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diff --git a/Documentation/power/swsusp-and-swap-files.rst b/Documentation/power/swsusp-and-swap-files.rst new file mode 100644 index 000000000..a33a2919d --- /dev/null +++ b/Documentation/power/swsusp-and-swap-files.rst @@ -0,0 +1,63 @@ +=============================================== +Using swap files with software suspend (swsusp) +=============================================== + + (C) 2006 Rafael J. Wysocki <rjw@sisk.pl> + +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 +areas: +(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 +consideration. + +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> +units. + +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) + +OR + +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.rst (this is the only method to suspend +to a swap file allowing the resume to be initiated from an initrd or initramfs +image). + +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. |