# Amazon EMR cluster error: HDFS replication factor error
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When you remove a core node from a core [instance group](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-uniform-instance-group.html) or [instance fleet](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-instance-fleet.html), Amazon EMR might run into an HDFS replication error. This error happens when you remove core nodes and the number core nodes falls below the configured [dfs.replication factor](https://docs.aws.amazon.com/emr/latest/ReleaseGuide/emr-hdfs-config.html) for the Hadoop Distributed File System (HDFS). As such, Amazon EMR can't safely perform the operation. To determine the default value of the `dfs.replication` configuration, [HDFS configuration](https://docs.aws.amazon.com/emr/latest/ReleaseGuide/emr-hdfs-config.html).

## Possible causes
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See the following for the possible causes of HDFS replication factor error:
+ If you [ manually resize](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-manage-resize.html) a core instance group or instance fleet below the configured `dfs.replication` factor.
+ Your policies for [ managed scaling](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-managed-scaling.html) or [ autoscaling](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-automatic-scaling.html) might allow for scaling to reduce the number of core nodes below the threshold of `dfs.replication`.
+ This error can also occur if Amazon EMR tries to [ replace](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-plan-node-replacement.html) an unhealthy core node when a cluster has the minimal number of core nodes defined by []().

## Solutions and best practices
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See the following for solutions and best practices:
+ When you manually resize an Amazon EMR cluster, don't scale down below the `dfs.replication` as Amazon EMR can't safely complete the resize.
+ When you use managed scaling or autoscaling, make sure that the minimum capacity of your cluster isn't lower than the `dfs.replication` factor.
+ The number of core instances should be at least `dfs.replication` plus one. This makes sure that Amazon EMR can successfully replace an unhealthy core node if you enabled unhealthy core replacement.

**Important**  
Failure of a single core node can lead to HDFS data loss if you set `dfs.replication` to 1. If your cluster has HDFS storage, we recommend that you configure the cluster with at least four core nodes for production workloads to avoid data loss and also set the `dfs.replication` factor to at least 2.