FAQ, but you probably 45should not do that.] 46 47If you want to limit the suspend image size to N bytes, do 48 49echo N > /sys/power/image_size 50 51before suspend (it is limited to 500 MB by default). 52 53. The resume process checks for the presence of the resume device, 54if found, it then checks the contents for the hibernation image signature. 55If both are found, it resumes the hibernation image. 56 57. The resume process may be triggered in two ways: 58 1) During lateinit: If resume=/dev/your_swap_partition is specified on 59 the kernel command line, lateinit runs the resume process. If the 60 resume device has not been probed yet, the resume process fails and 61 bootup continues. 62 2) Manually from an initrd or initramfs: May be run from 63 the init script by using the /sys/power/resume file. It is vital 64 that this be done prior to remounting any filesystems (even as 65 read-only) otherwise data may be corrupted. 66 67Article about goals and implementation of Software Suspend for Linux 68~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 69Author: Gábor Kuti 70Last revised: 2003-10-20 by Pavel Machek 71 72Idea and goals to achieve 73 74Nowadays it is common in several laptops that they have a suspend button. It 75saves the state of the machine to a filesystem or to a partition and switches 76to standby mode. Later resuming the machine the saved state is loaded back to 77ram and the machine can continue its work. It has two real benefits. First we 78save ourselves the time machine goes down and later boots up, energy costs 79are real high when running from batteries. The other gain is that we don't have to 80interrupt our programs so processes that are calculating something for a long 81time shouldn't need to be written interruptible. 82 83swsusp saves the state of the machine into active swaps and then reboots or 84powerdowns. You must explicitly specify the swap partition to resume from with 85``resume='' kernel option. If signature is found it loads and restores saved 86state. If the option ``noresume'' is specified as a boot parameter, it skips 87the resuming. If the option ``hibernate=nocompress'' is specified as a boot 88parameter, it saves hibernation image without compression. 89 90In the meantime while the system is suspended you should not add/remove any 91of the hardware, write to the filesystems, etc. 92 93Sleep states summary 94==================== 95 96There are three different interfaces you can use, /proc/acpi should 97work like this: 98 99In a really perfect world: 100echo 1 > /proc/acpi/sleep # for standby 101echo 2 > /proc/acpi/sleep # for suspend to ram 102echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative 103echo 4 > /proc/acpi/sleep # for suspend to disk 104echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system 105 106and perhaps 107echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios 108 109Frequently Asked Questions 110========================== 111 112Q: well, suspending a server is IMHO a really stupid thing, 113but... (Diego Zuccato): 114 115A: You bought new UPS for your server. How do you install it without 116bringing machine down? Suspend to disk, rearrange power cables, 117resume. 118 119You have your server on UPS. Power died, and UPS is indicating 30 120seconds to failure. What do you do? Suspend to disk. 121 122 123Q: Maybe I'm missing something, but why don't the regular I/O paths work? 124 125A: We do use the regular I/O paths. However we cannot restore the data 126to its original location as we load it. That would create an 127inconsistent kernel state which would certainly result in an oops. 128Instead, we load the image into unused memory and then atomically copy 129it back to it original location. This implies, of course, a maximum 130image size of half the amount of memory. 131 132There are two solutions to this: 133 134* require half of memory to be free during suspend. That way you can 135read "new" data onto free spots, then cli and copy 136 137* assume we had special "polling" ide driver that only uses memory 138between 0-640KB. That way, I'd have to make sure that 0-640KB is free 139during suspending, but otherwise it would work... 140 141suspend2 shares this fundamental limitation, but does not include user 142data and disk caches into "used memory" by saving them in 143advance. That means that the limitation goes away in practice. 144 145Q: Does linux support ACPI S4? 146 147A: Yes. That's what echo platform > /sys/power/disk does. 148 149Q: What is 'suspend2'? 150 151A: suspend2 is 'Software Suspend 2', a forked implementation of 152suspend-to-disk which is available as separate patches for 2.4 and 2.6 153kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB 154highmem and preemption. It also has a extensible architecture that 155allows for arbitrary transformations on the image (compression, 156encryption) and arbitrary backends for writing the image (eg to swap 157or an NFS share[Work In Progress]). Questions regarding suspend2 158should be sent to the mailing list available through the suspend2 159website, and not to the Linux Kernel Mailing List. We are working 160toward merging suspend2 into the mainline kernel. 161 162Q: What is the freezing of tasks and why are we using it? 163 164A: The freezing of tasks is a mechanism by which user space processes and some 165kernel threads are controlled during hibernation or system-wide suspend (on some 166architectures). See freezing-of-tasks.txt for details. 