Based on kernel version 4.1. Page generated on 2015-06-28 12:14 EST.
1 Some 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 27 You need to append resume=/dev/your_swap_partition to kernel command 28 line. Then you suspend by 29 30 echo 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 34 echo platform > /sys/power/disk; echo disk > /sys/power/state 35 36 . If you would like to write hibernation image to swap and then suspend 37 to RAM (provided your platform supports it), you can try 38 39 echo suspend > /sys/power/disk; echo disk > /sys/power/state 40 41 . If you have SATA disks, you'll need recent kernels with SATA suspend 42 support. For suspend and resume to work, make sure your disk drivers 43 are built into kernel -- not modules. [There's way to make 44 suspend/resume with modular disk drivers, see FAQ, but you probably 45 should not do that.] 46 47 If you want to limit the suspend image size to N bytes, do 48 49 echo N > /sys/power/image_size 50 51 before suspend (it is limited to 500 MB by default). 52 53 . The resume process checks for the presence of the resume device, 54 if found, it then checks the contents for the hibernation image signature. 55 If 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 67 Article about goals and implementation of Software Suspend for Linux 68 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 69 Author: Gábor Kuti 70 Last revised: 2003-10-20 by Pavel Machek 71 72 Idea and goals to achieve 73 74 Nowadays it is common in several laptops that they have a suspend button. It 75 saves the state of the machine to a filesystem or to a partition and switches 76 to standby mode. Later resuming the machine the saved state is loaded back to 77 ram and the machine can continue its work. It has two real benefits. First we 78 save ourselves the time machine goes down and later boots up, energy costs 79 are real high when running from batteries. The other gain is that we don't have to 80 interrupt our programs so processes that are calculating something for a long 81 time shouldn't need to be written interruptible. 82 83 swsusp saves the state of the machine into active swaps and then reboots or 84 powerdowns. You must explicitly specify the swap partition to resume from with 85 ``resume='' kernel option. If signature is found it loads and restores saved 86 state. If the option ``noresume'' is specified as a boot parameter, it skips 87 the resuming. If the option ``hibernate=nocompress'' is specified as a boot 88 parameter, it saves hibernation image without compression. 89 90 In the meantime while the system is suspended you should not add/remove any 91 of the hardware, write to the filesystems, etc. 92 93 Sleep states summary 94 ==================== 95 96 There are three different interfaces you can use, /proc/acpi should 97 work like this: 98 99 In a really perfect world: 100 echo 1 > /proc/acpi/sleep # for standby 101 echo 2 > /proc/acpi/sleep # for suspend to ram 102 echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative 103 echo 4 > /proc/acpi/sleep # for suspend to disk 104 echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system 105 106 and perhaps 107 echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios 108 109 Frequently Asked Questions 110 ========================== 111 112 Q: well, suspending a server is IMHO a really stupid thing, 113 but... (Diego Zuccato): 114 115 A: You bought new UPS for your server. How do you install it without 116 bringing machine down? Suspend to disk, rearrange power cables, 117 resume. 118 119 You have your server on UPS. Power died, and UPS is indicating 30 120 seconds to failure. What do you do? Suspend to disk. 121 122 123 Q: Maybe I'm missing something, but why don't the regular I/O paths work? 124 125 A: We do use the regular I/O paths. However we cannot restore the data 126 to its original location as we load it. That would create an 127 inconsistent kernel state which would certainly result in an oops. 128 Instead, we load the image into unused memory and then atomically copy 129 it back to it original location. This implies, of course, a maximum 130 image size of half the amount of memory. 131 132 There are two solutions to this: 133 134 * require half of memory to be free during suspend. That way you can 135 read "new" data onto free spots, then cli and copy 136 137 * assume we had special "polling" ide driver that only uses memory 138 between 0-640KB. That way, I'd have to make sure that 0-640KB is free 139 during suspending, but otherwise it would work... 140 141 suspend2 shares this fundamental limitation, but does not include user 142 data and disk caches into "used memory" by saving them in 143 advance. That means that the limitation goes away in practice. 