Regional MIG target distribution shape

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By default, a regional managed instance group (MIG) distributes its managed virtual machine (VM) instances evenly across selected zones. But if you need hardware that is not available in all zones or that might be temporarily unavailable in selected zones, or if you need to prioritize the use of zonal reservations, you might prefer a different distribution.

To configure how a regional MIG distributes your managed instances across selected zones within a region, set the MIG's target distribution shape. The following options are available:

• EVEN (default): the group creates and deletes VMs to achieve and maintain the same number of VMs across the selected zones. In an EVEN distribution, the number of VMs does not differ by more than 1 between any two zones. Recommended for highly available serving workloads.
• BALANCED: the group prioritizes creation of VMs in zones where resources are available, while distributing VMs as evenly as possible across selected zones to minimize the impact of zonal failure. Recommended for highly available serving or batch workloads.
• ANY: the group picks zones for creating VM instances to fulfill the requested number of VMs within present resource constraints and to maximize utilization of unused zonal reservations. Recommended for batch workloads that do not require high availability.
• ANY SINGLE ZONE: the group creates all VM instances within a single zone. The zone is chosen based on hardware support, current resource and quota availability, and matching reservations. Recommended in combination with a compact instance placement policy for workloads that require extensive communication between VMs.

Choose an option based on your workload requirements and which MIG capabilities you need. See the comparison table, use cases, and how distribution shapes work.

Comparison of shapes

For each possible target shape, the following table describes its intended workloads, purpose, distribution of managed instances, feature support, and a brief description of MIG behavior when faced with unavailable resources.

	EVEN (default)	BALANCED	ANY	ANY_SINGLE_ZONE
Intended workloads	Highly available serving workloads (stateless or stateful)	Highly available serving workloads (stateless or stateful) Highly available batch workloads	Batch workloads	Batch workloads requiring extensive communication between VMs
Purpose	Minimize the impact of zone-level failure, assuming sufficient availability of resources in each zone.	Minimize the impact of zone-level failure as much as possible considering availability of resources in each zone.	Prioritize resource acquisition and utilization of unused reservations.	Minimize network latency and costs between VMs by keeping all VMs in one zone.
Target distribution of managed instances across zones	Even. The number of managed instances does not differ by more than 1 between any two zones, regardless of resource availability.* Some managed instances might not be up and running in case of zonal capacity constraints.	As even as possible. No guarantee on discrepancies in the number of VMs across zones, which depends on current resource availability. When resources are available, the distribution is similar to EVEN. In the worst case of resource constraints, the distribution can take any shape.	Any. Each zone can have a different number of managed instances (including all or none).	Single zone. All instances are created within one zone. The MIG chooses the zone whenever it scales out from zero VMs.
Feature support	EVEN (default)	BALANCED	ANY	ANY_SINGLE_ZONE
Autoscaling
Canary updates
Instance flexibility		(Preview)		(Preview)
Proactive instance redistribution			Not applicable
Reservations	Maximally utilized within each zone independently. Reservations don't impact how instances are distributed.	Maximally utilized within each zone independently. If reservations are present they might help arriving at a balanced distribution.	Maximally utilized within the region. The group prioritizes using up reservations in the region.	Maximally utilized within the chosen zone. Whenever the group has no VMs and needs to create one or more VMs, the group prioritizes the zone with the most reservations if that zone also has sufficient resources and supports the hardware to fulfill the request.
Instance template and stateful configuration hardware requirements (machine type, CPU, GPU, existing disks)	Selected hardware must be available in all selected zones.	Selected hardware must be available in at least one selected zone.	Selected hardware must be available in at least one selected zone.	Selected hardware must be available in at least one selected zone.
Sole-tenant nodes
Handling failures	EVEN (default)	BALANCED	ANY	ANY_SINGLE_ZONE
Temporary unavailability of resources in a zone	Exposed Creates new managed instances in zones with fewer managed instances. Keeps retrying to create VM instances in a zone where resources are unavailable until it succeeds. Risk: Cannot create VMs in a zone with limited resources.	Resilient Creates new managed instances in zones where resources are available, while distributing instances as evenly as possible across zones. Risk: VMs might not be distributed evenly across zones.	Resilient Creates new managed instances in zones where resources are available and to maximize utilization of unused reservations. Risk: VMs might not be distributed evenly across zones.	Resilient on group creation and resizes from zero Creates new VM instances within a single zone, where resources are available. Risk: Cannot guarantee all additional instances are successfully created during scale-out requests if the chosen zone does not have enough resources.
Zone-level failure	Resilient Impact is minimized because instances in healthy zones keep serving. Impact is further minimized if you provision extra instances, sufficient to tolerate losing one zone.	Resilient Impact is minimized because instances in healthy zones keep serving. Impact is further minimized if you provision extra instances, sufficient to tolerate losing one zone.	Exposed Outage might happen if the majority or all instances are concentrated in a failed zone.	Exposed Outage is inevitable if the failure happens in the chosen zone.

