61 lines
2.7 KiB
Text
61 lines
2.7 KiB
Text
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Using swap files with software suspend (swsusp)
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(C) 2006 Rafael J. Wysocki <rjw@sisk.pl>
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The Linux kernel handles swap files almost in the same way as it handles swap
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partitions and there are only two differences between these two types of swap
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areas:
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(1) swap files need not be contiguous,
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(2) the header of a swap file is not in the first block of the partition that
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holds it. From the swsusp's point of view (1) is not a problem, because it is
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already taken care of by the swap-handling code, but (2) has to be taken into
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consideration.
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In principle the location of a swap file's header may be determined with the
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help of appropriate filesystem driver. Unfortunately, however, it requires the
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filesystem holding the swap file to be mounted, and if this filesystem is
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journaled, it cannot be mounted during resume from disk. For this reason to
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identify a swap file swsusp uses the name of the partition that holds the file
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and the offset from the beginning of the partition at which the swap file's
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header is located. For convenience, this offset is expressed in <PAGE_SIZE>
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units.
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In order to use a swap file with swsusp, you need to:
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1) Create the swap file and make it active, eg.
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# dd if=/dev/zero of=<swap_file_path> bs=1024 count=<swap_file_size_in_k>
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# mkswap <swap_file_path>
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# swapon <swap_file_path>
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2) Use an application that will bmap the swap file with the help of the
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FIBMAP ioctl and determine the location of the file's swap header, as the
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offset, in <PAGE_SIZE> units, from the beginning of the partition which
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holds the swap file.
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3) Add the following parameters to the kernel command line:
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resume=<swap_file_partition> resume_offset=<swap_file_offset>
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where <swap_file_partition> is the partition on which the swap file is located
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and <swap_file_offset> is the offset of the swap header determined by the
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application in 2) (of course, this step may be carried out automatically
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by the same application that determines the swap file's header offset using the
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FIBMAP ioctl)
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OR
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Use a userland suspend application that will set the partition and offset
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with the help of the SNAPSHOT_SET_SWAP_AREA ioctl described in
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Documentation/power/userland-swsusp.txt (this is the only method to suspend
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to a swap file allowing the resume to be initiated from an initrd or initramfs
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image).
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Now, swsusp will use the swap file in the same way in which it would use a swap
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partition. In particular, the swap file has to be active (ie. be present in
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/proc/swaps) so that it can be used for suspending.
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Note that if the swap file used for suspending is deleted and recreated,
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the location of its header need not be the same as before. Thus every time
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this happens the value of the "resume_offset=" kernel command line parameter
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has to be updated.
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