17.4. HA for Hosts

The user can specify a virtual machine as HA-enabled. By default, all virtual router VMs and Elastic Load Balancing VMs are automatically configured as HA-enabled. When an HA-enabled VM crashes, CloudStack detects the crash and restarts the VM automatically within the same Availability Zone. HA is never performed across different Availability Zones. CloudStack has a conservative policy towards restarting VMs and ensures that there will never be two instances of the same VM running at the same time. The Management Server attempts to start the VM on another Host in the same cluster.

HA features work with iSCSI or NFS primary storage. HA with local storage is not supported.

17.4.1. Dedicated HA Hosts

One or more hosts can be designated for use only by HA-enabled VMs that are restarting due to a host failure. Setting up a pool of such dedicated HA hosts as the recovery destination for all HA-enabled VMs is useful to:

Make it easier to determine which VMs have been restarted as part of the CloudStack high-availability function. If a VM is running on a dedicated HA host, then it must be an HA-enabled VM whose original host failed. (With one exception: It is possible for an administrator to manually migrate any VM to a dedicated HA host.).
Keep HA-enabled VMs from restarting on hosts which may be reserved for other purposes.

The dedicated HA option is set through a special host tag when the host is created. To allow the administrator to dedicate hosts to only HA-enabled VMs, set the global configuration variable ha.tag to the desired tag (for example, "ha_host"), and restart the Management Server. Enter the value in the Host Tags field when adding the host(s) that you want to dedicate to HA-enabled VMs.

Note

If you set ha.tag, be sure to actually use that tag on at least one host in your cloud. If the tag specified in ha.tag is not set for any host in the cloud, the HA-enabled VMs will fail to restart after a crash.