Based on kernel version 3.9. Page generated on 2013-05-02 23:12 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 54 Article about goals and implementation of Software Suspend for Linux 55 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 56 Author: GÃ¡bor Kuti 57 Last revised: 2003-10-20 by Pavel Machek 58 59 Idea and goals to achieve 60 61 Nowadays it is common in several laptops that they have a suspend button. It 62 saves the state of the machine to a filesystem or to a partition and switches 63 to standby mode. Later resuming the machine the saved state is loaded back to 64 ram and the machine can continue its work. It has two real benefits. First we 65 save ourselves the time machine goes down and later boots up, energy costs 66 are real high when running from batteries. The other gain is that we don't have to 67 interrupt our programs so processes that are calculating something for a long 68 time shouldn't need to be written interruptible. 69 70 swsusp saves the state of the machine into active swaps and then reboots or 71 powerdowns. You must explicitly specify the swap partition to resume from with 72 ``resume='' kernel option. If signature is found it loads and restores saved 73 state. If the option ``noresume'' is specified as a boot parameter, it skips 74 the resuming. If the option ``hibernate=nocompress'' is specified as a boot 75 parameter, it saves hibernation image without compression. 76 77 In the meantime while the system is suspended you should not add/remove any 78 of the hardware, write to the filesystems, etc. 79 80 Sleep states summary 81 ==================== 82 83 There are three different interfaces you can use, /proc/acpi should 84 work like this: 85 86 In a really perfect world: 87 echo 1 > /proc/acpi/sleep # for standby 88 echo 2 > /proc/acpi/sleep # for suspend to ram 89 echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative 90 echo 4 > /proc/acpi/sleep # for suspend to disk 91 echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system 92 93 and perhaps 94 echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios 95 96 Frequently Asked Questions 97 ========================== 98 99 Q: well, suspending a server is IMHO a really stupid thing, 100 but... (Diego Zuccato): 101 102 A: You bought new UPS for your server. How do you install it without 103 bringing machine down? Suspend to disk, rearrange power cables, 104 resume. 105 106 You have your server on UPS. Power died, and UPS is indicating 30 107 seconds to failure. What do you do? Suspend to disk. 108 109 110 Q: Maybe I'm missing something, but why don't the regular I/O paths work? 111 112 A: We do use the regular I/O paths. However we cannot restore the data 113 to its original location as we load it. That would create an 114 inconsistent kernel state which would certainly result in an oops. 115 Instead, we load the image into unused memory and then atomically copy 116 it back to it original location. This implies, of course, a maximum 117 image size of half the amount of memory. 118 119 There are two solutions to this: 120 121 * require half of memory to be free during suspend. That way you can 122 read "new" data onto free spots, then cli and copy 123 124 * assume we had special "polling" ide driver that only uses memory 125 between 0-640KB. That way, I'd have to make sure that 0-640KB is free 126 during suspending, but otherwise it would work... 127 128 suspend2 shares this fundamental limitation, but does not include user 129 data and disk caches into "used memory" by saving them in 130 advance. That means that the limitation goes away in practice. 131 132 Q: Does linux support ACPI S4? 133 134 A: Yes. That's what echo platform > /sys/power/disk does. 135 136 Q: What is 'suspend2'? 137 138 A: suspend2 is 'Software Suspend 2', a forked implementation of 139 suspend-to-disk which is available as separate patches for 2.4 and 2.6 140 kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB 141 highmem and preemption. It also has a extensible architecture that 142 allows for arbitrary transformations on the image (compression, 143 encryption) and arbitrary backends for writing the image (eg to swap 144 or an NFS share[Work In Progress]). Questions regarding suspend2 145 should be sent to the mailing list available through the suspend2 146 website, and not to the Linux Kernel Mailing List. We are working 147 toward merging suspend2 into the mainline kernel. 148 149 Q: What is the freezing of tasks and why are we using it? 150 151 A: The freezing of tasks is a mechanism by which user space processes and some 152 kernel threads are controlled during hibernation or system-wide suspend (on some 153 architectures). See freezing-of-tasks.txt for details. 154 155 Q: What is the difference between "platform" and "shutdown"? 156 157 A: 158 159 shutdown: save state in linux, then tell bios to powerdown 160 161 platform: save state in linux, then tell bios to powerdown and blink 162 "suspended led" 163 164 "platform" is actually right thing to do where supported, but 165 "shutdown" is most reliable (except on ACPI systems). 