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 have SATA disks, you'll need recent kernels with SATA suspend 37support. For suspend and resume to work, make sure your disk drivers 38are built into kernel -- not modules. [There's way to make 39suspend/resume with modular disk drivers, see FAQ, but you probably 40should not do that.] 41 42If you want to limit the suspend image size to N bytes, do 43 44echo N > /sys/power/image_size 45 46before suspend (it is limited to 500 MB by default). 47 48 49Article about goals and implementation of Software Suspend for Linux 50~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 51Author: G‚ábor Kuti 52Last revised: 2003-10-20 by Pavel Machek 53 54Idea and goals to achieve 55 56Nowadays it is common in several laptops that they have a suspend button. It 57saves the state of the machine to a filesystem or to a partition and switches 58to standby mode. Later resuming the machine the saved state is loaded back to 59ram and the machine can continue its work. It has two real benefits. First we 60save ourselves the time machine goes down and later boots up, energy costs 61are real high when running from batteries. The other gain is that we don't have to 62interrupt our programs so processes that are calculating something for a long 63time shouldn't need to be written interruptible. 64 65swsusp saves the state of the machine into active swaps and then reboots or 66powerdowns. You must explicitly specify the swap partition to resume from with 67``resume='' kernel option. If signature is found it loads and restores saved 68state. If the option ``noresume'' is specified as a boot parameter, it skips 69the resuming. If the option ``hibernate=nocompress'' is specified as a boot 70parameter, it saves hibernation image without compression. 71 72In the meantime while the system is suspended you should not add/remove any 73of the hardware, write to the filesystems, etc. 74 75Sleep states summary 76==================== 77 78There are three different interfaces you can use, /proc/acpi should 79work like this: 80 81In a really perfect world: 82echo 1 > /proc/acpi/sleep # for standby 83echo 2 > /proc/acpi/sleep # for suspend to ram 84echo 3 > /proc/acpi/sleep # for suspend to ram, but with more power conservative 85echo 4 > /proc/acpi/sleep # for suspend to disk 86echo 5 > /proc/acpi/sleep # for shutdown unfriendly the system 87 88and perhaps 89echo 4b > /proc/acpi/sleep # for suspend to disk via s4bios 90 91Frequently Asked Questions 92========================== 93 94Q: well, suspending a server is IMHO a really stupid thing, 95but... (Diego Zuccato): 96 97A: You bought new UPS for your server. How do you install it without 98bringing machine down? Suspend to disk, rearrange power cables, 99resume. 100 101You have your server on UPS. Power died, and UPS is indicating 30 102seconds to failure. What do you do? Suspend to disk. 103 104 105Q: Maybe I'm missing something, but why don't the regular I/O paths work? 106 107A: We do use the regular I/O paths. However we cannot restore the data 108to its original location as we load it. That would create an 109inconsistent kernel state which would certainly result in an oops. 110Instead, we load the image into unused memory and then atomically copy 111it back to it original location. This implies, of course, a maximum 112image size of half the amount of memory. 113 114There are two solutions to this: 115 116* require half of memory to be free during suspend. That way you can 117read "new" data onto free spots, then cli and copy 118 119* assume we had special "polling" ide driver that only uses memory 120between 0-640KB. That way, I'd have to make sure that 0-640KB is free 121during suspending, but otherwise it would work... 122 123suspend2 shares this fundamental limitation, but does not include user 124data and disk caches into "used memory" by saving them in 125advance. That means that the limitation goes away in practice. 126 127Q: Does linux support ACPI S4? 128 129A: Yes. That's what echo platform > /sys/power/disk does. 130 131Q: What is 'suspend2'? 132 133A: suspend2 is 'Software Suspend 2', a forked implementation of 134suspend-to-disk which is available as separate patches for 2.4 and 2.6 135kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB 136highmem and preemption. It also has a extensible architecture that 137allows for arbitrary transformations on the image (compression, 138encryption) and arbitrary backends for writing the image (eg to swap 139or an NFS share[Work In Progress]). Questions regarding suspend2 140should be sent to the mailing list available through the suspend2 141website, and not to the Linux Kernel Mailing List. We are working 142toward merging suspend2 into the mainline kernel. 143 144Q: What is the freezing of tasks and why are we using it? 145 146A: The freezing of tasks is a mechanism by which user space processes and some 147kernel threads are controlled during hibernation or system-wide suspend (on some 148architectures). See freezing-of-tasks.txt for details. 149 150Q: What is the difference between "platform" and "shutdown"? 