Before you define an autoscaling policy, note the following autoscaling behaviors.
- Clusters can perform one upscale or downscale operation at a time.
- A cluster will continue to accept jobs while it is running, regardless of any in-progress upscale or downscale operations.
- Only one autoscale policy type (either load-based or schedule-based) can be configured for a single host group, in a single cluster, at a time.
- Autoscaling is available only for host groups with nodes running YARN NodeManager (and optionally client/GATEWAY components).
- If there are not enough nodes available to match the requested scale operation, the operation will proceed on however many nodes are available (for example, during a request for a 10 node scale-up, if the cluster loses 1 node, the operation will proceed with scaling-up 9 nodes instead of 10).
- Autoscaling will be disabled if the cluster has any node failures on instances running YARN ResourceManager, or the ClouderaManager node.
- After scaling down, nodes will show up as UNHEALTHY in Cloudera Manager. This is expected. While scaling down, stopped nodes are put into maintenance mode, to suppress alerts.
Cluster management operations with load-based autoscaling enabled
You may receive a warning when you try to perform certain administrative operations on a
cluster with a load-based autoscaling policy enabled. When a cluster is running with stopped
instances (as a result of a downscale), operations like Data Hub upgrades or certificate
rotation are not allowed. To perform these operations, make sure that the cluster is in a
state where there are 0 stopped instances. You can do this by starting all instances that
are in a stopped state (temporarily disable autoscaling, then use the beta CLI command
cdp datahub start-instances --cluster CLUSTER --host-group HOST_GROUP
[--instance-count INSTANCE_COUNT]). Alternatively you can delete all instances in
a stopped state. Specific operations are affected as follows:
- Node repair: If there are STOPPED nodes in a cluster, and some other nodes need to be repaired - the operation can be performed by including all of the stopped nodes in the set of nodes to be repaired.
- Cluster stop/start: Cluster stop is supported. During a cluster start, all nodes in the host group will be started and recommissioned.
- Node delete: Node delete (non-forced) is supported, including for STOPPED instances. Forced node delete may require you to End Maintenance (Enable Alerts/Recommission) in Cloudera Manager after performing another upscale operation.
- Retry: Do not attempt retry on an autoscale operation. Instead, see Manually recovering from load-based scaling failures. The nature of load-based autoscaling is to automatically try again after some time, based on the load on the cluster. There are a few scenarios where this retry will be adequate to continue cluster operations.
Only a single management operation (including scaling) can be performed on a cluster at a time. For example, if an upgrade is in progress, scaling cannot be performed until the upgrade completes.
EBS versus ephemeral storage
YARN Decommission Timeout
- Schedule-based downscaling will attempt to remove nodes to reach the target count,
regardless of whether the node has running tasks, or data for a running job. You can use
YARN DecommissionTimeout to control the leeway available to jobs beyond the scheduled
decommission time. For example, with YARN decommission timeout set to one hour, a
scheduled downscale will result in the following behavior:
- Vacant nodes (no running tasks, no tasks previously run for running jobs) will be removed immediately.
- Other nodes will be marked as decommissioning, and will not accept new work.
- The remaining nodes will be terminated once any jobs complete, or at the one hour (YARN decommission timeout) mark.
For load-based autoscaling, the YARN DecommissionTimeout should be set to 30 seconds. See Configuring autoscaling for more details.