Saturday, October 17, 2009

Avoiding EC2 InsufficientInstanceCapacity: Insufficient Capacity Errors

Here's a quick tip from this thread on the AWS EC2 Developer Forums.

If you experience the InsufficientInstanceCapacity: Insufficient Capacity error, you'll be glad to know there are some strategies for working around it. Justin@AWS offers this advice:
There can be short periods of time when we are unable to accommodate instance requests that are targeted to a specific Availability Zone. When a particular instance type experiences unexpected demand in an Availability Zone, our system must react by shifting capacity from one instance type to another. This can result in short periods of insufficient capacity. We incorporate this data into our capacity planning and try to manage all zones to have adequate capacity at all times. The following steps will ensure that you will have the best experience launching Amazon EC2 instances when an initial insufficient capacity message is received:

1. Don't specify an Availability Zone in your request unless necessary. By targeting a specific Availability Zone you eliminate our ability to satisfy that request by using our other available Availability Zones. Please note that a single RunInstances call will allocate all instances within a single Availability Zone.

2. If you require a large number of instances for a particular job, please request them in batches. The best practice to follow here is to request 25% of your total cluster size at a time. For example, if you want to launch 200 instances, launching 50 instances at a time will result in a better experience.

3. Try using a different instance type. As capacity varies across instance types, attempting to launch difference instance types provides spill over capacity should your primary instance type be temporarily unavailable.

Unfortunately, these techniques require that you be willing to accept higher bandwidth costs for cross-availability-zone traffic.

And, none of these tips help if you're using Auto Scaling. A single Auto Scaling Group must be in a specific availability zone, so #1 won't help. You can try using smaller numbers of instances when a trigger is reached by choosing a smaller LowerBreachScaleIncrement or UpperBreachScaleIncrement (which control by how many instances or by what percent to scale in each direction), as per #2, but this is only helpful if you've planned in advance. And #3 is only possible if you've already noticed an auto scaling activity failure and changed the Launch Configuration - which defeats the purpose of Auto Scaling.

Auto Scaling's error reporting and recovery is very limited currently. Are you listening, AWS?

Update 18 October 2009: AWS is listening. The following post by John@AWS appears in this thread:
AutoScaling currently reports [...] InsufficientInstanceCapacity [...] as a generic Internal Error. This is unintentional, and will be remedied in our next release.

Update 19 October 2009: Auto Scaling Groups can now be configured to support more than one Availability Zone. Here is the salient quote from the updated documentation:
Instance Distribution and Balance across Multiple Zones

Amazon Auto Scaling attempts to distribute instances evenly between the Availability Zones that are enabled for your AutoScalingGroup. Auto Scaling uses the Availability Zone with the least number of instances when launching new instances. However, if an Availabilty Zone has insufficient capacity or if Amazon EC2 is unable to launch new instances in it, then Auto Scaling launches instances in another Availability Zone to satisfy the required capacity for your group.

Certain operations and conditions can cause your AutoScalingGroup to become unbalanced. Auto Scaling compensates by creating a rebalancing activity under any of the following conditions:

  1. You issue a request to change the Availability Zones for your group.

  2. You call TerminateInstanceInAutoScalingGroup, which causes the group to become unbalanced.

  3. An Availability Zone that previously had insufficient capacity recovers and has additional capacity available.

Auto Scaling always launches new instances before attempting to terminate old ones, so a rebalancing activity will not compromise the performance or availability of your application.

Multi-Zone Instance Counts when Approaching Capacity

Because Auto Scaling always attempts to launch new instances before terminating old ones, being at or near the specified maximum capacity could impede or completely halt rebalancing activities. To avoid this problem, the system can temporarily exceed the specified maximum capacity of a group by a 10% margin during a rebalancing activity. The margin is only extended if the group is at or near maximum capacity and needs rebalancing (either as a result of user-requested rezoning or to compensate for zone availability issues). The extension only lasts as long as needed to re-balanced the group (typically a few minutes).


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