167 168Q: What is the difference between "platform" and "shutdown"? 169 170A: 171 172shutdown: save state in linux, then tell bios to powerdown 173 174platform: save state in linux, then tell bios to powerdown and blink 175 "suspended led" 176 177"platform" is actually right thing to do where supported, but 178"shutdown" is most reliable (except on ACPI systems). 179 180Q: I do not understand why you have such strong objections to idea of 181selective suspend. 182 183A: Do selective suspend during runtime power management, that's okay. But 184it's useless for suspend-to-disk. (And I do not see how you could use 185it for suspend-to-ram, I hope you do not want that). 186 187Lets see, so you suggest to 188 189* SUSPEND all but swap device and parents 190* Snapshot 191* Write image to disk 192* SUSPEND swap device and parents 193* Powerdown 194 195Oh no, that does not work, if swap device or its parents uses DMA, 196you've corrupted data. You'd have to do 197 198* SUSPEND all but swap device and parents 199* FREEZE swap device and parents 200* Snapshot 201* UNFREEZE swap device and parents 202* Write 203* SUSPEND swap device and parents 204 205Which means that you still need that FREEZE state, and you get more 206complicated code. (And I have not yet introduce details like system 207devices). 208 209Q: There don't seem to be any generally useful behavioral 210distinctions between SUSPEND and FREEZE. 211 212A: Doing SUSPEND when you are asked to do FREEZE is always correct, 213but it may be unnecessarily slow. If you want your driver to stay simple, 214slowness may not matter to you. It can always be fixed later. 215 216For devices like disk it does matter, you do not want to spindown for 217FREEZE. 218 219Q: After resuming, system is paging heavily, leading to very bad interactivity. 220 221A: Try running 222 223cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u | while read file 224do 225 test -f "$file" && cat "$file" > /dev/null 226done 227 228after resume. swapoff -a; swapon -a may also be useful. 229 230Q: What happens to devices during swsusp? They seem to be resumed 231during system suspend? 232 233A: That's correct. We need to resume them if we want to write image to 234disk. Whole sequence goes like 235 236 Suspend part 237 ~~~~~~~~~~~~ 238 running system, user asks for suspend-to-disk 239 240 user processes are stopped 241 242 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 243 with state snapshot 244 245 state snapshot: copy of whole used memory is taken with interrupts disabled 246 247 resume(): devices are woken up so that we can write image to swap 248 249 write image to swap 250 251 suspend(PMSG_SUSPEND): suspend devices so that we can power off 252 253 turn the power off 254 255 Resume part 256 ~~~~~~~~~~~ 257 (is actually pretty similar) 258 259 running system, user asks for suspend-to-disk 260 261 user processes are stopped (in common case there are none, but with resume-from-initrd, no one knows) 262 263 read image from disk 264 265 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 266 with image restoration 267 268 image restoration: rewrite memory with image 269 270 resume(): devices are woken up so that system can continue 271 272 thaw all user processes 273 274Q: What is this 'Encrypt suspend image' for? 275 276A: First of all: it is not a replacement for dm-crypt encrypted swap. 277It cannot protect your computer while it is suspended. Instead it does 278protect from leaking sensitive data after resume from suspend. 279 280Think of the following: you suspend while an application is running 281that keeps sensitive data in memory. The application itself prevents 282the data from being swapped out. Suspend, however, must write these 283data to swap to be able to resume later on. Without suspend encryption 284your sensitive data are then stored in plaintext on disk. This means 285that after resume your sensitive data are accessible to all 286applications having direct access to the swap device which was used 287for suspend. If you don't need swap after resume these data can remain 288on disk virtually forever. Thus it can happen that your system gets 289broken in weeks later and sensitive data which you thought were 290encrypted and protected are retrieved and stolen from the swap device. 291To prevent this situation you should use 'Encrypt suspend image'. 292 293During suspend a temporary key is created and this key is used to 294encrypt the data written to disk. When, during resume, the data was 295read back into memory the temporary key is destroyed which simply 296means that all data written to disk during suspend are then 297inaccessible so they can't be stolen later on. The only thing that 298you must then take care of is that you call 'mkswap' for the swap 299partition used for suspend as early as possible during regular 300boot. This asserts that any temporary key from an oopsed suspend or 301from a failed or aborted resume is erased from the swap device. 302 303As a rule of thumb use encrypted swap to protect your data while your 304system is shut down or suspended. Additionally use the encrypted 305suspend image to prevent sensitive data from being stolen after 306resume. 