1============ 2Swap suspend 3============ 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 some 20 problems. If your disk driver does not support suspend... (IDE does), 21 it may cause some problems, too. If you change kernel command line 22 between suspend and resume, it may do something wrong. If you change 23 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 though 30 you have unplugged the USB devices with mounted filesystems on them; 31 see the FAQ below for details. (This is not true for more traditional 32 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 command 36 line or specify it using /sys/power/resume. 37 38Swap file: 39 If using a swapfile you can also specify a resume offset using 40 resume_offset=<number> on the kernel command line or specify it 41 in /sys/power/resume_offset. 42 43After preparing then you suspend by:: 44 45 echo shutdown > /sys/power/disk; echo disk > /sys/power/state 46 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/state 50 51- If you would like to write hibernation image to swap and then suspend 52 to RAM (provided your platform supports it), you can try:: 53 54 echo suspend > /sys/power/disk; echo disk > /sys/power/state 55 56- If you have SATA disks, you'll need recent kernels with SATA suspend 57 support. For suspend and resume to work, make sure your disk drivers 58 are built into kernel -- not modules. [There's way to make 59 suspend/resume with modular disk drivers, see FAQ, but you probably 60 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_size 65 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 on 75 the kernel command line, lateinit runs the resume process. If the 76 resume device has not been probed yet, the resume process fails and 77 bootup continues. 78 2) Manually from an initrd or initramfs: May be run from 79 the init script by using the /sys/power/resume file. It is vital 80 that this be done prior to remounting any filesystems (even as 81 read-only) otherwise data may be corrupted. 82 83Article about goals and implementation of Software Suspend for Linux 84==================================================================== 85 86Author: Gábor Kuti 87Last revised: 2003-10-20 by Pavel Machek 88 89Idea and goals to achieve 90------------------------- 91 92Nowadays it is common in several laptops that they have a suspend button. It 93saves the state of the machine to a filesystem or to a partition and switches 94to standby mode. Later resuming the machine the saved state is loaded back to 95ram and the machine can continue its work. It has two real benefits. First we 96save ourselves the time machine goes down and later boots up, energy costs 97are real high when running from batteries. The other gain is that we don't have 98to interrupt our programs so processes that are calculating something for a long 99time shouldn't need to be written interruptible. 100 101swsusp saves the state of the machine into active swaps and then reboots or 102powerdowns. You must explicitly specify the swap partition to resume from with 103`resume=` kernel option. If signature is found it loads and restores saved 104state. If the option `noresume` is specified as a boot parameter, it skips 105the resuming. If the option `hibernate=nocompress` is specified as a boot 106parameter, it saves hibernation image without compression. 107 108In the meantime while the system is suspended you should not add/remove any 109of the hardware, write to the filesystems, etc. 110 111Sleep states summary 112==================== 113 114There are three different interfaces you can use, /proc/acpi should 115work like this: 116 117In a really perfect world:: 118 119 echo 1 > /proc/acpi/sleep # for standby 120 echo 2 > /proc/acpi/sleep # for suspend to ram 121 echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative 122 echo 4 > /proc/acpi/sleep # for suspend to disk 123 echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system 124 125and perhaps:: 126 127 echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios 128 129Frequently Asked Questions 130========================== 131 132Q: 133 well, suspending a server is IMHO a really stupid thing, 134 but... (Diego Zuccato): 135 136A: 137 You bought new UPS for your server. How do you install it without 138 bringing machine down? Suspend to disk, rearrange power cables, 139 resume. 