systemd-soft-reboot.service — Userspace reboot operation
systemd-soft-reboot.service
systemd-soft-reboot.service is a system service that is pulled in by
soft-reboot.target and is responsible for performing a userspace-only reboot
operation. When invoked, it will send the SIGTERM signal to any processes left
running (but does not follow up with SIGKILL, and does not wait for the processes to
exit). If the /run/nextroot/ directory exists (which may be a regular directory, a
directory mount point or a symlink to either) then it will switch the file system root to it. It then
reexecutes the service manager off the (possibly now new) root file system, which will enqueue a new boot
transaction as in a normal reboot.
Such a userspace-only reboot operation permits updating or resetting the entirety of userspace with minimal downtime, as the reboot operation does not transition through:
The second phase of regular shutdown, as implemented by systemd-shutdown(8).
The third phase of regular shutdown, i.e. the return to the initrd context
The hardware reboot operation
The firmware initialization
The boot loader initialization
The kernel initialization
The initrd initialization
However this form of reboot comes with drawbacks as well:
The OS update remains incomplete, as the kernel is not reset and continues running.
Kernel settings (such as /proc/sys/ settings, a.k.a. "sysctl", or
/sys/ settings) are not reset.
These limitations may be addressed by various means, which are outside of the scope of this
documentation, such as kernel live-patching and sufficiently comprehensive
/etc/sysctl.d/ files.
Various runtime OS resources can passed from a system runtime to the next, through the userspace reboot operation. Specifically:
File descriptors placed in the file descriptor store of services that remain active
until the very end are passed to the next boot, where they are placed in the file descriptor store of
the same unit. For this to work, units must declare DefaultDependencies=no (and
avoid a manual Conflicts=shutdown.target or similar) to ensure they are not
terminated as usual during the system shutdown operation. Alternatively, use
FileDescriptorStorePreserve= to allow the file descriptor store to remain pinned
even when the unit is down. See
systemd.service(5) for
details about the file descriptor store.
Similar to this, file descriptors associated with .socket units
remain open (and connectible) if the units are not stopped during the transition. (Achieved by
DefaultDependencies=no.)
The /run/ file system remains mounted and populated and may be
used to pass state information between such userspace reboot cycles.
Service processes may continue to run over the transition, if they are placed in
services that remain active until the very end of shutdown (which again is achieved via
DefaultDependencies=no). They must also be set up to avoid being killed by the
aforementioned SIGTERM spree (as per systemd and Storage Daemons for the Root File
System).
File system mounts may remain mounted during the transition, and complex storage
attached, if configured to remain until the very end of the shutdown process. (Also achieved via
DefaultDependencies=no, and by avoiding
Conflicts=umount.target)
Even though passing resources from one soft reboot cycle to the next is possible this way, we
strongly suggest to use this functionality sparingly only, as it creates a more fragile system as
resources from different versions of the OS and applications might be mixed with unforeseen
consequences. In particular it's recommended to avoid allowing processes to survive
the soft reboot operation, as this means code updates will necessarily be incomplete, and processes
typically pin various other resources (such as the file system they are backed by), thus increasing
memory usage (as two versions of the OS/application/file system might be kept in memory). Leaving
processes running during a soft-reboot operation requires disconnecting the service comprehensively from
the rest of the OS, i.e. minimizing IPC and reducing sharing of resources with the rest of the OS. A
possible mechanism to achieve this is the concept of Portable Services, but make sure no resource from the
host's OS filesystems is pinned via BindPaths= or similar unit settings, otherwise the
old, originating filesystem will remain mounted as long as the unit is running.
Note that because
systemd-shutdown(8) is
not executed, the executables in /usr/lib/systemd/system-shutdown/ are not executed
either.
Note that systemd-soft-reboot.service (and related units) should never be
executed directly. Instead, trigger system shutdown with a command such as "systemctl
soft-reboot".