151 152A: 153 154shutdown: save state in linux, then tell bios to powerdown 155 156platform: save state in linux, then tell bios to powerdown and blink 157 "suspended led" 158 159"platform" is actually right thing to do where supported, but 160"shutdown" is most reliable (except on ACPI systems). 161 162Q: I do not understand why you have such strong objections to idea of 163selective suspend. 164 165A: Do selective suspend during runtime power management, that's okay. But 166it's useless for suspend-to-disk. (And I do not see how you could use 167it for suspend-to-ram, I hope you do not want that). 168 169Lets see, so you suggest to 170 171* SUSPEND all but swap device and parents 172* Snapshot 173* Write image to disk 174* SUSPEND swap device and parents 175* Powerdown 176 177Oh no, that does not work, if swap device or its parents uses DMA, 178you've corrupted data. You'd have to do 179 180* SUSPEND all but swap device and parents 181* FREEZE swap device and parents 182* Snapshot 183* UNFREEZE swap device and parents 184* Write 185* SUSPEND swap device and parents 186 187Which means that you still need that FREEZE state, and you get more 188complicated code. (And I have not yet introduce details like system 189devices). 190 191Q: There don't seem to be any generally useful behavioral 192distinctions between SUSPEND and FREEZE. 193 194A: Doing SUSPEND when you are asked to do FREEZE is always correct, 195but it may be unnecessarily slow. If you want your driver to stay simple, 196slowness may not matter to you. It can always be fixed later. 197 198For devices like disk it does matter, you do not want to spindown for 199FREEZE. 200 201Q: After resuming, system is paging heavily, leading to very bad interactivity. 202 203A: Try running 204 205cat `cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u` > /dev/null 206 207after resume. swapoff -a; swapon -a may also be useful. 208 209Q: What happens to devices during swsusp? They seem to be resumed 210during system suspend? 211 212A: That's correct. We need to resume them if we want to write image to 213disk. Whole sequence goes like 214 215 Suspend part 216 ~~~~~~~~~~~~ 217 running system, user asks for suspend-to-disk 218 219 user processes are stopped 220 221 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 222 with state snapshot 223 224 state snapshot: copy of whole used memory is taken with interrupts disabled 225 226 resume(): devices are woken up so that we can write image to swap 227 228 write image to swap 229 230 suspend(PMSG_SUSPEND): suspend devices so that we can power off 231 232 turn the power off 233 234 Resume part 235 ~~~~~~~~~~~ 236 (is actually pretty similar) 237 238 running system, user asks for suspend-to-disk 239 240 user processes are stopped (in common case there are none, but with resume-from-initrd, no one knows) 241 242 read image from disk 243 244 suspend(PMSG_FREEZE): devices are frozen so that they don't interfere 245 with image restoration 246 247 image restoration: rewrite memory with image 248 249 resume(): devices are woken up so that system can continue 250 251 thaw all user processes 252 253Q: What is this 'Encrypt suspend image' for? 254 255A: First of all: it is not a replacement for dm-crypt encrypted swap. 256It cannot protect your computer while it is suspended. Instead it does 257protect from leaking sensitive data after resume from suspend. 258 259Think of the following: you suspend while an application is running 260that keeps sensitive data in memory. The application itself prevents 261the data from being swapped out. Suspend, however, must write these 262data to swap to be able to resume later on. Without suspend encryption 263your sensitive data are then stored in plaintext on disk. This means 264that after resume your sensitive data are accessible to all 265applications having direct access to the swap device which was used 266for suspend. If you don't need swap after resume these data can remain 267on disk virtually forever. Thus it can happen that your system gets 268broken in weeks later and sensitive data which you thought were 269encrypted and protected are retrieved and stolen from the swap device. 270To prevent this situation you should use 'Encrypt suspend image'. 271 272During suspend a temporary key is created and this key is used to 273encrypt the data written to disk. When, during resume, the data was 274read back into memory the temporary key is destroyed which simply 275means that all data written to disk during suspend are then 276inaccessible so they can't be stolen later on. The only thing that 277you must then take care of is that you call 'mkswap' for the swap 278partition used for suspend as early as possible during regular 279boot. This asserts that any temporary key from an oopsed suspend or 280from a failed or aborted resume is erased from the swap device. 281 282As a rule of thumb use encrypted swap to protect your data while your 283system is shut down or suspended. Additionally use the encrypted 284suspend image to prevent sensitive data from being stolen after 285resume. 286 287Q: Can I suspend to a swap file? 288 289A: Generally, yes, you can. However, it requires you to use the "resume=" and 290"resume_offset=" kernel command line parameters, so the resume from a swap file 291cannot be initiated from an initrd or initramfs image. See 292swsusp-and-swap-files.txt for details. 