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