166 167 Q: I do not understand why you have such strong objections to idea of 168 selective suspend. 169 170 A: Do selective suspend during runtime power management, that's okay. But 171 it's useless for suspend-to-disk. (And I do not see how you could use 172 it for suspend-to-ram, I hope you do not want that). 173 174 Lets see, so you suggest to 175 176 * SUSPEND all but swap device and parents 177 * Snapshot 178 * Write image to disk 179 * SUSPEND swap device and parents 180 * Powerdown 181 182 Oh no, that does not work, if swap device or its parents uses DMA, 183 you've corrupted data. You'd have to do 184 185 * SUSPEND all but swap device and parents 186 * FREEZE swap device and parents 187 * Snapshot 188 * UNFREEZE swap device and parents 189 * Write 190 * SUSPEND swap device and parents 191 192 Which means that you still need that FREEZE state, and you get more 193 complicated code. (And I have not yet introduce details like system 194 devices). 195 196 Q: There don't seem to be any generally useful behavioral 197 distinctions between SUSPEND and FREEZE. 198 199 A: Doing SUSPEND when you are asked to do FREEZE is always correct, 200 but it may be unnecessarily slow. If you want your driver to stay simple, 201 slowness may not matter to you. It can always be fixed later. 202 203 For devices like disk it does matter, you do not want to spindown for 204 FREEZE. 205 206 Q: After resuming, system is paging heavily, leading to very bad interactivity. 207 208 A: Try running 209 210 cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null 211 212 after resume. swapoff -a; swapon -a may also be useful. 213 214 Q: What happens to devices during swsusp? They seem to be resumed 215 during system suspend? 216 217 A: That's correct. We need to resume them if we want to write image to 218 disk. Whole sequence goes like 219 220 Suspend part 221 ~~~~~~~~~~~~ 222 running system, user asks for suspend-to-disk 223 224 user processes are stopped 225 226 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 227 with state snapshot 228 229 state snapshot: copy of whole used memory is taken with interrupts disabled 230 231 resume(): devices are woken up so that we can write image to swap 232 233 write image to swap 234 235 suspend(PMSG_SUSPEND): suspend devices so that we can power off 236 237 turn the power off 238 239 Resume part 240 ~~~~~~~~~~~ 241 (is actually pretty similar) 242 243 running system, user asks for suspend-to-disk 244 245 user processes are stopped (in common case there are none, but with resume-from-initrd, no one knows) 246 247 read image from disk 248 249 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 250 with image restoration 251 252 image restoration: rewrite memory with image 253 254 resume(): devices are woken up so that system can continue 255 256 thaw all user processes 257 258 Q: What is this 'Encrypt suspend image' for? 259 260 A: First of all: it is not a replacement for dm-crypt encrypted swap. 261 It cannot protect your computer while it is suspended. Instead it does 262 protect from leaking sensitive data after resume from suspend. 263 264 Think of the following: you suspend while an application is running 265 that keeps sensitive data in memory. The application itself prevents 266 the data from being swapped out. Suspend, however, must write these 267 data to swap to be able to resume later on. Without suspend encryption 268 your sensitive data are then stored in plaintext on disk. This means 269 that after resume your sensitive data are accessible to all 270 applications having direct access to the swap device which was used 271 for suspend. If you don't need swap after resume these data can remain 272 on disk virtually forever. Thus it can happen that your system gets 273 broken in weeks later and sensitive data which you thought were 274 encrypted and protected are retrieved and stolen from the swap device. 275 To prevent this situation you should use 'Encrypt suspend image'. 276 277 During suspend a temporary key is created and this key is used to 278 encrypt the data written to disk. When, during resume, the data was 279 read back into memory the temporary key is destroyed which simply 280 means that all data written to disk during suspend are then 281 inaccessible so they can't be stolen later on. The only thing that 282 you must then take care of is that you call 'mkswap' for the swap 283 partition used for suspend as early as possible during regular 284 boot. This asserts that any temporary key from an oopsed suspend or 285 from a failed or aborted resume is erased from the swap device. 286 287 As a rule of thumb use encrypted swap to protect your data while your 288 system is shut down or suspended. Additionally use the encrypted 289 suspend image to prevent sensitive data from being stolen after 290 resume. 291 292 Q: Can I suspend to a swap file? 293 294 A: Generally, yes, you can. However, it requires you to use the "resume=" and 295 "resume_offset=" kernel command line parameters, so the resume from a swap file 296 cannot be initiated from an initrd or initramfs image. See 297 swsusp-and-swap-files.txt for details. 