61 lines
2.7 KiB
Plaintext
61 lines
2.7 KiB
Plaintext
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.txt (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.
|