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
122                                  # conservative
123  echo 4 > /proc/acpi/sleep       # for suspend to disk
124  echo 5 > /proc/acpi/sleep       # for shutdown unfriendly the system
125
126and perhaps::
127
128  echo 4b > /proc/acpi/sleep      # for suspend to disk via s4bios
129
130Frequently Asked Questions
131==========================
132
133Q:
134  well, suspending a server is IMHO a really stupid thing,
135  but... (Diego Zuccato):
136
137A:
138  You bought new UPS for your server. How do you install it without
139  bringing machine down? Suspend to disk, rearrange power cables,
140  resume.
141
142  You have your server on UPS. Power died, and UPS is indicating 30
143  seconds to failure. What do you do? Suspend to disk.
144
145
146Q:
147  Maybe I'm missing something, but why don't the regular I/O paths work?
148
149A:
150  We do use the regular I/O paths. However we cannot restore the data
151  to its original location as we load it. That would create an
152  inconsistent kernel state which would certainly result in an oops.
153  Instead, we load the image into unused memory and then atomically copy
154  it back to it original location. This implies, of course, a maximum
155  image size of half the amount of memory.
156
157  There are two solutions to this:
158
159  * require half of memory to be free during suspend. That way you can
160    read "new" data onto free spots, then cli and copy
161
162  * assume we had special "polling" ide driver that only uses memory
163    between 0-640KB. That way, I'd have to make sure that 0-640KB is free
164    during suspending, but otherwise it would work...
165
166  suspend2 shares this fundamental limitation, but does not include user
167  data and disk caches into "used memory" by saving them in
168  advance. That means that the limitation goes away in practice.
169
170Q:
171  Does linux support ACPI S4?
172
173A:
174  Yes. That's what echo platform > /sys/power/disk does.
175
176Q:
177  What is 'suspend2'?
178
179A:
180  suspend2 is 'Software Suspend 2', a forked implementation of
181  suspend-to-disk which is available as separate patches for 2.4 and 2.6
182  kernels from swsusp.sourceforge.net. It includes support for SMP, 4GB
183  highmem and preemption. It also has a extensible architecture that
184  allows for arbitrary transformations on the image (compression,
185  encryption) and arbitrary backends for writing the image (eg to swap
186  or an NFS share[Work In Progress]). Questions regarding suspend2
187  should be sent to the mailing list available through the suspend2
188  website, and not to the Linux Kernel Mailing List. We are working
189  toward merging suspend2 into the mainline kernel.
190
191Q:
192  What is the freezing of tasks and why are we using it?
193
194A:
195  The freezing of tasks is a mechanism by which user space processes and some
196  kernel threads are controlled during hibernation or system-wide suspend (on
197  some architectures).  See freezing-of-tasks.txt for details.
198
199Q:
200  What is the difference between "platform" and "shutdown"?
201
202A:
203  shutdown:
204	save state in linux, then tell bios to powerdown
205
206  platform:
207	save state in linux, then tell bios to powerdown and blink
208        "suspended led"
209
210  "platform" is actually right thing to do where supported, but
211  "shutdown" is most reliable (except on ACPI systems).
212
213Q:
214  I do not understand why you have such strong objections to idea of
215  selective suspend.
216
217A:
218  Do selective suspend during runtime power management, that's okay. But
219  it's useless for suspend-to-disk. (And I do not see how you could use
220  it for suspend-to-ram, I hope you do not want that).
221
222  Lets see, so you suggest to
223
224  * SUSPEND all but swap device and parents
225  * Snapshot
226  * Write image to disk
227  * SUSPEND swap device and parents
228  * Powerdown
229
230  Oh no, that does not work, if swap device or its parents uses DMA,
231  you've corrupted data. You'd have to do
232
233  * SUSPEND all but swap device and parents
234  * FREEZE swap device and parents
235  * Snapshot
236  * UNFREEZE swap device and parents
237  * Write
238  * SUSPEND swap device and parents
239
240  Which means that you still need that FREEZE state, and you get more
241  complicated code. (And I have not yet introduce details like system
242  devices).
243
244Q:
245  There don't seem to be any generally useful behavioral
246  distinctions between SUSPEND and FREEZE.
247
248A:
249  Doing SUSPEND when you are asked to do FREEZE is always correct,
250  but it may be unnecessarily slow. If you want your driver to stay simple,
251  slowness may not matter to you. It can always be fixed later.
