Troubleshooting managed instance groups

There are several issues that can prevent a managed instance group (MIG) from successfully creating or recreating a VM instance.

If your MIG cannot create or recreate instances

The problem might be related to the following situations.

The boot disk already exists

By default, a new boot persistent disk is created when you create an instance. The name of the boot disk matches the name of the VM. If you name a VM my-instance, the disk is also named my-instance. If a persistent disk already exists with that name, the request fails. To resolve this issue, you can optionally take a snapshot, and then delete the existing persistent disk.

The instance template is not valid

If you updated your instance template recently, there could be an invalid property that causes the MIG to fail VM creation. Examine the properties for these common errors:

  • You specified a resource that doesn't exist, such as a source image.
  • You misspelled a resource name.
  • You tried to attach an existing non-boot persistent disk in read/write mode but your group contains more than one VM. For groups with more than one VM, any additional disks you want to share between all of the VMs in the group can be attached only in read-only mode.

If your MIG continually tries to recreate instances

The problem might be related to the following situation.

Health check probes cannot reach the instance

If you configured an autohealing policy but you did not configure—or misconfigured—the firewall rule that lets the health check probes reach your application, then your VMs appear unhealthy, and the MIG continuously tries to recreate them. For information about how to configure a health check firewall rule, see Example health check set up.