Dynamically scale up storage on Amazon EMR clusters
In a managed Apache Hadoop environment—like an Amazon EMR cluster—when the storage capacity on your cluster fills up, there is no convenient solution to deal with it. This situation occurs because you set up Amazon Elastic Block Store (Amazon EBS) volumes and configure mount points when the cluster is launched, so it’s difficult to modify the storage capacity after the cluster is running. The feasible solutions usually involve adding more nodes to your cluster, backing up your data to a data lake, and then launching a new cluster with a higher storage capacity. Or, if the data that occupies the storage is expendable, removing the excess data is usually the way to go.
To help you deal with this issue in a manageable way in Amazon EMR, I will show you how to dynamically scale up the storage using the elastic volumes feature of Amazon EBS. With this feature, you can increase the volume size, adjust the performance, or change the volume type while the volume is in use. You can continue to use your EMR cluster to run big data applications while the changes take effect.
How HDFS and YARN use disk space on an Amazon EMR cluster
When you create an Amazon EMR cluster, by default, HDFS (Hadoop Distributed File System) and YARN (Yet Another Resource Negotiator) are configured to use the local disk storage available on all the core/task nodes. You configure this inside the yarn-site.xml and hdfs-site.xml configuration files.
Specifically, for HDFS, the dfs.datanode.data.dir parameter is configured to use local storage, where it stores the data blocks for the files maintained by the NameNode daemon. And for YARN, the yarn.nodemanager.local-dirs parameter is configured to store the intermediate files needed for the NodeManager daemon to run the YARN containers.
For example, when the cluster is running a MapReduce job, the map tasks store their output files inside the directories defined by yarn.nodemanager.local-dirs. Additionally, the yarn.nodemanager.log-dirs parameter is configured to store the container logs on the core and task nodes after the YARN application has finished.
General best practices for avoiding storage issues
As you plan to run your jobs on an Amazon EMR cluster, the following are some helpful tips to avoid exceeding the available storage on your cluster.
Estimate your future storage needs
Plan ahead regarding the storage needs of your jobs. When you launch a cluster using the default storage configuration, it might not be sufficient for your workloads, and you might have issues while your jobs are running. It is a good practice to estimate how much intermediate storage your jobs will need. And based on that, you can customize the storage configuration when launching a new cluster.
Store passive data in a data lake
Try to design your workloads in such a way that all your passive data is stored in a data lake like Amazon Simple Storage Service (Amazon S3). This way, you use your cluster only for data processing, performing miscellaneous computation tasks, and storing the results back to the data lake for persistent storage. This approach minimizes the storage requirements on the running cluster.
Plan for more capacity
If your use case dictates that the input or output data should be stored locally on the cluster (HDFS or local storage), you should plan the size of your cluster accordingly. For example, if you are using HDFS, you can create a cluster with a higher number of core nodes that will be enough to store your data. Or, you can customize the core instance group to have more EBS storage capacity than what the default configuration provides.
Possible issues if the storage reaches its maximum capacity
As the EMR cluster is being used to run different data processing applications, at some point, the storage capacity on the cluster might be used up. In that case, the following are some of the issues that can affect your cluster.
Issues from a YARN perspective
If the directories that are defined by the yarn.nodemanager.local-dirs or yarn.nodemanager.log-dirs parameters are filled up to at least 90 percent of the total storage of the volume, the NodeManager marks that particular disk as unhealthy. This action then causes the NodeManager to mark the node that has those disks as unhealthy also. So, if the node is unhealthy, the ResourceManager will not assign any containers to the node.
Additionally, if the termination protection feature is turned off on your EMR cluster, the EMR service eventually terminates the node from your cluster.
Issues from an HDFS perspective
If the HDFS usage on the cluster increases, it corresponds to an increase in the usage of local storage on EBS volumes also. In EMR, the HDFS data directories are configured under the same mount point as the YARN local and log directories. So, if the usage of the mount point exceeds the storage threshold (90 percent) due to HDFS, that again causes YARN to mark that disk as unhealthy, and the ResourceManager blacklists the node.
Dynamically resize the storage space on core and task nodes
To scale up the storage of core and task nodes on your cluster, use the following bootstrap action script:
Additionally, the EC2 instance profile of your cluster must have the ec2:ModifyVolume permissions to be able to resize the volume.
The script runs on all the nodes of an EMR cluster. It configures a cron job on the nodes and performs a disk utilization check every 2 minutes. On the master node, it performs the check on the root volume and the volumes that are storing the logs of various master daemons. On core and task nodes, it performs the check on volumes that YARN and HDFS use and determines whether there is a need to scale up the storage.
When it determines that a volume has exceeded 90 percent of its usage, the size of the volume is expanded by the percentage specified by the “--scaling-factor” parameter. During the resize process, the partition of the volume is expanded, and the file system is extended to reflect the updated capacity. All of this happens without affecting the applications that are running on the cluster.
Consider the following caveats before using this solution:
- You can scale up the storage capacity of nodes in an EMR cluster only if the cluster uses EBS volumes as its storage backend. Certain EC2 instance types use only instance store volumes or both instance store and EBS volumes. You can’t resize the storage capacity on clusters that use such EC2 instance types.
- While you are deciding on the scaling factor option of the script, plan ahead to increase the volume so that the updated configuration will last for quite some time. The scaling up of storage has to wait at least 6 hours before applying further modifications to the same volume.
In this post, I explained how HDFS and YARN use the local storage on Amazon EMR cluster nodes. I covered how you can scale up the storage on an EMR cluster using the elastic volumes feature of Amazon EBS. You can use this feature to increase volume size, adjust performance, or change volume type while the volume is in use. You can then continue to use your EMR cluster to run big data applications while the changes are being applied.
If you have any questions or suggestions, please leave a comment below.
About the Author
Jigar Mistry is a Hadoop Systems Engineer with Amazon Web Services. He works with customers to provide them architectural guidance and technical support for processing large datasets in the cloud using open-source applications. In his spare time, he enjoys going for camping and exploring different restaurants in the Seattle area.