Use cases

Review the feature support, then choose a distribution shape based on your use case.

Prioritize workload resilience with an even distribution

If you run a highly available serving application that must survive zone-level failure without degraded performance, use the EVEN target distribution shape with an over-provisioned group size. Overprovisioning the number of instances in a group protects your workload from zone-level failure.

Depending on your workload, consider creating an autoscaler to automatically add or remove instances to your group when load increases or decreases.

To learn more about the EVEN target distribution shape, see the comparison of target shapes and read How the EVEN target shape works.

For more information about deploying highly available workloads on regional MIGs, see the following sections:

• If you need special hardware that is only available in specific zones, select zones for your VMs
• Provision the correct managed instance group size.
• If you want to prevent unintended deletion of instances when you manually delete or abandon instances from a regional group, turn off cross-zone instance redistribution
• Configure autoscaling for a regional MIG, which tries to preserve an even balance across zones on group resizes and scales out to available zones in case of zone-level failure.
• Configure load balancing, which shifts load to healthy zones in case of zone-level failure and, if autoscaling is configured, leads to an increase in the number of VMs in zones where load grows.
• Configure a stateful regional MIG, which can preserve the unique state of each of your MIG's instances, including VM instance name, persistent disks, and metadata, on machine restart, recreation, autohealing, or update events.

Balance resource acquisition with an even distribution

If you run a highly available serving or batch workload and need to balance acquisition of resources against an even distribution of VM instances across selected zones in a region, use the BALANCED target distribution shape.

The BALANCED shape prioritizes acquisition of resources—the group creates instances in zones where resources are available—while distributing instances as evenly as possible across zones to minimize the impact of zone-level failure.

If you run a batch workload that doesn't need to be protected against zone-level failure, use the ANY target shape instead. The ANY shape prioritizes acquisition of resources as well as use of zonal reservations.

With the shape set to BALANCED or to ANY, you don't need to manually verify whether specific hardware is available in a particular zone. You can select all zones in a region and the group automatically deploys instances in zones where your required hardware is available.

To learn more about the BALANCED target distribution shape, see the comparison of target shapes and read How the BALANCED target distribution shape works.

Prioritize resource acquisition

If you run batch workloads and if getting the requested number of instances to perform the processing is more important for you than workload resilience to zone-level failures, use the ANY target distribution shape.

If you have matching reservations, set your target shape to ANY to prioritize the use of zones that contain the matching reservations. To learn how to configure reservations in an instance template, see Consuming instances from a specific reservation.

Similar to the BALANCED target shape, the ANY shape is useful when your batch workload requires any of the following features:

• VMs with special hardware, such as a specific CPU platform or GPU model. The group will deploy instances to the zones that support the requested hardware, according to resource availability, and with a preference for zones that have matching reservations.
• Preemptible VMs. You won't need to explore which zones have preemptible capacity available. The group will deploy to zones with preemptible capacity automatically.
• VMs with a large number of cores. The group will get large machines where they are available, with a preference for zones that have matching reservations.