307 308Q: Can I suspend to a swap file? 309 310A: Generally, yes, you can. However, it requires you to use the "resume=" and 311"resume_offset=" kernel command line parameters, so the resume from a swap file 312cannot be initiated from an initrd or initramfs image. See 313swsusp-and-swap-files.txt for details. 314 315Q: Is there a maximum system RAM size that is supported by swsusp? 316 317A: It should work okay with highmem. 318 319Q: Does swsusp (to disk) use only one swap partition or can it use 320multiple swap partitions (aggregate them into one logical space)? 321 322A: Only one swap partition, sorry. 323 324Q: If my application(s) causes lots of memory & swap space to be used 325(over half of the total system RAM), is it correct that it is likely 326to be useless to try to suspend to disk while that app is running? 327 328A: No, it should work okay, as long as your app does not mlock() 329it. Just prepare big enough swap partition. 330 331Q: What information is useful for debugging suspend-to-disk problems? 332 333A: Well, last messages on the screen are always useful. If something 334is broken, it is usually some kernel driver, therefore trying with as 335little as possible modules loaded helps a lot. I also prefer people to 336suspend from console, preferably without X running. Booting with 337init=/bin/bash, then swapon and starting suspend sequence manually 338usually does the trick. Then it is good idea to try with latest 339vanilla kernel. 340 341Q: How can distributions ship a swsusp-supporting kernel with modular 342disk drivers (especially SATA)? 343 344A: Well, it can be done, load the drivers, then do echo into 345/sys/power/resume file from initrd. Be sure not to mount 346anything, not even read-only mount, or you are going to lose your 347data. 348 349Q: How do I make suspend more verbose? 350 351A: If you want to see any non-error kernel messages on the virtual 352terminal the kernel switches to during suspend, you have to set the 353kernel console loglevel to at least 4 (KERN_WARNING), for example by 354doing 355 356 # save the old loglevel 357 read LOGLEVEL DUMMY < /proc/sys/kernel/printk 358 # set the loglevel so we see the progress bar. 359 # if the level is higher than needed, we leave it alone. 360 if [ $LOGLEVEL -lt 5 ]; then 361 echo 5 > /proc/sys/kernel/printk 362 fi 363 364 IMG_SZ=0 365 read IMG_SZ < /sys/power/image_size 366 echo -n disk > /sys/power/state 367 RET=$? 368 # 369 # the logic here is: 370 # if image_size > 0 (without kernel support, IMG_SZ will be zero), 371 # then try again with image_size set to zero. 372 if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size 373 echo 0 > /sys/power/image_size 374 echo -n disk > /sys/power/state 375 RET=$? 376 fi 377 378 # restore previous loglevel 379 echo $LOGLEVEL > /proc/sys/kernel/printk 380 exit $RET 381 382Q: Is this true that if I have a mounted filesystem on a USB device and 383I suspend to disk, I can lose data unless the filesystem has been mounted 384with "sync"? 385 386A: That's right ... if you disconnect that device, you may lose data. 387In fact, even with "-o sync" you can lose data if your programs have 388information in buffers they haven't written out to a disk you disconnect, 389or if you disconnect before the device finished saving data you wrote. 390 391Software suspend normally powers down USB controllers, which is equivalent 392to disconnecting all USB devices attached to your system. 393 394Your system might well support low-power modes for its USB controllers 395while the system is asleep, maintaining the connection, using true sleep 396modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the 397/sys/power/state file; write "standby" or "mem".) We've not seen any 398hardware that can use these modes through software suspend, although in 399theory some systems might support "platform" modes that won't break the 400USB connections. 401 402Remember that it's always a bad idea to unplug a disk drive containing a 403mounted filesystem. That's true even when your system is asleep! The 404safest thing is to unmount all filesystems on removable media (such USB, 405Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays) 406before suspending; then remount them after resuming. 407 408There is a work-around for this problem. For more information, see 409Documentation/usb/persist.txt. 410 411Q: Can I suspend-to-disk using a swap partition under LVM? 412 413A: No. You can suspend successfully, but you'll not be able to 414resume. uswsusp should be able to work with LVM. See suspend.sf.net. 415 416Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were 417compiled with the similar configuration files. Anyway I found that 418suspend to disk (and resume) is much slower on 2.6.16 compared to 4192.6.15. Any idea for why that might happen or how can I speed it up? 420 421A: This is because the size of the suspend image is now greater than 422for 2.6.15 (by saving more data we can get more responsive system 423after resume). 424 425There's the /sys/power/image_size knob that controls the size of the 426image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as 427root), the 2.6.15 behavior should be restored. If it is still too 428slow, take a look at suspend.sf.net -- userland suspend is faster and 429supports LZF compression to speed it up further. 430