In OpenBSD tool , designed for automatic system upgrades to a new release or snapshot of the CURRENT branch.
Sysupgrade downloads the necessary files for the update, verifies them using , copies bsd.rd (a special ramdisk that operates entirely from RAM, used for installation, upgrading, and recovery of the system) to bsd.upgrade, and initiates a system reboot. The bootloader, upon detecting bsd.upgrade, starts its automatic loading (which can be canceled by the user) and automatically updates the system to the previously loaded version.
Currently, sysupgrade can be used for automatic updates to the latest daily snapshots of CURRENT, and starting with OpenBSD 6.6, it is expected to be used for upgrades from release to release. Before sysupgrade, similar tasks had to be performed manually or automated independently.
For installing security updates and bug fixes on stable releases of OpenBSD, it is still recommended to use the utility , which applies binary patches with fixes to the base system.
Bron: opennet.ru
