You can soft or hard reboot a running instance. A soft reboot attempts a graceful shut down and restart of the instance. A hard reboot power cycles the instance.
By default, when you reboot an instance, it is a soft reboot.
$ nova reboot SERVER
To perform a hard reboot, pass the –hard parameter, as follows:
$ nova reboot --hard SERVER
It is also possible to reboot a running instance into rescue mode. For example, this operation may be required, if a filesystem of an instance becomes corrupted with prolonged use.
Pause, suspend, and stop operations are not allowed when an instance is running in rescue mode, as triggering these actions causes the loss of the original instance state, and makes it impossible to unrescue the instance.
Rescue mode provides a mechanism for access, even if an image renders the instance inaccessible. By default, it starts an instance from the initial image attaching the current boot disk as a secondary one.
To perform an instance reboot into rescue mode, run the following command:
$ nova rescue SERVER
On running the nova rescue command, an instance performs a soft shutdown first. This means that the guest operating system has a chance to perform a controlled shutdown before the instance is powered off. The shutdown behavior is configured by the
shutdown_timeoutparameter that can be set in the
nova.conffile. Its value stands for the overall period (in seconds) a guest operation system is allowed to complete the shutdown. The default timeout is 60 seconds. See Description of Compute configuration options for details.
The timeout value can be overridden on a per image basis by means of
os_shutdown_timeoutthat is an image metadata setting allowing different types of operating systems to specify how much time they need to shut down cleanly.
To restart the instance from the normal boot disk, run the following command:
$ nova unrescue SERVER
If you want to rescue an instance with a specific image, rather than the default one, use the –rescue_image_ref parameter:
$ nova rescue --rescue_image_ref IMAGE_ID SERVER