You don't need to manually verify whether specific hardware is available in a particular zone. You can select all zones in a region and the group automatically deploys instances in zones where your required hardware is available.

You can selectively delete batch job worker instances that have completed calculations without affecting other workers. Unlike a group with an EVEN target shape and proactive redistribution, a group with ANY target shape doesn't have to achieve an even balance and won't trigger redistribution.

To learn more details about the ANY target distribution shape, see the comparison of target shapes and read how the ANY target distribution shape works.

Minimize networking across VMs

If you run a batch workload and want to place all VMs in a single zone to reduce VM-to-VM network latency and costs, and if you don't have a specific zone requirement, set the group's target shape to ANY_SINGLE_ZONE. You can also create a compact placement policy and apply it to your MIG so that the VMs in the MIG are located closer to each other and on the same network infrastructure.

When you create a MIG with at least one VM, and whenever a MIG with no VMs needs to scale out again, the ANY_SINGLE_ZONE shape picks the optimal zone based on your reservations, quotas, and hardware requirements.

Similar to the BALANCED and ANY target shapes, the ANY_SINGLE_ZONE shape is useful when your batch workload requires any of the following features:

• VMs with special hardware, such as a specific CPU platform or GPU model. The group will deploy instances to a zone that supports the requested hardware, according to resource availability, and with a preference for the zone that has matching reservations.
• Preemptible VMs. You won't need to explore which zones have preemptible capacity available. The group will deploy to a zone with preemptible capacity automatically.
• VMs with a large number of cores. The group will get large machines where they are available, with a preference for the zone that has matching reservations.

You don't need to manually verify whether specific hardware is available in a particular zone. When creating the MIG, select all zones in a region and the group automatically deploys instances in a zone where your required hardware is available.

To learn more details about the ANY_SINGLE_ZONE target distribution, see the comparison of target shapes and read how the ANY_SINGLE_ZONE target distribution shape works.

If you have specific zone requirements and don't want your MIG to switch zones under any circumstance, use a zonal MIG instead.

How it works

This section describes how each target distribution shape works in the following situations:

• When you resize the MIG
• In case resources are temporarily unavailable in a zone
• In case of zonal failure

The EVEN distribution shape

With a target distribution shape set to EVEN and proactive redistribution enabled, the number of managed instances in a regional MIG does not differ by more than 1 between any two zones, regardless of resource availability. But a managed instance might not be up and running if its zone lacks the resources to provision an actual VM.

Resizing a MIG that has an EVEN distribution shape

A group with an EVEN target shape picks zones for adding or deleting instances in a way that preserves or converges to an even balance of managed instances across zones.

For example, the following diagram shows how a group adds and removes managed instances.

Impact of temporarily unavailable resources

Resources might be temporarily unavailable in a zone when you create the group or increase the number of instances. For example, if you request preemptible instances or specialized hardware in a limited supply, those resources might not be available at the time of your request.

With the goal of maintaining an even distribution of instances across zones, the group continues attempting to create VM instances in zones where the resources are temporarily unavailable. Eventually, the group does acquire the full number of running VM instances after the resources become available.

For example, the following diagram shows what happens if one of the zones cannot fulfill your request due to a temporary unavailability of resources.

Impact of zone-level failure

If you use the EVEN (or BALANCED) target distribution shape, you can provision extra instances to minimize impact of a zone-level failure.

In case of zone-level failure, a regional MIG that is deployed to 3 zones with an EVEN (or BALANCED) target distribution shape might lose 1/3 of its instances. You can ensure sufficient capacity to serve your load in case of zone-level failure by provisioning more VMs, 2/3 of which are required by the load.

For example, if you require 8 instances to process requests across 3 zones and you want to protect your workload against zone-level failure, you should create a regional group with 12 instances. The following diagram shows what happens if one zone fails.

The EVEN target distribution shape works well with autoscaling and load balancing under such circumstances. In case of a zone-level failure, the load balancer starts sending traffic to instances in the two remaining zones to accommodate traffic from the failed zone.

