I have computer with battery power supply that allows running the computer for approximately one minute after power loss. I want to trigger suspend-to-disk immediately after power loss so it can be resumed later. The initrd (default Devuan initrd) looks for suspend signature in the swap partition and resumes from it when the signature is found. I am not sure what happens when power is completely interrupted while writing data to the swap partition. That could happen when the battery fails or the system hangs while suspending. Will the system resume from the corrupted swap partition or it will just ignore the swap partition? I consider the second option better – it is better to have incorrectly unmounted filesystem than corrupted system state.
Is the signature written to the swap partition after or before the other data? Does it use checksums?
Best Answer
If power is lost prior to explicitly entering S4 or S5 state (hereafter just referred to as "hibernation state" for simplicity), then the partially filled data in the swap partition will be ignored completely, because there's no hibernation state persisted. Swap partitions and files are also volatile, and the data in it will be ignored after a reboot without hibernation state.
In the kernel, restoration from hibernation is requested by the configured
platform_hibernation_ops->leave, which is only called on resumption from hibernation state. For example, on most modern platforms where S5 is supported, we configure a reboot notifier.Losing power prior to hibernation state being entered (and thus the hibernation file being completely written) won't have configured any hibernation to resume from, so there's no chance it will try to thaw using the partially-filled swap space. As such, you don't have to worry about the kernel trying to restore from a partially complete hibernation.