140 141 You have your server on UPS. Power died, and UPS is indicating 30 142 seconds to failure. What do you do? Suspend to disk. 143 144 145Q: 146 Maybe I'm missing something, but why don't the regular I/O paths work? 147 148A: 149 We do use the regular I/O paths. However we cannot restore the data 150 to its original location as we load it. That would create an 151 inconsistent kernel state which would certainly result in an oops. 152 Instead, we load the image into unused memory and then atomically copy 153 it back to it original location. This implies, of course, a maximum 154 image size of half the amount of memory. 155 156 There are two solutions to this: 157 158 * require half of memory to be free during suspend. That way you can 159 read "new" data onto free spots, then cli and copy 160 161 * assume we had special "polling" ide driver that only uses memory 162 between 0-640KB. That way, I'd have to make sure that 0-640KB is free 163 during suspending, but otherwise it would work... 164 165 suspend2 shares this fundamental limitation, but does not include user 166 data and disk caches into "used memory" by saving them in 167 advance. That means that the limitation goes away in practice. 168 169Q: 170 Does linux support ACPI S4? 171 172A: 173 Yes. That's what echo platform > /sys/power/disk does. 174 175Q: 176 What is 'suspend2'? 177 178A: 179 suspend2 is 'Software Suspend 2', a forked implementation of 180 suspend-to-disk which is available as separate patches for 2.4 and 2.6 181 kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB 182 highmem and preemption. It also has a extensible architecture that 183 allows for arbitrary transformations on the image (compression, 184 encryption) and arbitrary backends for writing the image (eg to swap 185 or an NFS share[Work In Progress]). Questions regarding suspend2 186 should be sent to the mailing list available through the suspend2 187 website, and not to the Linux Kernel Mailing List. We are working 188 toward merging suspend2 into the mainline kernel. 189 190Q: 191 What is the freezing of tasks and why are we using it? 192 193A: 194 The freezing of tasks is a mechanism by which user space processes and some 195 kernel threads are controlled during hibernation or system-wide suspend (on some 196 architectures). See freezing-of-tasks.txt for details. 197 198Q: 199 What is the difference between "platform" and "shutdown"? 200 201A: 202 shutdown: 203 save state in linux, then tell bios to powerdown 204 205 platform: 206 save state in linux, then tell bios to powerdown and blink 207 "suspended led" 208 209 "platform" is actually right thing to do where supported, but 210 "shutdown" is most reliable (except on ACPI systems). 211 212Q: 213 I do not understand why you have such strong objections to idea of 214 selective suspend. 215 216A: 217 Do selective suspend during runtime power management, that's okay. But 218 it's useless for suspend-to-disk. (And I do not see how you could use 219 it for suspend-to-ram, I hope you do not want that). 220 221 Lets see, so you suggest to 222 223 * SUSPEND all but swap device and parents 224 * Snapshot 225 * Write image to disk 226 * SUSPEND swap device and parents 227 * Powerdown 228 229 Oh no, that does not work, if swap device or its parents uses DMA, 230 you've corrupted data. You'd have to do 231 232 * SUSPEND all but swap device and parents 233 * FREEZE swap device and parents 234 * Snapshot 235 * UNFREEZE swap device and parents 236 * Write 237 * SUSPEND swap device and parents 238 239 Which means that you still need that FREEZE state, and you get more 240 complicated code. (And I have not yet introduce details like system 241 devices). 242 243Q: 244 There don't seem to be any generally useful behavioral 245 distinctions between SUSPEND and FREEZE. 246 247A: 248 Doing SUSPEND when you are asked to do FREEZE is always correct, 249 but it may be unnecessarily slow. If you want your driver to stay simple, 250 slowness may not matter to you. It can always be fixed later. 251 252 For devices like disk it does matter, you do not want to spindown for 253 FREEZE. 254 255Q: 256 After resuming, system is paging heavily, leading to very bad interactivity. 257 258A: 259 Try running:: 260 261 cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u | while read file 262 do 263 test -f "$file" && cat "$file" > /dev/null 264 done 265 266 after resume. swapoff -a; swapon -a may also be useful. 267 268Q: 269 What happens to devices during swsusp? They seem to be resumed 270 during system suspend? 