252
253  For devices like disk it does matter, you do not want to spindown for
254  FREEZE.
255
256Q:
257  After resuming, system is paging heavily, leading to very bad interactivity.
258
259A:
260  Try running::
261
262    cat /proc/[0-9]*/maps | grep / | sed 's:.* /:/:' | sort -u | while read file
263    do
264      test -f "$file" && cat "$file" > /dev/null
265    done
266
267  after resume. swapoff -a; swapon -a may also be useful.
268
269Q:
270  What happens to devices during swsusp? They seem to be resumed
271  during system suspend?
272
273A:
274  That's correct. We need to resume them if we want to write image to
275  disk. Whole sequence goes like
276
277      **Suspend part**
278
279      running system, user asks for suspend-to-disk
280
281      user processes are stopped
282
283      suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
284      with state snapshot
285
286      state snapshot: copy of whole used memory is taken with interrupts
287      disabled
288
289      resume(): devices are woken up so that we can write image to swap
290
291      write image to swap
292
293      suspend(PMSG_SUSPEND): suspend devices so that we can power off
294
295      turn the power off
296
297      **Resume part**
298
299      (is actually pretty similar)
300
301      running system, user asks for suspend-to-disk
302
303      user processes are stopped (in common case there are none,
304      but with resume-from-initrd, no one knows)
305
306      read image from disk
307
308      suspend(PMSG_FREEZE): devices are frozen so that they don't interfere
309      with image restoration
310
311      image restoration: rewrite memory with image
312
313      resume(): devices are woken up so that system can continue
314
315      thaw all user processes
316
317Q:
318  What is this 'Encrypt suspend image' for?
319
320A:
321  First of all: it is not a replacement for dm-crypt encrypted swap.
322  It cannot protect your computer while it is suspended. Instead it does
323  protect from leaking sensitive data after resume from suspend.
324
325  Think of the following: you suspend while an application is running
326  that keeps sensitive data in memory. The application itself prevents
327  the data from being swapped out. Suspend, however, must write these
328  data to swap to be able to resume later on. Without suspend encryption
329  your sensitive data are then stored in plaintext on disk.  This means
330  that after resume your sensitive data are accessible to all
331  applications having direct access to the swap device which was used
332  for suspend. If you don't need swap after resume these data can remain
333  on disk virtually forever. Thus it can happen that your system gets
334  broken in weeks later and sensitive data which you thought were
335  encrypted and protected are retrieved and stolen from the swap device.
336  To prevent this situation you should use 'Encrypt suspend image'.
337
338  During suspend a temporary key is created and this key is used to
339  encrypt the data written to disk. When, during resume, the data was
340  read back into memory the temporary key is destroyed which simply
341  means that all data written to disk during suspend are then
342  inaccessible so they can't be stolen later on.  The only thing that
343  you must then take care of is that you call 'mkswap' for the swap
344  partition used for suspend as early as possible during regular
345  boot. This asserts that any temporary key from an oopsed suspend or
346  from a failed or aborted resume is erased from the swap device.
347
348  As a rule of thumb use encrypted swap to protect your data while your
349  system is shut down or suspended. Additionally use the encrypted
350  suspend image to prevent sensitive data from being stolen after
351  resume.
352
353Q:
354  Can I suspend to a swap file?
355
356A:
357  Generally, yes, you can.  However, it requires you to use the "resume=" and
358  "resume_offset=" kernel command line parameters, so the resume from a swap
359  file cannot be initiated from an initrd or initramfs image.  See
360  swsusp-and-swap-files.txt for details.
361
362Q:
363  Is there a maximum system RAM size that is supported by swsusp?
364
365A:
366  It should work okay with highmem.
367
368Q:
369  Does swsusp (to disk) use only one swap partition or can it use
370  multiple swap partitions (aggregate them into one logical space)?
371
372A:
373  Only one swap partition, sorry.
374
375Q:
376  If my application(s) causes lots of memory & swap space to be used
377  (over half of the total system RAM), is it correct that it is likely
378  to be useless to try to suspend to disk while that app is running?
379
380A:
381  No, it should work okay, as long as your app does not mlock()
382  it. Just prepare big enough swap partition.
383
384Q:
385  What information is useful for debugging suspend-to-disk problems?
386
387A:
388  Well, last messages on the screen are always useful. If something
389  is broken, it is usually some kernel driver, therefore trying with as
390  little as possible modules loaded helps a lot. I also prefer people to
391  suspend from console, preferably without X running. Booting with
392  init=/bin/bash, then swapon and starting suspend sequence manually
393  usually does the trick. Then it is good idea to try with latest
394  vanilla kernel.