144 145 Q: Does linux support ACPI S4? 146 147 A: Yes. That's what echo platform > /sys/power/disk does. 148 149 Q: What is 'suspend2'? 150 151 A: suspend2 is 'Software Suspend 2', a forked implementation of 152 suspend-to-disk which is available as separate patches for 2.4 and 2.6 153 kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB 154 highmem and preemption. It also has a extensible architecture that 155 allows for arbitrary transformations on the image (compression, 156 encryption) and arbitrary backends for writing the image (eg to swap 157 or an NFS share[Work In Progress]). Questions regarding suspend2 158 should be sent to the mailing list available through the suspend2 159 website, and not to the Linux Kernel Mailing List. We are working 160 toward merging suspend2 into the mainline kernel. 161 162 Q: What is the freezing of tasks and why are we using it? 163 164 A: The freezing of tasks is a mechanism by which user space processes and some 165 kernel threads are controlled during hibernation or system-wide suspend (on some 166 architectures). See freezing-of-tasks.txt for details. 167 168 Q: What is the difference between "platform" and "shutdown"? 169 170 A: 171 172 shutdown: save state in linux, then tell bios to powerdown 173 174 platform: 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). 179 180 Q: I do not understand why you have such strong objections to idea of 181 selective suspend. 182 183 A: Do selective suspend during runtime power management, that's okay. But 184 it's useless for suspend-to-disk. (And I do not see how you could use 185 it for suspend-to-ram, I hope you do not want that). 186 187 Lets 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 194 195 Oh no, that does not work, if swap device or its parents uses DMA, 196 you'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 204 205 Which means that you still need that FREEZE state, and you get more 206 complicated code. (And I have not yet introduce details like system 207 devices). 208 209 Q: There don't seem to be any generally useful behavioral 210 distinctions between SUSPEND and FREEZE. 211 212 A: Doing SUSPEND when you are asked to do FREEZE is always correct, 213 but it may be unnecessarily slow. If you want your driver to stay simple, 214 slowness may not matter to you. It can always be fixed later. 215 216 For devices like disk it does matter, you do not want to spindown for 217 FREEZE. 218 219 Q: After resuming, system is paging heavily, leading to very bad interactivity. 220 221 A: Try running 222 223 cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u | while read file 224 do 225 test -f "$file" && cat "$file" > /dev/null 226 done 227 228 after resume. swapoff -a; swapon -a may also be useful. 229 230 Q: What happens to devices during swsusp? They seem to be resumed 231 during system suspend? 232 233 A: That's correct. We need to resume them if we want to write image to 234 disk. 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 273 274 Q: What is this 'Encrypt suspend image' for? 275 276 A: First of all: it is not a replacement for dm-crypt encrypted swap. 277 It cannot protect your computer while it is suspended. Instead it does 278 protect from leaking sensitive data after resume from suspend. 279 280 Think of the following: you suspend while an application is running 281 that keeps sensitive data in memory. The application itself prevents 282 the data from being swapped out. Suspend, however, must write these 283 data to swap to be able to resume later on. Without suspend encryption 284 your sensitive data are then stored in plaintext on disk. This means 285 that after resume your sensitive data are accessible to all 286 applications having direct access to the swap device which was used 287 for suspend. If you don't need swap after resume these data can remain 288 on disk virtually forever. Thus it can happen that your system gets 289 broken in weeks later and sensitive data which you thought were 290 encrypted and protected are retrieved and stolen from the swap device. 291 To prevent this situation you should use 'Encrypt suspend image'. 292 293 During suspend a temporary key is created and this key is used to 294 encrypt the data written to disk. When, during resume, the data was 295 read back into memory the temporary key is destroyed which simply 296 means that all data written to disk during suspend are then 297 inaccessible so they can't be stolen later on. The only thing that 298 you must then take care of is that you call 'mkswap' for the swap 299 partition used for suspend as early as possible during regular 300 boot. This asserts that any temporary key from an oopsed suspend or 301 from a failed or aborted resume is erased from the swap device. 