For more information about how a regional MIG works with an autoscaler, see Autoscaling a regional MIG.

The BALANCED distribution shape

A regional MIG with a BALANCED target shape might not achieve an even distribution across zones, specifically when the requested resources are not available in a zone.

The MIG prioritizes provisioning the requested number of VMs by creating VMs in zones where resources are available. When resources are available, the distribution is similar to EVEN. In the worst case of resource constraints, the distribution can take any shape.

Resizing a MIG that has a BALANCED distribution shape

Increasing group size

With a BALANCED target shape, the group chooses zones for creating new instances based on the current availability of the resources that you specified in the MIG's instance template.

• When resources are sufficiently available in all selected zones, the group maintains an even distribution across zones on size increases, the same way as the EVEN target shape.
• When zonal capacity constraints make it impossible to achieve an even distribution, the group creates instances in the zones where resources are available, while still trying to maximize balance.

For example, you might observe capacity constraints and an uneven distribution if you request a specialized CPU platform, GPU model, or preemptible VMs that are not uniformly available in all zones.

Decreasing group size

When decreasing its size, a Regional MIG with a BALANCED target shape removes instances in the following sequence in order to limit disruption to your workload:

1. Instances that are not running; that is, instances that for any reason can't be created or are being created or autohealed.
2. Instances in zones where the group has more VMs, to converge to an evenly distributed state.

Impact of temporarily unavailable resources or zonal failure

With a BALANCED target distribution shape, the group deploys instances to zones where capacity is available. During temporary zonal capacity constraints, this can lead to an uneven distribution of instances across zones.

If in such a situation a zone with the largest number of VM instances fails, your workload might lose a significant share of your serving capacity. If the healthy zones have temporary capacity constraints, the group tries to recreate failed instances in the original location (a failed zone) and this attempt might fail.

To protect your workload against such an extreme case:

• Overprovision the size of your regional MIG, so that your workload has sufficient serving capacity in the case of a zonal failure.
• Reserve a sufficient amount of resources in each zone to cover peak load, to overprovision, and to maintain an even distribution across zones. This tactic helps ensure that you can get an even distribution of instances across zones, which minimizes capacity loss in case of a zonal failure.

The following diagram shows how a scenario with temporary zonal capacity constraints, followed by a zonal failure, might evolve.

If your request cannot be fulfilled in any zone in the region, the group schedules VM creation in zones with temporarily unavailable resources. The group continues attempting to create the scheduled instances within the zones where their creation was originally scheduled. If the resources become available in other zones sooner than in the original zone where a VM was scheduled, the group won't try creation in those other zones. You can schedule new instances in zones with available capacity manually by deleting the managed instances that failed to create and resizing the group up to its target size.

If VM creation is unsuccessful, you can list managed instances to review the error message in the corresponding managed VM instance or list recent errors.

In case of a zonal failure, the BALANCED target distribution shape works well with autoscaling and load balancing. To accommodate traffic from the failed zone, the load balancer sends traffic to instances in the remaining zones. An autoscaler responds to the increased utilization in the zones and automatically creates capacity in healthy zones. For more information, see Autoscaling a regional MIG.

The ANY distribution shape

With a target distribution shape set to ANY, a regional MIG prioritizes resource acquisition by creating managed instances in zones where resources are available. This means that all of the instances might be created in one zone, or evenly distributed across all zones, or anything between those two scenarios.

Resizing a MIG that has an ANY distribution shape

Increasing group size

When you increase group size, the group picks any zone where capacity is available.

If you have matching reservations in one or more zones, the group prioritizes utilization of those reservations. However, if you decrease the group size, it might take a few minutes for any consumed reservation to be available again for consumption. During this period when previously consumed reservations are not available yet, if you increase the group size and there are no matching reservations, then the group creates VM instances in a zone where resources are available.

