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Documentation / power / swsusp-and-swap-files.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	Using swap files with software suspend (swsusp)
2		(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
3	
4	The Linux kernel handles swap files almost in the same way as it handles swap
5	partitions and there are only two differences between these two types of swap
6	areas:
7	(1) swap files need not be contiguous,
8	(2) the header of a swap file is not in the first block of the partition that
9	holds it.  From the swsusp's point of view (1) is not a problem, because it is
10	already taken care of by the swap-handling code, but (2) has to be taken into
11	consideration.
12	
13	In principle the location of a swap file's header may be determined with the
14	help of appropriate filesystem driver.  Unfortunately, however, it requires the
15	filesystem holding the swap file to be mounted, and if this filesystem is
16	journaled, it cannot be mounted during resume from disk.  For this reason to
17	identify a swap file swsusp uses the name of the partition that holds the file
18	and the offset from the beginning of the partition at which the swap file's
19	header is located.  For convenience, this offset is expressed in <PAGE_SIZE>
20	units.
21	
22	In order to use a swap file with swsusp, you need to:
23	
24	1) Create the swap file and make it active, eg.
25	
26	# dd if=/dev/zero of=<swap_file_path> bs=1024 count=<swap_file_size_in_k>
27	# mkswap <swap_file_path>
28	# swapon <swap_file_path>
29	
30	2) Use an application that will bmap the swap file with the help of the
31	FIBMAP ioctl and determine the location of the file's swap header, as the
32	offset, in <PAGE_SIZE> units, from the beginning of the partition which
33	holds the swap file.
34	
35	3) Add the following parameters to the kernel command line:
36	
37	resume=<swap_file_partition> resume_offset=<swap_file_offset>
38	
39	where <swap_file_partition> is the partition on which the swap file is located
40	and <swap_file_offset> is the offset of the swap header determined by the
41	application in 2) (of course, this step may be carried out automatically
42	by the same application that determines the swap file's header offset using the
43	FIBMAP ioctl)
44	
45	OR
46	
47	Use a userland suspend application that will set the partition and offset
48	with the help of the SNAPSHOT_SET_SWAP_AREA ioctl described in
49	Documentation/power/userland-swsusp.txt (this is the only method to suspend
50	to a swap file allowing the resume to be initiated from an initrd or initramfs
51	image).
52	
53	Now, swsusp will use the swap file in the same way in which it would use a swap
54	partition.  In particular, the swap file has to be active (ie. be present in
55	/proc/swaps) so that it can be used for suspending.
56	
57	Note that if the swap file used for suspending is deleted and recreated,
58	the location of its header need not be the same as before.  Thus every time
59	this happens the value of the "resume_offset=" kernel command line parameter
60	has to be updated.
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