302 303 As a rule of thumb use encrypted swap to protect your data while your 304 system is shut down or suspended. Additionally use the encrypted 305 suspend image to prevent sensitive data from being stolen after 306 resume. 307 308 Q: Can I suspend to a swap file? 309 310 A: 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 312 cannot be initiated from an initrd or initramfs image. See 313 swsusp-and-swap-files.txt for details. 314 315 Q: Is there a maximum system RAM size that is supported by swsusp? 316 317 A: It should work okay with highmem. 318 319 Q: Does swsusp (to disk) use only one swap partition or can it use 320 multiple swap partitions (aggregate them into one logical space)? 321 322 A: Only one swap partition, sorry. 323 324 Q: 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 326 to be useless to try to suspend to disk while that app is running? 327 328 A: No, it should work okay, as long as your app does not mlock() 329 it. Just prepare big enough swap partition. 330 331 Q: What information is useful for debugging suspend-to-disk problems? 332 333 A: Well, last messages on the screen are always useful. If something 334 is broken, it is usually some kernel driver, therefore trying with as 335 little as possible modules loaded helps a lot. I also prefer people to 336 suspend from console, preferably without X running. Booting with 337 init=/bin/bash, then swapon and starting suspend sequence manually 338 usually does the trick. Then it is good idea to try with latest 339 vanilla kernel. 340 341 Q: How can distributions ship a swsusp-supporting kernel with modular 342 disk drivers (especially SATA)? 343 344 A: Well, it can be done, load the drivers, then do echo into 345 /sys/power/resume file from initrd. Be sure not to mount 346 anything, not even read-only mount, or you are going to lose your 347 data. 348 349 Q: How do I make suspend more verbose? 350 351 A: If you want to see any non-error kernel messages on the virtual 352 terminal the kernel switches to during suspend, you have to set the 353 kernel console loglevel to at least 4 (KERN_WARNING), for example by 354 doing 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 381 382 Q: Is this true that if I have a mounted filesystem on a USB device and 383 I suspend to disk, I can lose data unless the filesystem has been mounted 384 with "sync"? 385 386 A: That's right ... if you disconnect that device, you may lose data. 387 In fact, even with "-o sync" you can lose data if your programs have 388 information in buffers they haven't written out to a disk you disconnect, 389 or if you disconnect before the device finished saving data you wrote. 390 391 Software suspend normally powers down USB controllers, which is equivalent 392 to disconnecting all USB devices attached to your system. 393 394 Your system might well support low-power modes for its USB controllers 395 while the system is asleep, maintaining the connection, using true sleep 396 modes 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 398 hardware that can use these modes through software suspend, although in 399 theory some systems might support "platform" modes that won't break the 400 USB connections. 401 402 Remember that it's always a bad idea to unplug a disk drive containing a 403 mounted filesystem. That's true even when your system is asleep! The 404 safest thing is to unmount all filesystems on removable media (such USB, 405 Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays) 406 before suspending; then remount them after resuming. 407 408 There is a work-around for this problem. For more information, see 409 Documentation/usb/persist.txt. 410 411 Q: Can I suspend-to-disk using a swap partition under LVM? 412 413 A: No. You can suspend successfully, but you'll not be able to 414 resume. uswsusp should be able to work with LVM. See suspend.sf.net. 415 416 Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were 417 compiled with the similar configuration files. Anyway I found that 418 suspend to disk (and resume) is much slower on 2.6.16 compared to 419 2.6.15. Any idea for why that might happen or how can I speed it up? 420 421 A: This is because the size of the suspend image is now greater than 422 for 2.6.15 (by saving more data we can get more responsive system 423 after resume). 424 425 There's the /sys/power/image_size knob that controls the size of the 426 image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as 427 root), the 2.6.15 behavior should be restored. If it is still too 428 slow, take a look at suspend.sf.net -- userland suspend is faster and 429 supports LZF compression to speed it up further.