293 294Q: Is there a maximum system RAM size that is supported by swsusp? 295 296A: It should work okay with highmem. 297 298Q: Does swsusp (to disk) use only one swap partition or can it use 299multiple swap partitions (aggregate them into one logical space)? 300 301A: Only one swap partition, sorry. 302 303Q: If my application(s) causes lots of memory & swap space to be used 304(over half of the total system RAM), is it correct that it is likely 305to be useless to try to suspend to disk while that app is running? 306 307A: No, it should work okay, as long as your app does not mlock() 308it. Just prepare big enough swap partition. 309 310Q: What information is useful for debugging suspend-to-disk problems? 311 312A: Well, last messages on the screen are always useful. If something 313is broken, it is usually some kernel driver, therefore trying with as 314little as possible modules loaded helps a lot. I also prefer people to 315suspend from console, preferably without X running. Booting with 316init=/bin/bash, then swapon and starting suspend sequence manually 317usually does the trick. Then it is good idea to try with latest 318vanilla kernel. 319 320Q: How can distributions ship a swsusp-supporting kernel with modular 321disk drivers (especially SATA)? 322 323A: Well, it can be done, load the drivers, then do echo into 324/sys/power/disk/resume file from initrd. Be sure not to mount 325anything, not even read-only mount, or you are going to lose your 326data. 327 328Q: How do I make suspend more verbose? 329 330A: If you want to see any non-error kernel messages on the virtual 331terminal the kernel switches to during suspend, you have to set the 332kernel console loglevel to at least 4 (KERN_WARNING), for example by 333doing 334 335 # save the old loglevel 336 read LOGLEVEL DUMMY < /proc/sys/kernel/printk 337 # set the loglevel so we see the progress bar. 338 # if the level is higher than needed, we leave it alone. 339 if [ $LOGLEVEL -lt 5 ]; then 340 echo 5 > /proc/sys/kernel/printk 341 fi 342 343 IMG_SZ=0 344 read IMG_SZ < /sys/power/image_size 345 echo -n disk > /sys/power/state 346 RET=$? 347 # 348 # the logic here is: 349 # if image_size > 0 (without kernel support, IMG_SZ will be zero), 350 # then try again with image_size set to zero. 351 if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size 352 echo 0 > /sys/power/image_size 353 echo -n disk > /sys/power/state 354 RET=$? 355 fi 356 357 # restore previous loglevel 358 echo $LOGLEVEL > /proc/sys/kernel/printk 359 exit $RET 360 361Q: Is this true that if I have a mounted filesystem on a USB device and 362I suspend to disk, I can lose data unless the filesystem has been mounted 363with "sync"? 364 365A: That's right ... if you disconnect that device, you may lose data. 366In fact, even with "-o sync" you can lose data if your programs have 367information in buffers they haven't written out to a disk you disconnect, 368or if you disconnect before the device finished saving data you wrote. 369 370Software suspend normally powers down USB controllers, which is equivalent 371to disconnecting all USB devices attached to your system. 372 373Your system might well support low-power modes for its USB controllers 374while the system is asleep, maintaining the connection, using true sleep 375modes like "suspend-to-RAM" or "standby". (Don't write "disk" to the 376/sys/power/state file; write "standby" or "mem".) We've not seen any 377hardware that can use these modes through software suspend, although in 378theory some systems might support "platform" modes that won't break the 379USB connections. 380 381Remember that it's always a bad idea to unplug a disk drive containing a 382mounted filesystem. That's true even when your system is asleep! The 383safest thing is to unmount all filesystems on removable media (such USB, 384Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays) 385before suspending; then remount them after resuming. 386 387There is a work-around for this problem. For more information, see 388Documentation/usb/persist.txt. 389 390Q: Can I suspend-to-disk using a swap partition under LVM? 391 392A: No. You can suspend successfully, but you'll not be able to 393resume. uswsusp should be able to work with LVM. See suspend.sf.net. 394 395Q: I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were 396compiled with the similar configuration files. Anyway I found that 397suspend to disk (and resume) is much slower on 2.6.16 compared to 3982.6.15. Any idea for why that might happen or how can I speed it up? 399 400A: This is because the size of the suspend image is now greater than 401for 2.6.15 (by saving more data we can get more responsive system 402after resume). 403 404There's the /sys/power/image_size knob that controls the size of the 405image. If you set it to 0 (eg. by echo 0 > /sys/power/image_size as 406root), the 2.6.15 behavior should be restored. If it is still too 407slow, take a look at suspend.sf.net -- userland suspend is faster and 408supports LZF compression to speed it up further. 409