298 299 Q: Is there a maximum system RAM size that is supported by swsusp? 300 301 A: It should work okay with highmem. 302 303 Q: Does swsusp (to disk) use only one swap partition or can it use 304 multiple swap partitions (aggregate them into one logical space)? 305 306 A: Only one swap partition, sorry. 307 308 Q: If my application(s) causes lots of memory & swap space to be used 309 (over half of the total system RAM), is it correct that it is likely 310 to be useless to try to suspend to disk while that app is running? 311 312 A: No, it should work okay, as long as your app does not mlock() 313 it. Just prepare big enough swap partition. 314 315 Q: What information is useful for debugging suspend-to-disk problems? 316 317 A: Well, last messages on the screen are always useful. If something 318 is broken, it is usually some kernel driver, therefore trying with as 319 little as possible modules loaded helps a lot. I also prefer people to 320 suspend from console, preferably without X running. Booting with 321 init=/bin/bash, then swapon and starting suspend sequence manually 322 usually does the trick. Then it is good idea to try with latest 323 vanilla kernel. 324 325 Q: How can distributions ship a swsusp-supporting kernel with modular 326 disk drivers (especially SATA)? 327 328 A: Well, it can be done, load the drivers, then do echo into 329 /sys/power/disk/resume file from initrd. Be sure not to mount 330 anything, not even read-only mount, or you are going to lose your 331 data. 332 333 Q: How do I make suspend more verbose? 334 335 A: If you want to see any non-error kernel messages on the virtual 336 terminal the kernel switches to during suspend, you have to set the 337 kernel console loglevel to at least 4 (KERN_WARNING), for example by 338 doing 339 340 # save the old loglevel 341 read LOGLEVEL DUMMY < /proc/sys/kernel/printk 342 # set the loglevel so we see the progress bar. 343 # if the level is higher than needed, we leave it alone. 344 if [ $LOGLEVEL -lt 5 ]; then 345 echo 5 > /proc/sys/kernel/printk 346 fi 347 348 IMG_SZ=0 349 read IMG_SZ < /sys/power/image_size 350 echo -n disk > /sys/power/state 351 RET=$? 352 # 353 # the logic here is: 354 # if image_size > 0 (without kernel support, IMG_SZ will be zero), 355 # then try again with image_size set to zero. 356 if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size 357 echo 0 > /sys/power/image_size 358 echo -n disk > /sys/power/state 359 RET=$? 360 fi 361 362 # restore previous loglevel 363 echo $LOGLEVEL > /proc/sys/kernel/printk 364 exit $RET 365 366 Q: Is this true that if I have a mounted filesystem on a USB device and 367 I suspend to disk, I can lose data unless the filesystem has been mounted 368 with "sync"? 369 370 A: That's right ... if you disconnect that device, you may lose data. 371 In fact, even with "-o sync" you can lose data if your programs have 372 information in buffers they haven't written out to a disk you disconnect, 373 or if you disconnect before the device finished saving data you wrote. 374 375 Software suspend normally powers down USB controllers, which is equivalent 376 to disconnecting all USB devices attached to your system. 377 378 Your system might well support low-power modes for its USB controllers 379 while the system is asleep, maintaining the connection, using true sleep 380 modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the 381 /sys/power/state file; write "standby" or "mem".) We've not seen any 382 hardware that can use these modes through software suspend, although in 383 theory some systems might support "platform" modes that won't break the 384 USB connections. 385 386 Remember that it's always a bad idea to unplug a disk drive containing a 387 mounted filesystem. That's true even when your system is asleep! The 388 safest thing is to unmount all filesystems on removable media (such USB, 389 Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays) 390 before suspending; then remount them after resuming. 391 392 There is a work-around for this problem. For more information, see 393 Documentation/usb/persist.txt. 394 395 Q: Can I suspend-to-disk using a swap partition under LVM? 396 397 A: No. You can suspend successfully, but you'll not be able to 398 resume. uswsusp should be able to work with LVM. See suspend.sf.net. 399 400 Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were 401 compiled with the similar configuration files. Anyway I found that 402 suspend to disk (and resume) is much slower on 2.6.16 compared to 403 2.6.15. Any idea for why that might happen or how can I speed it up? 404 405 A: This is because the size of the suspend image is now greater than 406 for 2.6.15 (by saving more data we can get more responsive system 407 after resume). 408 409 There's the /sys/power/image_size knob that controls the size of the 410 image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as 411 root), the 2.6.15 behavior should be restored. If it is still too 412 slow, take a look at suspend.sf.net -- userland suspend is faster and 413 supports LZF compression to speed it up further.