suspendtodisk

Suspend to disk, also known as hibernation, is a power management state in which a computer saves the contents of its volatile memory to non-volatile storage and then powers off. When resumed, the system restores the saved memory image and returns to the previous user session and state. This allows a full session restore without power consumption, unlike suspend-to-RAM which keeps memory active.

The typical process involves freezing active processes, writing the RAM contents to a swap device or a dedicated hibernation image, and then powering down. Upon startup, the bootloader or initramfs locates the saved image, loads it back into memory, and reinitializes devices and services to resume operation where the user left off. A resume parameter (such as a device UUID or an offset) guides the kernel to the correct image. Adequate swap space (equal to or larger than the system RAM) is usually required, and some setups use an encrypted or compressed image to improve security and space efficiency.

Requirements and limitations include having a accessible and correctly configured swap area, enough disk space, and compatible hardware/firmware. If the swap area is damaged, moved, or unavailable, hibernation can fail. Some devices or drivers may not reliably restore state, and modern fast-boot configurations can interfere with resume. Enabling hibernation may involve configuring the kernel command line with a resume option and ensuring the initramfs can access the image.

In practice, suspend to disk is one option among sleep states, alongside suspend to RAM and hybrid sleep. It is commonly used to preserve user sessions across power-downs while enabling a completely powered-off state.