271 272A: 273 That's correct. We need to resume them if we want to write image to 274 disk. Whole sequence goes like 275 276 **Suspend part** 277 278 running system, user asks for suspend-to-disk 279 280 user processes are stopped 281 282 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 283 with state snapshot 284 285 state snapshot: copy of whole used memory is taken with interrupts disabled 286 287 resume(): devices are woken up so that we can write image to swap 288 289 write image to swap 290 291 suspend(PMSG_SUSPEND): suspend devices so that we can power off 292 293 turn the power off 294 295 **Resume part** 296 297 (is actually pretty similar) 298 299 running system, user asks for suspend-to-disk 300 301 user processes are stopped (in common case there are none, 302 but with resume-from-initrd, no one knows) 303 304 read image from disk 305 306 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 307 with image restoration 308 309 image restoration: rewrite memory with image 310 311 resume(): devices are woken up so that system can continue 312 313 thaw all user processes 314 315Q: 316 What is this 'Encrypt suspend image' for? 317 318A: 319 First of all: it is not a replacement for dm-crypt encrypted swap. 320 It cannot protect your computer while it is suspended. Instead it does 321 protect from leaking sensitive data after resume from suspend. 322 323 Think of the following: you suspend while an application is running 324 that keeps sensitive data in memory. The application itself prevents 325 the data from being swapped out. Suspend, however, must write these 326 data to swap to be able to resume later on. Without suspend encryption 327 your sensitive data are then stored in plaintext on disk. This means 328 that after resume your sensitive data are accessible to all 329 applications having direct access to the swap device which was used 330 for suspend. If you don't need swap after resume these data can remain 331 on disk virtually forever. Thus it can happen that your system gets 332 broken in weeks later and sensitive data which you thought were 333 encrypted and protected are retrieved and stolen from the swap device. 334 To prevent this situation you should use 'Encrypt suspend image'. 335 336 During suspend a temporary key is created and this key is used to 337 encrypt the data written to disk. When, during resume, the data was 338 read back into memory the temporary key is destroyed which simply 339 means that all data written to disk during suspend are then 340 inaccessible so they can't be stolen later on. The only thing that 341 you must then take care of is that you call 'mkswap' for the swap 342 partition used for suspend as early as possible during regular 343 boot. This asserts that any temporary key from an oopsed suspend or 344 from a failed or aborted resume is erased from the swap device. 345 346 As a rule of thumb use encrypted swap to protect your data while your 347 system is shut down or suspended. Additionally use the encrypted 348 suspend image to prevent sensitive data from being stolen after 349 resume. 350 351Q: 352 Can I suspend to a swap file? 353 354A: 355 Generally, yes, you can. However, it requires you to use the "resume=" and 356 "resume_offset=" kernel command line parameters, so the resume from a swap file 357 cannot be initiated from an initrd or initramfs image. See 358 swsusp-and-swap-files.txt for details. 359 360Q: 361 Is there a maximum system RAM size that is supported by swsusp? 362 363A: 364 It should work okay with highmem. 365 366Q: 367 Does swsusp (to disk) use only one swap partition or can it use 368 multiple swap partitions (aggregate them into one logical space)? 369 370A: 371 Only one swap partition, sorry. 372 373Q: 374 If my application(s) causes lots of memory & swap space to be used 375 (over half of the total system RAM), is it correct that it is likely 376 to be useless to try to suspend to disk while that app is running? 377 378A: 379 No, it should work okay, as long as your app does not mlock() 380 it. Just prepare big enough swap partition. 381 382Q: 383 What information is useful for debugging suspend-to-disk problems? 384 385A: 386 Well, last messages on the screen are always useful. If something 387 is broken, it is usually some kernel driver, therefore trying with as 388 little as possible modules loaded helps a lot. I also prefer people to 389 suspend from console, preferably without X running. Booting with 390 init=/bin/bash, then swapon and starting suspend sequence manually 391 usually does the trick. Then it is good idea to try with latest 392 vanilla kernel. 