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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: Gábor 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.