Why Is a Managed Instance Group Failing to Create New Instances?

ACE Last updated

Managing compute resources.

Short answer

A Create a new valid instance template for the managed instance group and delete any persistent disks whose names match the instance names.

Question

Your managed instance group raised an alert stating that new instance creation has failed to create new instances. You need to solve the instance creation problem. What should you do?

  1. Create an instance template that contains valid syntax which will be used by the instance group. Delete any persistent disks with the same name as instance names. Correct Answer
  2. Create an instance template that contains valid syntax that will be used by the instance group. Verify that the instance name and persistent disk name values are not the same in the template.
  3. Verify that the instance template being used by the instance group contains valid syntax. Delete any persistent disks with the same name as instance names. Set the disks.autoDelete property to true in the instance template.
  4. Delete the current instance template and replace it with a new instance template. Verify that the instance name and persistent disk name values are not the same in the template. Set the disks.autoDelete property to true in the instance template.

Key Takeaways

The exam tests that Compute Engine instance templates are immutable and that leftover persistent disks with the same names as instances can block MIG creation; the trap is trying to edit or delete the existing template.

Managed instance groups create VMs from immutable instance templates, so failed instance creation often requires a new valid template plus cleanup of conflicting persistent disks. This page establishes that option A is the correct ACE troubleshooting answer.

Common mistake: The most common wrong pick is C: setting disks.autoDelete sounds like proper prevention, but you cannot modify an existing instance template, so the described fix is not executable.

Expert Analysis

Why the Answer Is Correct

Option A is correct because the managed instance group must be fed a usable, newly created instance template; existing templates are immutable, so repairing the current one is not an option. The second required action is clearing persistent disks whose names collide with the instance names, which is a common reason MIG instance creation fails even when the template syntax is valid. Once those blockers are removed and the MIG uses the new valid template, it can create replacements. This matches Google's operating model for MIGs and the exam's emphasis on troubleshooting compute resource provisioning.

Why the Other Options Are Wrong

Option B creates a valid template but only verifies that disk and instance names differ; it does not delete any already-existing persistent disk with the colliding name, so the provisioning failure can persist. Option C wrongly assumes you can verify and then set disks.autoDelete on the template already used by the MIG; Compute Engine instance templates cannot be edited after creation. Option D is worse because it proposes deleting a template that is still referenced by the MIG, which is not allowed, and it also relies on modifying template behavior that would require a new template. The exam trap is choosing the preventive-sounding autoDelete action while ignoring instance-template immutability.

Community Comment Notes

scanner2's explanation is the key: C fails because "you cannot update or modify an existing instance template" and D fails because a template referenced by a MIG cannot simply be deleted. Dewansh and 3arle reinforced the immutability rule, with 3arle explicitly changing to A after realizing templates cannot be updated. Learners who chose C, such as 0d2fdfa, focused on disks.autoDelete as future prevention, but that does not make C executable against the current template. happydays and scanner2 align with the documented behavior, so the community's useful signal supports A.

Official Reference

Exam Strategy

For MIG creation failures, first separate template validation from template mutation: instance templates are immutable, so the fix must create a replacement. Then check for orphaned persistent disks or name collisions, because those are resource-level blockers that a new template alone does not clear.

Frequently Asked Questions

Why can't I just update the existing instance template to set disks.autoDelete to true?

Compute Engine instance templates are immutable; you must create a new template and update the MIG to use it. Setting autoDelete requires a replacement template.

Why delete persistent disks with the same name as the instance names?

A leftover disk with a matching name can block MIG instance creation; deleting it clears the name collision so the new template can create instances.

More ACE FAQ →

Discussion 12 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded.

From public community threads 12

  1. scanner2 👍 20 Answered A
    C is eliminated because you cannot update or modify an existing instance template. It is immutable. https://cloud.google.com/compute/docs/instance-templates#how_to_update_instance_templates D is eliminated because you cannot delete an instance template if a managed instance group references it. https://cloud.google.com/compute/docs/instance-templates/get-list-delete-instance-templates#delete_an_instance_template B is eliminated because you cannot set different/custom name for persistent disk in an instance template. https://cloud.google.com/sdk/gcloud/reference/compute/instance-templates/create#--create-disk So we left with A. Therefore A is the correct answer. Although, it should set the disks.autoDelete property to true for completeness.
  2. Ell89 👍 5
    repeated question
  3. recluse7 👍 3 Answered C
    When a managed instance group raises an alert indicating that new instance creation has failed, it's important to investigate the issue. This issue is typically related to the configuration of the instance template used by the group. Therefore, the first step is to verify that the instance template being utilized by the instance group contains valid syntax. This includes checking all settings such as machine type, boot disk, and any custom configurations. Valid syntax ensures that the instances created adhere to the specified configurations.
  4. Dewansh 👍 3 Answered A
    instance templates are immutable so can not modify or update
  5. 3arle 👍 2
    i was wrong, templates are immutable so you cannot update them, so option A is valid
Show 7 more comments
  1. Enamfrancis 👍 2 Answered C
    C is corrct answer
  2. happydays 👍 2 Answered A
    THIS IS A
  3. yodaforce 👍 1
    A is correct answer auto-delete If set to yes, the persistent disk is automatically deleted when the instance is deleted. However, if you detach the disk from the instance, deleting the instance doesn't delete the disk. The default value is yes. https://cloud.google.com/sdk/gcloud/reference/compute/instance-templates/create#--create-disk
  4. 681230f 👍 1 Answered C
    crucial is to have disks.autoDelete: true, whithout this in a future you will have the same problem, for this reaison , option C is true, verify an instance template is ok because it is only a checkpoint not a change
  5. wooyourdaddy 👍 1 Answered C
    You can update the MIG to use a new template with disks.autoDelete = true using this syntax: gcloud compute instance-groups managed set-instance-template \ --instance-group <group> \ --template <new-template>
  6. 3arle 👍 1 Answered C
    Existing instance group has already template so no requirement to create new one. Problem is with persistent disk so delete it and configure autodelete solves problem now and in future
  7. 0d2fdfa 👍 1 Answered C
    A Correctly suggests deleting conflicting disks, but doesn't include the autoDelete setting to prevent it from happening again.

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