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ecbd0da1 | 2 | Using swap files with software suspend (swsusp) |
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3 | =============================================== |
4 | ||
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5 | (C) 2006 Rafael J. Wysocki <rjw@sisk.pl> |
6 | ||
7 | The Linux kernel handles swap files almost in the same way as it handles swap | |
8 | partitions and there are only two differences between these two types of swap | |
9 | areas: | |
10 | (1) swap files need not be contiguous, | |
11 | (2) the header of a swap file is not in the first block of the partition that | |
12 | holds it. From the swsusp's point of view (1) is not a problem, because it is | |
13 | already taken care of by the swap-handling code, but (2) has to be taken into | |
14 | consideration. | |
15 | ||
16 | In principle the location of a swap file's header may be determined with the | |
17 | help of appropriate filesystem driver. Unfortunately, however, it requires the | |
18 | filesystem holding the swap file to be mounted, and if this filesystem is | |
19 | journaled, it cannot be mounted during resume from disk. For this reason to | |
20 | identify a swap file swsusp uses the name of the partition that holds the file | |
21 | and the offset from the beginning of the partition at which the swap file's | |
22 | header is located. For convenience, this offset is expressed in <PAGE_SIZE> | |
23 | units. | |
24 | ||
25 | In order to use a swap file with swsusp, you need to: | |
26 | ||
151f4e2b | 27 | 1) Create the swap file and make it active, eg.:: |
ecbd0da1 | 28 | |
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29 | # dd if=/dev/zero of=<swap_file_path> bs=1024 count=<swap_file_size_in_k> |
30 | # mkswap <swap_file_path> | |
31 | # swapon <swap_file_path> | |
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32 | |
33 | 2) Use an application that will bmap the swap file with the help of the | |
34 | FIBMAP ioctl and determine the location of the file's swap header, as the | |
35 | offset, in <PAGE_SIZE> units, from the beginning of the partition which | |
36 | holds the swap file. | |
37 | ||
151f4e2b | 38 | 3) Add the following parameters to the kernel command line:: |
ecbd0da1 | 39 | |
151f4e2b | 40 | resume=<swap_file_partition> resume_offset=<swap_file_offset> |
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41 | |
42 | where <swap_file_partition> is the partition on which the swap file is located | |
43 | and <swap_file_offset> is the offset of the swap header determined by the | |
bf73bae6 | 44 | application in 2) (of course, this step may be carried out automatically |
01dd2fbf | 45 | by the same application that determines the swap file's header offset using the |
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46 | FIBMAP ioctl) |
47 | ||
48 | OR | |
49 | ||
50 | Use a userland suspend application that will set the partition and offset | |
51 | with the help of the SNAPSHOT_SET_SWAP_AREA ioctl described in | |
151f4e2b | 52 | Documentation/power/userland-swsusp.rst (this is the only method to suspend |
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53 | to a swap file allowing the resume to be initiated from an initrd or initramfs |
54 | image). | |
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55 | |
56 | Now, swsusp will use the swap file in the same way in which it would use a swap | |
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57 | partition. In particular, the swap file has to be active (ie. be present in |
58 | /proc/swaps) so that it can be used for suspending. | |
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59 | |
60 | Note that if the swap file used for suspending is deleted and recreated, | |
61 | the location of its header need not be the same as before. Thus every time | |
62 | this happens the value of the "resume_offset=" kernel command line parameter | |
63 | has to be updated. |