395
396Q:
397  How can distributions ship a swsusp-supporting kernel with modular
398  disk drivers (especially SATA)?
399
400A:
401  Well, it can be done, load the drivers, then do echo into
402  /sys/power/resume file from initrd. Be sure not to mount
403  anything, not even read-only mount, or you are going to lose your
404  data.
405
406Q:
407  How do I make suspend more verbose?
408
409A:
410  If you want to see any non-error kernel messages on the virtual
411  terminal the kernel switches to during suspend, you have to set the
412  kernel console loglevel to at least 4 (KERN_WARNING), for example by
413  doing::
414
415	# save the old loglevel
416	read LOGLEVEL DUMMY < /proc/sys/kernel/printk
417	# set the loglevel so we see the progress bar.
418	# if the level is higher than needed, we leave it alone.
419	if [ $LOGLEVEL -lt 5 ]; then
420	        echo 5 > /proc/sys/kernel/printk
421		fi
422
423        IMG_SZ=0
424        read IMG_SZ < /sys/power/image_size
425        echo -n disk > /sys/power/state
426        RET=$?
427        #
428        # the logic here is:
429        # if image_size > 0 (without kernel support, IMG_SZ will be zero),
430        # then try again with image_size set to zero.
431	if [ $RET -ne 0 -a $IMG_SZ -ne 0 ]; then # try again with minimal image size
432                echo 0 > /sys/power/image_size
433                echo -n disk > /sys/power/state
434                RET=$?
435        fi
436
437	# restore previous loglevel
438	echo $LOGLEVEL > /proc/sys/kernel/printk
439	exit $RET
440
441Q:
442  Is this true that if I have a mounted filesystem on a USB device and
443  I suspend to disk, I can lose data unless the filesystem has been mounted
444  with "sync"?
445
446A:
447  That's right ... if you disconnect that device, you may lose data.
448  In fact, even with "-o sync" you can lose data if your programs have
449  information in buffers they haven't written out to a disk you disconnect,
450  or if you disconnect before the device finished saving data you wrote.
451
452  Software suspend normally powers down USB controllers, which is equivalent
453  to disconnecting all USB devices attached to your system.
454
455  Your system might well support low-power modes for its USB controllers
456  while the system is asleep, maintaining the connection, using true sleep
457  modes like "suspend-to-RAM" or "standby".  (Don't write "disk" to the
458  /sys/power/state file; write "standby" or "mem".)  We've not seen any
459  hardware that can use these modes through software suspend, although in
460  theory some systems might support "platform" modes that won't break the
461  USB connections.
462
463  Remember that it's always a bad idea to unplug a disk drive containing a
464  mounted filesystem.  That's true even when your system is asleep!  The
465  safest thing is to unmount all filesystems on removable media (such USB,
466  Firewire, CompactFlash, MMC, external SATA, or even IDE hotplug bays)
467  before suspending; then remount them after resuming.
468
469  There is a work-around for this problem.  For more information, see
470  Documentation/driver-api/usb/persist.rst.
471
472Q:
473  Can I suspend-to-disk using a swap partition under LVM?
474
475A:
476  Yes and No.  You can suspend successfully, but the kernel will not be able
477  to resume on its own.  You need an initramfs that can recognize the resume
478  situation, activate the logical volume containing the swap volume (but not
479  touch any filesystems!), and eventually call::
480
481    echo -n "$major:$minor" > /sys/power/resume
482
483  where $major and $minor are the respective major and minor device numbers of
484  the swap volume.
485
486  uswsusp works with LVM, too.  See http://suspend.sourceforge.net/
487
488Q:
489  I upgraded the kernel from 2.6.15 to 2.6.16. Both kernels were
490  compiled with the similar configuration files. Anyway I found that
491  suspend to disk (and resume) is much slower on 2.6.16 compared to
492  2.6.15. Any idea for why that might happen or how can I speed it up?
493
494A:
495  This is because the size of the suspend image is now greater than
496  for 2.6.15 (by saving more data we can get more responsive system
497  after resume).
498
499  There's the /sys/power/image_size knob that controls the size of the
500  image.  If you set it to 0 (eg. by echo 0 > /sys/power/image_size as
501  root), the 2.6.15 behavior should be restored.  If it is still too
502  slow, take a look at suspend.sf.net -- userland suspend is faster and
503  supports LZF compression to speed it up further.
504