Decreasing group size

When you decrease group size, the group deletes the VM instances in the following order:

1. VMs that are not running for any reason
2. VMs that are not yet updated to the intended version
3. VMs chosen nondeterministically

If you need to decrease group size in specific zones or remove specific VM instances, for example workers that finished their job, you can delete specific instances from the group.

Impact of temporarily unavailable resources

With a target distribution shape set to ANY, the group schedules VM instance creation in zones where the requested resources are available and avoids zones with temporarily unavailable resources.

If your request cannot be fulfilled in any zone in the region, the group schedules VM creation in zones with temporarily unavailable resources. The group will keep trying to create the scheduled instances within the zones where their creation was originally scheduled. If the resources become available in other zones sooner than in the original zone where a VM was scheduled, the group won't try creation in those other zones. You can manually schedule new instances in zones with available capacity by deleting the non-running managed instances and resizing the group up to its target size.

If VM creation is unsuccessful, you can list managed instances to review the error message in the corresponding VM instance or list recent errors.

For example, the following diagram shows how a regional group schedules instances when a zone cannot fulfill your request.

Impact of zone-level failure

With its target distribution shape set to ANY, the group might deploy the majority or all of its instances in a single zone. In the event of failure in that zone, most or all of the group's instances could become unavailable for the duration of the failure.

In case of a zone-level failure or resources becoming temporarily unavailable, or when for any reason your VM instances are not running, you can delete the individual non-running instances then resize the group back to its necessary size in order to try to get replacement instances in zones with available capacity.

The ANY_SINGLE_ZONE distribution shape

A regional MIG with the ANY_SINGLE_ZONE target distribution shape automatically selects the optimal zone when the first VM in the group is created. After the first VM is created, all other VMs are created in the same zone.

The MIG can select a different zone only when it is scaled back to zero VMs and it starts creating its first VM again.

Choosing the optimal zone

When only one of the selected zones supports the group's hardware requirements then Compute Engine chooses this zone.

When multiple selected zones support the group's hardware requirements then Compute Engine chooses a zone that has enough available resources to fit all of the regional MIG's VMs, with a preference for the zone with the most matching reservations.

If none of the selected zones has enough available resources or matching reservations to accommodate all VMs then, in order to create as many VMs as possible, Compute Engine chooses the zone with the most available resources and matching reservations, with a preference for the zone with the most matching reservations. The group continues to try to create the remainder of the VMs in the same zone even if resources become available sooner in another zone.

Resizing a MIG that has an ANY_SINGLE_ZONE distribution shape

Increasing group size

If a MIG already has VMs in it and has its target distribution shape set to ANY_SINGLE_ZONE, then for all scale-out operations, the MIG places new VMs within the same zone as existing VMs. If there are not enough available resources or reservations to accommodate all additional VMs, the MIG creates as many as possible.

If the MIG has no VMs, then when it is scaled out, it chooses the optimal zone that supports the group's hardware requirements and that utilizes matching reservations.

Decreasing group size

When scaling in, a regional MIG with the ANY_SINGLE_ZONE distribution shape removes VMs in the following order:

• VMs that are not in RUNNING state are removed first to limit the disruption to your workload. A not-running VM is a VM that for any reason can't be created or is being created or repaired.
• VMs that don't use the group's latest configuration.
• VMs chosen nondeterministically

Impact of temporarily unavailable resources

A regional MIG with its target distribution shape set to ANY_SINGLE_ZONE is susceptible to resource shortages in the selected zone.

If resources become temporarily unavailable in the MIG's chosen zone, the MIG does not automatically switch zones. This means that scale-out and update processes can become disrupted until sufficient resources become available.

Impact of zone-level failure

A regional MIG with its target distribution shape set to ANY_SINGLE_ZONE is susceptible to zonal failures.

In the unlikely event of a zonal failure in the zone that hosts your regional MIG's VMs, all of the MIG's VMs might become unable to process your workload.

What's next

• To learn how to set the target shape on a MIG, see Setting a policy for distributing VMs across zones.
• Read more about Regional MIGs.
• Learn about zonal reservations of Compute Engine resources.