393 394Q: 395 How can distributions ship a swsusp-supporting kernel with modular 396 disk drivers (especially SATA)? 397 398A: 399 Well, it can be done, load the drivers, then do echo into 400 /sys/power/resume file from initrd. Be sure not to mount 401 anything, not even read-only mount, or you are going to lose your 402 data. 403 404Q: 405 How do I make suspend more verbose? 406 407A: 408 If you want to see any non-error kernel messages on the virtual 409 terminal the kernel switches to during suspend, you have to set the 410 kernel console loglevel to at least 4 (KERN_WARNING), for example by 411 doing:: 412 413 # save the old loglevel 414 read LOGLEVEL DUMMY < /proc/sys/kernel/printk 415 # set the loglevel so we see the progress bar. 416 # if the level is higher than needed, we leave it alone. 417 if [ $LOGLEVEL -lt 5 ]; then 418 echo 5 > /proc/sys/kernel/printk 419 fi 420 421 IMG_SZ=0 422 read IMG_SZ < /sys/power/image_size 423 echo -n disk > /sys/power/state 424 RET=$? 425 # 426 # the logic here is: 427 # if image_size > 0 (without kernel support, IMG_SZ will be zero), 428 # then try again with image_size set to zero. 429 if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size 430 echo 0 > /sys/power/image_size 431 echo -n disk > /sys/power/state 432 RET=$? 433 fi 434 435 # restore previous loglevel 436 echo $LOGLEVEL > /proc/sys/kernel/printk 437 exit $RET 438 439Q: 440 Is this true that if I have a mounted filesystem on a USB device and 441 I suspend to disk, I can lose data unless the filesystem has been mounted 442 with "sync"? 443 444A: 445 That's right ... if you disconnect that device, you may lose data. 446 In fact, even with "-o sync" you can lose data if your programs have 447 information in buffers they haven't written out to a disk you disconnect, 448 or if you disconnect before the device finished saving data you wrote. 449 450 Software suspend normally powers down USB controllers, which is equivalent 451 to disconnecting all USB devices attached to your system. 452 453 Your system might well support low-power modes for its USB controllers 454 while the system is asleep, maintaining the connection, using true sleep 455 modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the 456 /sys/power/state file; write "standby" or "mem".) We've not seen any 457 hardware that can use these modes through software suspend, although in 458 theory some systems might support "platform" modes that won't break the 459 USB connections. 460 461 Remember that it's always a bad idea to unplug a disk drive containing a 462 mounted filesystem. That's true even when your system is asleep! The 463 safest thing is to unmount all filesystems on removable media (such USB, 464 Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays) 465 before suspending; then remount them after resuming. 466 467 There is a work-around for this problem. For more information, see 468 Documentation/driver-api/usb/persist.rst. 469 470Q: 471 Can I suspend-to-disk using a swap partition under LVM? 472 473A: 474 Yes and No. You can suspend successfully, but the kernel will not be able 475 to resume on its own. You need an initramfs that can recognize the resume 476 situation, activate the logical volume containing the swap volume (but not 477 touch any filesystems!), and eventually call:: 478 479 echo -n "$major:$minor" > /sys/power/resume 480 481 where $major and $minor are the respective major and minor device numbers of 482 the swap volume. 483 484 uswsusp works with LVM, too. See http://suspend.sourceforge.net/ 485 486Q: 487 I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were 488 compiled with the similar configuration files. Anyway I found that 489 suspend to disk (and resume) is much slower on 2.6.16 compared to 490 2.6.15. Any idea for why that might happen or how can I speed it up? 491 492A: 493 This is because the size of the suspend image is now greater than 494 for 2.6.15 (by saving more data we can get more responsive system 495 after resume). 496 497 There's the /sys/power/image_size knob that controls the size of the 498 image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as 499 root), the 2.6.15 behavior should be restored. If it is still too 500 slow, take a look at suspend.sf.net -- userland suspend is faster and 501 supports LZF compression to speed it up further. 502